Developments in Magnetic Pulse Welding

(1)

i

Diogo Jorge de Oliveira Andrade Pereira

Mestrado Integrado em Engenharia Mecânica

Developments in Magnetic Pulse Welding

Dissertação para obtenção do Grau de Doutor em Engenharia Mecânica

Orientador: Rosa Maria Mendes Miranda, Professora Associada com Agregação Aposentada da Faculdade de Ciências

e Tecnologias da Universidade Nova de Lisboa Co-orientador: Telmo Jorge Gomes dos Santos, Professor

Associado da Faculdade de Ciências e Tecnologias da Universidade Nova de Lisboa

Presidente: Prof. Doutor Jorge Joaquim Pamies Teixeira Arguente(s): Prof. Doutor Altino de Jesus Roque Loureiro Prof. Doutora Ana Rosanete Lourenço Reis

Vogais: Prof. Doutora Maria Luísa Coutinho Gomes de Almeida

Prof. Doutor Telmo Jorge Gomes dos Santos

Julho de 2018

(2)

ii

(3)

i

Developments in Magnetic Pulse Welding

Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa

A Faculdade de Ciências e Tecnologia e a Universidade Nova de Lisboa têm o direito, perpétuo e sem limites geográficos, de arquivar e publicar esta dissertação através de exemplares impressos reproduzidos em papel ou de forma digital, ou por qualquer outro meio conhecido ou que venha a ser inventado, e de a divulgar através de repositórios científicos e de admitir a sua cópia e distribuição com objetivos educacionais ou de investigação, não comerciais, desde que seja dado crédito ao autor e editor.

(4)

ii

(5)

iii

RESUMO

A soldadura por impulso magnético é um processo de soldadura, por impacto, no estado sólido que permite produzir juntas sobrepostas tanto em geometria tubular como em planar. Recentemente, tem atraído a atenção da indústria por permitir soldar combinações dissimilares (metálicos e não metálicos) que tendem a formar intermetálicos quando soldados por tecnologias à base de fusão.

No entanto, os equipamentos não têm sofrido alterações e são normalmente dimensionados para resistir a várias descargas o que compromete a sua eficiência. Na maioria dos casos estão equipados com grandes bancos de condensadores, mas por vezes a energia é insuficiente para fazer ligações dissimilares.

Os equipamentos existentes foram analisados neste estudo para identificar os componentes mais críticos a otimizar. Foi desenvolvido e construído um protótipo com o objectivo de a aumentar a eficiência da descarga dos condensadores. O equipamento foi testado e validado na produção de juntas tubulares devido às potenciais aplicações industriais identificadas na indústria aeroespacial e ainda pelo facto de ser mais fácil produzir as bobinas.

Esta tecnologia é caracterizada por necessitar de um material bom condutor para permitir um bom acoplamento elétrico para maximizar a energia transferida promovendo a soldadura aquando do impacto.

(6)

iv

Assim, produziram-se juntas tubulares, tubo a tubo e tubo a varão, em AA6063 nas ligações similares e Ti6Al4V nas dissimilares. Foram ainda realizadas com sucesso ligações de AA7075 a tubos poliméricos reforçados com fibra de carbono, sem e com intercamadas de Cu ou Ni na interface.

O protótipo desenvolvido foi comparado com máquinas comercializadas para verificar a otimização conseguida bem como a qualidade das juntas produzidas. Para isso as juntas foram caracterizadas mecânica e micro-estruturalmente.

O protótipo demonstrou ter uma eficiência superior, necessitando de apenas 15 % da energia utilizada numa máquina comercial para produzir resultados semelhantes em juntas similares de alumínio (a energia diminuiu de 16 kJ para 2 kJ). O protótipo também demonstrou eficiência a produzir juntas dissimilares de alumínio a titânio bem como ligações metal não-metal.

(7)

v

ABSTRACT

Magnetic Pulse Welding is a solid state joining technology based on impact, which allows to produce overlap joints both in planar and tubular geometries. The technology has seen an increased interest in recent years, especially as a result of the industrial need to joint dissimilar materials (metallic and non-metallic) which easily form brittle intermetallic phases when welded by fusion-based processes.

However, no significant improvements on existing equipments have been reported, which are normally sized for endurance, compromising the machine efficiency.

In fact these are normally equipped with large storage capacitors banks, which are sometimes insufficient for dissimilar material combinations that require more energy to weld

In this study existing equipments were analysed to understand the key components aiming at its optimization. A prototype machine was developed and assembled envisaging higher discharge energies efficiency. The equipment was tested and validated in tubular transitions due to the facility to produce the coils in laboratory facilities but also due to the industrial applications identified.

This joining process is known to need a conductive flyer material to allow inducing current for the magnetic interaction which projects the flyer against the target to produce a weld.

Thus, tube to tube and tube to rod welds were produced in AA6063 in similar and dissimilar metallic joints to Ti6A4V. AA7075 to carbon fibre reinforced polymer tubes

(8)

vi

transitions were also successfully produced especially when Cu or Ni ductile interlayers were used.

The developed prototype equipment was compared to a commercial machine to identify the optimization achieved and to compare characteristics of the welds produced.

For this, the joints were characterized both structural and mechanically.

The prototype machine proved to have a higher efficiency needing less than 15%

of the energy required on the commercial machine to produce similar aluminium transitions (reducing from 16 kJ to 2 kJ). The machine also proved to be efficient in producing dissimilar joints, such as aluminium to titanium transitions and metal to non- metal transitions.

(9)

vii

PALAVRAS-CHAVE

Soldadura por Impulso Magnético Ligações metálicas similares Ligações metálicas não similares Transições metal – não-metal Caracterização Mecânica Caracterização microestrutural

KEYWORDS

Magnetic Pulse Welding Similar metal joining Dissimilar metal joining Metal to non-Metal transitions Mechanical Characterization Microstructural Characterization

(10)

viii

(11)

ix

AGRADECIMENTOS

Quero agradecer à minha orientadora, Professora Rosa Maria Mendes Miranda, por me ter incentivado para a realização do doutoramento, e pelo apoio, empenho e partilha de conhecimentos que em muito contribuíram para o trabalho desenvolvido.

Um sentido agradecimento ao meu coorientador, Professor Telmo Jorge Gomes dos Santos, pela partilha de conhecimentos bem como por todo o apoio prestado ao longo deste projeto.

Um agradecimento à Omnidea, Lda, em especial ao Eng. Tiago Pardal e à Eng. Filipa Lourenço, pelo apoio quer no financiamento quer na gestão indispensável para o bom desenvolvimento dos projetos. Também por parte da Omnidea, ao Eng. Renato Salles e ao Eng. João Couto pelo suporte técnico que permitiu obter os resultados alcançados.

Um profundo obrigado ao doutorado João Pedro de Sousa Oliveira pela ajuda indispensável na caracterização das amostras e análise dos resultados.

Ao Professor Rui Jorge Cordeiro Silva pelo apoio nas análises metalográficas e SEM, um sincero obrigado.

Ao Professor Jorge Joaquim Pamies Teixeira um agradecimento pela ajuda essencial na resolução de problemas encontrados bem como o conhecimento transmitido.

Um agradecimento especial ao Sr. António Guinapo Campos e Paulo M. G.

Magalhães por toda a assistência na resolução dos mais diversos desafios.

(12)

x

Aos meus colegas e futuros Doutorados Patrick Inácio, Miguel Machado, Valdemar Duarte e André Silva, entre muitos outros, que me acompanharam durante todo o curso e em especial na realização deste projeto um profundo obrigado pela amizade que se preservou e desenvolveu bem como pela ajuda prestada.

À minha família e amigos um agradecimento especial pelo apoio e incentivos permanentes que ajudaram ao bom desenvolvimento deste estudo.

Por fim agradeço o financiamento para este trabalho do Project SpIM, (Centro- 01-0247-FEDER-010605), enquadrado no Fundo Europeu de Desenvolvimento Regional (FEDER), Programa Operacional Regional do Centro 2020, e aos projetos financiados pela ESA que possibilitaram o bom desenvolvimento desta tese.

(13)

xi

ACKNOWLEDGEMENTS

I want to thank my supervisor, Professor Maria Rosa Mendes Miranda, for the incentive to proceed to a PhD and for the support, commitment and knowledge shared which greatly contributed to the research developed.

Sincere thanks to my co-supervisor Professor Telmo Jorge Gomes dos Santos, for knowledge sharing as well as for all his support throughout this project.

Acknowledgement to Omnidea, Lda, to Eng. Tiago Pardal and to Eng. Filipa Lourenço, for the financial and project management support which allow a good development of the involved projects. Also, from Omnidea, to Eng. Renato Salles and Eng. João Couto, for the technical support which made possible to achieve the presented results.

The author deep gratitude to Doctor João Pedro de Sousa Oliveira for the indispensable aid in in the samples characterization and results analysis.

The author acknowledges Professor Rui Jorge Cordeiro Silva for helping in metallography and SEM analysis.

To Professor Jorge Joaquim Pamies Teixeira a thank you for the help provided overcome the challenges and the knowledge shared.

Special thanks to Mr. António Guinapo Campos and Paulo M. G. Magalhães for all the assistance in solving various challenges.

To my colleagues and Doctors Patrick Inacio, Miguel Machado, Valdemar Duarte and André Silva, among many others, who accompanied me throughout the course and

(14)

xii

especially in this project thanks for a deep friendship that has preserved and developed as well as the assistance provided.

To my family and friends, a special thank for the support and permanent incentives that lead to the good development of this study.

Finally, I acknowledge Project SpIM, (Centro-01-0247-FEDER-010605), supported by Fundo Europeu de Desenvolvimento Regional (FEDER), Programa Operacional Regional do Centro 2020, as well as, projects funded by ESA for the financial support which made this thesis possible.

(15)

xiii

TABLES INDEX

Table 2.1 – Summary of material combinations joined by MPW: geometry, discharge

energy, observations and existing works ... 20

Table 3.1 – Circuit parameters tested ... 33

Table 3.2 – Advantages and disadvantages between the three different capacitor designs tested ... 45

Table 3.3 – Melinex® characteristics ... 45

Table 3.4 – NWL Capacitor Properties ... 47

Table 3.5 – Dielectric rupture trigger geometries... 50

Table 3.6 – Spark gap geometry design inductance results ... 51

Table 3.7 – Transmission line design comparison ... 52

Table 3.8 – Transmission line designs results ... 53

Table 3.9 – Parameters from breadboard testing ... 57

Table 3.10 – Radial deformation comparison ... 58

Table 4.1 – Materials, shapes and dimensions ... 74

Table 4.2 – Chemical composition of the materials under study ... 74

Table 4.3 – Mechanical properties of the materials under study ... 74

Table 4.4 –Electrical properties of the MPW machines used... 75

Table 5.1 – Weld parameters matrix: Sample reference, welding parameters varied and result of leak test ... 84

Table 5.2 – Mechanical properties of aluminium tubes ... 94

Table 5.3 – Lap shear tests results ... 95

Table 5.4 – Weld parameters matrix (commercial machine): Sample reference, welding parameters varied and result of leak test ... 99

Table 5.5 – Welded length measured along the joint axis ... 104

(20)

xviii

Table 5.6 – Range of processing parameters tested... 105

Table 5.8 – Results of lap shear tests ... 114

Table 6.1 – Parameter set used to study the CFRP tube behaviour to impact ... 134

Table 6.2 – Samples references and joining parameters tested. ... 135

Table 6.3 – Parameter sets of the cracked samples ... 145

Table 6.4 – Parameter set for joining Al to CFRP with interlayers... 156

(21)

xix

FIGURES INDEX

Figure 2.1 – Schematic of Magnetic Pulse Welding machine architecture ... 8

Figure 2.2 – Uniform pressure actuator schematic for planar geometries [25] ... 9

Figure 2.3 – Welding window representation [1] ... 11

Figure 2.4 – Wavy interface formation mechanism [29] ... 12

Figure 2.5 – Visual aspect of welds produced with: a) low discharge energy (9.6 kJ); b) high discharge energy (21.6 kJ) [38] ... 14

Figure 2.6 – Wave morphology for different impact angles: a) 8º; b) 12º, c) 16º and d) 20º [30]. ... 18

Figure 2.7 – Cross section of the produced laminates: a) before heat treatment; b) after 5 h annealing [61] ... 19

Figure 2.8 – AC capacitor for the automotive industry produced with MPW: a) Assembled capacitor; b) Welded interface of the capacitor [4]. ... 23

Figure 2.9 – DANA transmission shaft welded with MPW [18] ... 24

Figure 3.1 – Magnetic Pulse Welding equivalent circuit [92] ... 28

Figure 3.2 – Discharge current over time for a series RLC circuit ... 29

Figure 3.3 – transferred energy ratio for varying resistance and inductance ... 30

Figure 3.4 – Typical current discharge profile in EMF. ... 31

Figure 3.5 – Electric circuit designed to generate pulse discharge. ... 33

Figure 3.6 – Geometry and mesh used in the simulations ... 34

Figure 3.7 – Magnetic pressure for Low capacity and varying inductance ... 36

Figure 3.8 – Magnetic pressure for High capacity and varying inductance ... 36

Figure 3.9 – Magnetic pressure for Low inductance and varying capacitance ... 37

Figure 3.10 – Magnetic pressure for High inductance and varying capacitance ... 37

Figure 3.11 – Deformation for Low capacity and varying inductance ... 38

(22)

xx

Figure 3.12 – Deformation for High capacity and varying inductance ... 38 Figure 3.13 – Deformation for Low inductance and varying capacitance ... 38 Figure 3.14 – Deformation for High inductance and varying capacitance ... 39 Figure 3.15 – Speed deformation for low capacity and varying inductance ... 40 Figure 3.16 – Speed deformation for High capacity and varying inductance ... 40 Figure 3.17 – Speed deformation for Low inductance and varying capacitance ... 41 Figure 3.18 – Speed deformation for High inductance and varying capacitance ... 41 Figure 3.19 – Extended foil roll capacitor design ... 43 Figure 3.20 – Transmission line roll capacitor design... 44 Figure 3.21 – Planar extended foil capacitor design ... 44 Figure 3.22 – MPW voltage discharge waveform (adapted from [94]) ... 46 Figure 3.23 – Schematic of the cross-section steps for the magnetic simulation ... 48 Figure 3.24 – Impact position marked with the dash circle for a coil with: a) one step; b)

two steps; c) three steps ... 49 Figure 3.25 – Current distribution for: a) Single Turn Coil; b) Multi Turn Coil ... 49 Figure 3.26 – Gap switch design ... 51 Figure 3.27 – Cross section of transmission line. Current density distribution. ... 54 Figure 3.28 – Transmission line used in the MPW prototype machine: a) Design, b)

Prototype built ... 54 Figure 3.29 – High Voltage probe up to 15 kV, 50 MHz bandwidth ... 55 Figure 3.30 – Rogowski coil for current measuring... 56 Figure 3.31 – Data system connected to the machine. ... 56 Figure 3.32 – Tube deformation tests ... 57 Figure 3.33 – Radial deformation simulation results ... 57 Figure 3.34 – High power prototype ... 59 Figure 3.35 – Discharge waveform schematic of the prototype machine: yellow curve –

discharge voltage; blue curve – discharge current ... 60 Figure 3.36 – Capacitor bank ... 61 Figure 3.37 – Positioning System a) Experimental Welding Setup; b) 3d cad model

section view of the assembly with the coil at the Edge ... 62 Figure 3.38 – Coil Support – a) Assembly overview; b) Open Support; c) Coil Support

individual parts ... 63 Figure 3.39 – Coil support assembled to the machine ... 63 Figure 3.40 – New welding interface design ... 64

(23)

xxi Figure 3.41 – MPW machine developed ... 64 Figure 3.42 – Steel holder... 65 Figure 3.43 – Dielectric rupture trigger with solenoid. ... 65 Figure 3.44 – Trigger system. a) Transmission line detail with reinforcement disc of

Copper Tungsten Alloy; b) Copper-Tungsten actuator puncture with the insulator support ... 66 Figure 3.45 – Linear step-motor implemented for the actuator. ... 66 Figure 3.46 – Laser trigger System: a) assembly overview; b) Laser top view;

c) Laser front view ... 67 Figure 3.47 – Updated trigger geometry for higher discharge voltage ... 68 Figure 3.48 – Digital remote control ... 68 Figure 3.49 – Transmission line detail (actuator zone) ... 69 Figure 3.50 – Protection Circuit. Mechanical short-circuit. ... 70 Figure 3.51 – Discharge dumper resistor ... 70 Figure 3.52 – PSU for the small-scale prototype ... 71 Figure 3.53 – PSU with digital control ... 71 Figure 4.1 – Bmax MPW 25/25 used in the experimental procedure ... 75 Figure 4.2 – Prototype machine developed ... 76 Figure 4.3 – Schematic of the samples positioning to determine the impact angle, when

the coil was placed at the tube edge (a) and at mid tube length (b). ... 77 Figure 4.4 – Schematic of relative positioning of parts and field shaper in the commercial

machine ... 78 Figure 5.1 – Non-destructive Testing dye application (under black light) ... 86 Figure 5.2 – Dye penetrant test results on samples: a) Weld with coil at tube edge;

b) Weld with coil at tube centre. ... 86 Figure 5.3 – Macrograph and micrographic details of weld A7 cross section.

Welding parameters: DE=1 kJ, SD=1 mm, IA=6.8º (8-turn coil at tube edge) ... 87 Figure 5.4 – Macrograph and micrographic details of weld A16 cross section.

Welding parameter: DE=1 kJ, SD=1 mm, IA=13.4º (8-turn coil at tube centre) ... 87 Figure 5.5 – Macrograph and micrographic details of weld A2 cross section

Welding parameters: DE=1.5 kJ, SD=1 mm, IA=13.4º (4-turn coil at tube centre) ... 88

(24)

xxii

Figure 5.6 – Macrograph and micrographic details of weld A2 cross section after etching Welding parameters: DE=1.5 kJ, SD=1 mm, IA=13.4º (4-turn coil at tube centre)) ... 89 Figure 5.7 – SEM images of tube-to-rod interface of welds ref.:

a) A2: DE=1.5 kJ, SD=1 mm IA=13.4º (4-turn coil at tube centre),

b) A3 (DE=2 kJ, SD=1 mm, IA=13.4º (4-turn coil at tube centre)) ... 90 Figure 5.8 – Fracture surface of lap shear test of sample A2:

Welding Parameters: DE=1.5 kJ, SD=1 mm, IA=13.4º (4-turn coil at tube centre) ... 91 Figure 5.9 – SEM image of entrapped jetting on the welding zone of sample A2,

parameters: DE=1.5 kJ, SD=1 mm, IA=13.4º (4-turn coil at tube centre);b) Magnification of region marked in red; c) detail of b); d) higher magnification of region marked in c) ... 92 Figure 5.10 – SEM image of the interface evidencing the directionality of the jet of

specimen A3 Welding parameters: DE=2 kJ, SD=1 mm, IA13.4º (4-turn coil at tube centre) ... 93 Figure 5.11 – Fracture surface evidencing high plastic deformation before breaking on

specimen A3 Welding parameters: DE=2 kJ, SD=1 mm, IA13.4º (4-turn coil at tube centre) ... 93 Figure 5.12 – SEM image of a jetting pocket on the weld interface a), and at higher

magnification in b) ... 94 Figure 5.13 – Tensile testes samples: a) Specimens with the weld at the tube edge and

b) at the tube centre... 95 Figure 5.14 – Stress-Strain curves for welds:

a) A2 (DE=1.5 kJ, SD=1 mm, IA=13.4º (4-turn coil at tube centre));

b) A3 (DE=2 kJ, SD=1 mm, IA=13.4º (4-turn coil at tube centre));

c) A12 (DE=2 kJ, SD=1 mm, IA=25.5º (4-turn coil at tube centre));

d) A15 (DE=2 kJ, SD=1 mm, 17.6º (6-turn coil at tube centre)) ... 96 Figure 5.15 – Cross-section of the specimens produced ink coated rod to analyse the

material flow. ... 97 Figure 5.16 – Sample B22.: a) joint interface; b) rod detail.

Welding parameters: DE=16 kJ, SD=1.5 mm, IA=10.0º. ... 99 Figure 5.17 – Macrograph and micrograph details of weld A63.

Welding parameters: DE=10.2 kJ, SD=2 mm, IA=13.2º. ... 100

(25)

xxiii Figure 5.18 – Macrograph of weld A43. Welding parameters: DE=12.4 kJ, SD=2 mm, IA=13.2º. ... 101 Figure 5.19 –Details of the weld interface of weld A43.

Welding parameters: DE=12.4 kJ, SD=2 mm, IA=13.2º. ... 101 Figure 5.20 – Macrograph of weld B23. Welding parameters: DE=16 kJ, SD=1.5 mm,

IA=13.8º. ... 102 Figure 5.21 – Micrograph Details of the weld interface in weld B23.

Welding parameters: DE=16 kJ, SD=1.5 mm, IA=13.8º. ... 102 Figure 5.22 – Macrograph of weld C23. Welding Parameters: DE=16 kJ, SD=1 mm,

IA=13.1º. ... 103 Figure 5.23 – Micrograph details of the weld interface in weld C23.

Welding parameters: DE=16 kJ, SD=1 mm, IA=13.1º ... 104 Figure 5.24 – Macrograph of welds cross-section:

a) C1 parameters: DE=1.5 kJ, SD=1.5 mm, IA=13.6º (D4 coil at tube edge);

b) C2 parameters: DE=2 kJ, SD=1.5 mm, IA=13.6º (D4 coil at tube edge);

c) C3 parameters: DE=1.5 kJ, SD=1 mm, IA=9.2º (D4 coil at tube edge);

d) C10 parameters: DE=2 kJ, SD=1 mm, IA=9.2º (D4 coil at tube edge);

e) C11 parameters: DE=1.5 kJ, SD=0.5 mm, IA=4.6º (D4 coil at tube edge);

f) C12 parameters: DE=2 kJ, SD=0.5 mm, IA=4.6º (D4 coil at tube edge).

... 107 Figure 5.25 – Macrograph of welds cross-section: a) C8 parameters: DE=2 kJ,

SD=1.5 mm, IA=17.0º (6-turn coil at tube edge); b) C31 parameters:

DE=2 kJ, SD=1.5 mm, IA=23.2º (4-turn coil at tube edge); c) C22 parameters: DE=2 kJ, SD=1 mm, IA=11.5º (6-turn coil at tube edge); d) C26 parameters: DE=2 kJ, SD=1 mm, IA=15.9º (4-turn coil at tube edge); e) C24 parameters: DE=2 kJ, SD=0.5 mm, IA=5.8º (6-turn coil at tube edge); f) C28 parameters: DE=2 kJ, SD=0.5 ,mm IA=8.1º (4-turn coil at tube edge). ... 108 Figure 5.26 – Longitudinal macrograph of cross-section of weld classified as:

a) Not airtight sample C26 parameters: DE=2 kJ, SD=1 mm, IA=15.9º (4- turn coil at tube edge); b) Airtight sample C31 parameters: DE=2 kJ, SD=1.5 mm, IA=23.2º (4-turn coil at tube edge). ... 109 Figure 5.27 – Micrographs of the interface in weld C7. Welding parameters: DE=1.5 kJ,

SD=1.5 mm, IA=17.0º (6-turn coil at tube edge) ... 110

(26)

xxiv

Figure 5.28 – Macrograph of weld C8 cross section. Welding parameters: DE=2 kJ, SD=1.5 mm, IA=17.0º (6-turn coil at tube edge) ... 110 Figure 5.29 – Micrographs of the interface in weld C8. Welding parameters: DE=2 kJ,

SD=1.5 mm, IA=17.0º (6-turn coil at tube edge) ... 111 Figure 5.30 – Macrograph of weld C22 cross section. Welding parameters: DE=2 kJ,

SD=1 mm, IA=11.5º (6-turn coil at tube edge) ... 112 Figure 5.31 – Macrograph of weld C34 cross section. Welding parameters: DE=2 kJ,

SD=1 mm, IA=15.3º (6-turn coil at tube centre) ... 112 Figure 5.32 – Microhardness profiles. ... 113 Figure 5.33 – Tensile test results of the Tube base material: stress vs nominal strain . 115 Figure 5.34 – Fractured joints in samples: a) C7 parameters: DE=1.5 kJ, SD=1.5 mm,

IA=17.0º and 6-turn coil at tube edge; b) C8 parameters: DE=2 kJ, SD=1.5 mm, IA=17.0º and 6-turn coil at tube edge. ... 115 Figure 5.35 – Lap shear stress curve for weld C7. Welding parameters: DE=1.5 kJ,

SD=1.5 mm, IA=17.0º and 6-turn coil at tube edge ... 116 Figure 5.36 – Lap shear stress curve for weld C8. Welding parameters: DE=2 kJ,

SD=1.5 mm, IA=17.0º and 6-turn coil at tube edge. ... 116 Figure 5.37 – Lap shear stress curve for weld C22. Welding parameters: DE=2 kJ,

SD=1 mm, IA=11.5º and 6-turn coil at tube edge ... 117 Figure 5.38 – Lap shear stress curve for weld C34. Welding parameters: DE=2 kJ,

SD=1 mm, IA=15.3º and 6-turn coil at tube centre ... 117 Figure 5.39 – Specimen C34_T2 after rupture ... 117 Figure 5.40 – Macrograph and micrograph of specimen B2. Welding parameters:

DE=2 kJ, SD=1 mm, IA=17.4º and 4-turn coil at tube edge. ... 123 Figure 5.41 – Macrograph and micrograph of specimen B2. Welding Parameters:

DE=2 kJ, SD=1 mm, IA=17.4º and 4-turn coil at tube edge. ... 124 Figure 5.42 – Macrograph and micrograph of specimen B4. Welding parameters:

DE=2 kJ, SD=1 mm, IA=24.4º (4-turn coil at tube centre) ... 124 Figure 5.43 – Macrograph and micrograph of specimen B4. Welding parameters:

DE=2 kJ, SD=1 mm, IA=24.4º and 4-turn coil at tube centre ... 125 Figure 5.44 – Specimen B6 lap shear tested: a) rupture zone; b) detail of the previous.

Welding parameters: DE=2 kJ, SD=1 mm, IA=17.4º and 4-turn coil at tube edge. ... 126 Figure 5.45 – Lap shear tests: force versus displacement ... 127

(27)

xxv Figure 5.46 – SEM image of sample B6 fracture zone: a) SE image; b) BSE image.

Welding parameters: DE=2 kJ, SD=1 mm, IA=17.4º and 4-turn coil at tube edge. ... 127 Figure 5.47 – Delamination planes on the Aluminium attached to the titanium rod. .. 128 Figure 5.48 – Attached Aluminium layer to the titanium rod: thickness measurements.

... 128 Figure 5.49 – Jetting in Aluminium to Titanium welding ... 129 Figure 5.50 – EDS analysis of the three different zones of Sample B6.

Welding parameters: DE=2 kJ, SD=1 mm, IA=17.4º and a 4-turn coil at tube edge)) ... 129 Figure 6.1 – Sample G1 cross section. Parameters: DE=1 kJ; SD=1 mm; 6-turn coil

without mandrel ... 134 Figure 6.2 – Schematic of the welding set-up: impact angle calculation with the coil at

the tube centre ... 135 Figure 6.3 – Discharge profiles for 3 kJ of DE and a SD of: a) 0.5 mm; b) 1 mm ... 136 Figure 6.4 – Radiographic images of joints produced with: DE=2.5 kJ an 8-turn coil and

varying SD. Mandrel type: HDPE (details a; d and g); Epoxy Resin (details b; e and h); Aluminium (details c; f and i)... 137 Figure 6.5 – Radiographic images of joints produced with: SD=1 mm, a 6-turn coil and

varying DE. Mandrel type: HDPE (details a; c and f); Epoxy Resin (details b;

d and g); Aluminium (details e and h). ... 138 Figure 6.6 – Radiographic images of joints produced with: SD=1 mm, DE=2.5 kJ and

varying number of turns in the coil. Mandrel type: HDPE (details a and d);

Epoxy Resin (details b and e); Aluminium (details c and f)... 139 Figure 6.7 – Ashby diagram of Young’s modulus vs density for mandrel material

selection [97]. ... 140 Figure 6.8 –Details from sample G7 interface. Welding parameters: DE=2.5 kJ;

SD=1 mm; 6-turn coil and HDPE mandrel. ... 141 Figure 6.9 – Cross section view of sample G17. The arrows mark the cracks formed.

Welding parameters: DE=2.5 kJ; SD=1 mm; 8-turn coil and epoxy resin mandrel. ... 141 Figure 6.10 – Cross section view of sample G31. The arrows mark the cracks formed.

Welding parameters: DE=2.5 kJ; SD=1 mm; 8-turn coil and aluminium mandrel. ... 142

(28)

xxvi

Figure 6.11 – Micrograph details of the regions highlighted in Figure 6.10; a), b) and c) respectively. of Sample G31 welding parameters: DE=2.5 kJ; SD=1 mm; 8- turn coil and aluminium mandrel. ... 142 Figure 6.12 – Cross section of sample G29 parameters: DE=2.5 kJ; SD=1.5 mm;8-turn

coil with aluminium mandrel ... 143 Figure 6.13 – SEM image of the aluminium to CFRP interface of specimen G35

(DE=3 kJ; SD=1 mm; 8-turn coil with aluminium support mandrel) ... 143 Figure 6.14 – Example of a Al-CFRP sample produced by MPW. ... 144 Figure 6.15 – Example of a cracked developed after welding ... 145 Figure 6.16 – Micrograph details of sample G10. Welding parameters: DE=2.5 kJ;

SD=1 mm; 8-turn coil with HDPE mandrel: a) Location of cracks initiation points; b) Crack propagation detail... 146 Figure 6.17 – Fracture surface of a crack on the aluminium G16 parameters: 2 kJ of

discharge energy; 1 mm of stand-off distance; 8-turn coil with epoxy resin mandrel. ... 146 Figure 6.18 – Interface Al fracture analysis G16 parameters: DE=2 kJ; SD=1 mm; 8-turn

coil with epoxy resin mandrel: a) Secondary Electrons; b) Backscattered Electrons. ... 147 Figure 6.19 – Base material: a) Macrograph from BS with the hardness profiles marked;

b) inner surface; c) outer surface. ... 147 Figure 6.20 – Sample G7 cross section view and hardness profile. Welding parameters:

DE=2.5 kJ; SD=1 mm and 6-turn coil with HDPE mandrel. The lines in the macrograph correspond the hardness profiles. ... 148 Figure 6.21 – Sample G11 cross section view and hardness profile. Welding parameters:

DE=2.5 kJ; SD=1 mm; 8-turn coil with HDPE mandrel. The lines in the macrograph correspond the hardness profiles. ... 149 Figure 6.22 – Aluminium base material hardness profile ... 149 Figure 6.23 – Force-Displacement curves for two different samples... 151 Figure 6.24 – Scheme of the joint contact: Dc, is the contact diameter; Ac is the contact

area; Lc is the effective contact length; Aal is the Aluminium resistant section;

tal is the aluminium wall thickness ... 152 Figure 6.25 – Tensile test specimen after testing ... 154 Figure 6.26 – EDS analysis of the CFRP outer surface near the impact zone. a) section

analysed Elements dispersion b) C; c) O; d) Al; e) Si; f) Zn. ... 155

(29)

xxvii Figure 6.27 – Interface details from samples with 1.5 mm thick wall flyer:

a) G32 parameters: DE=2.5 kJ; SD=1 mm; 6-turn coil and aluminium mandrel); b) G33 parameters: DE=2.5 kJ; SD=1 mm; 8-turn coil and aluminium mandrel)... 157 Figure 6.28 – Details from samples with 50 μm thick copper coating on the CFRP tube:

a) G38 parameters: DE=2.5 kJ; SD=1 mm; 6-turn coil and aluminium mandrel; b) G39 parameters: DE=3 kJ; SD=1 mm; 6-turn coil and aluminium mandrel. ... 158 Figure 6.29 – Cross section of sample G38 welding parameters: DE=2.5 kJ; SD=1 mm;

6-turn coil, aluminium mandrel and 50 μm thick copper coating on the CFRP tube) ... 158 Figure 6.30 – Cross section of sample G39 welding parameters: DE=3 kJ; SD=1 mm; 6- turn coil, aluminium mandrel and 50 μm thick copper coating on the CFRP tube ... 158 Figure 6.31 – Details from sample G40 welding parameters: DE=3 kJ; SD=1 mm; 6-turn

coil, aluminium mandrel and a 80 μm thick nickel coating on the CFRP tube ... 159 Figure 6.32 – SEM image of the interface of specimen G40 Welding parameters:

DE=3 kJ; SD=1 mm; 6-turn coil and aluminium mandrel ... 160 Figure 6.33 – EDS profiles from the nickel aluminium interface of specimen G40

Welding parameters: DE=3 kJ; SD=1 mm; 6-turn coil and aluminium mandrel ... 160

(30)

xxviii

(31)

xxix

ABBREVIATIONS

AA Aluminium Alloy

ARCR As Received Consumable Rod

BM Base Material

BSE Back-scattered electrons

DE Discharge Energy

DEMI Departamento de Engenharia Mecânica e Industrial

E Young’s Modulus

EDS Energy Dispersive Spectroscopy

ESA European Space Agency

FCT Faculdade de Ciências e Tecnologias

FGM Functionally Graded Material(s)

FPCB Flexible Printed Circuit Board

FSW Friction Stir Welding

HAZ Heat Affected Zone

IA Impact Angle

IACS International Annealed Copper Standard

ID Inner Diameter

MMC Metal Matrix Composites

MPW Magnetic Pulse Welding

MS Mild steel

N.E. Non-Existent

N.S. Not stable

OD Outer Diameter

(32)

xxx

OM Optical Microscopy

SE Secondary electrons

SEM Scanning electron microscopy

SD Stand-off Distance

SS Stainless steel

ST Steel

UFG Ultra-Fine Grain

UNL Universidade Nova de Lisboa

UTS Ultimate Tensile Stress

WT Wall thickness

(33)

1

CHAPTER 1 INTRODUCTION

With the intensification of multi-materials in welding, with different mechanical and thermo-physical properties, solid-state processes have seen an increased potential of application.

Several applications are being reported, especially involving light materials, as the ones used in aerospace industry, but also in automotive industry in small components involving aluminium and titanium alloys. This combination is particularly difficult to accomplish by fusion-based welding processes due to the facility to form brittle intermetallic components. Amongst industrialised solid-state processes, friction stir welding is not adequate to orbital welding of small diameters. Adhesives are applicable when mechanical resistance is not a major requirement, or the joints are not under shear stress solicitations. Explosion welding is not an environmentally friendly process due to the high levels of noise and particles and, additionally it is not adequate for small size components.

Magnetic pulse welding (MPW) is a solid-state process that has recently attracted attention of researchers and industrials due to its peculiar characteristics. As a solid-state process, it can virtually weld any material combination providing that one of the materials

(34)

2

is a good electrical conductor (the flyer) and the other has enough impact resistance to avoid cracking (the target).

MPW is similar to explosive welding or laser impact welding, but in this process the driving force is the interaction force between the magnetic field generated after the discharge of the capacitors bank and the induced eddy current on the flyer material. This driving force is normally denoted as pressure, since the force is applied on an area restricted to the overlap zone between the flyer and the coil.

Upon impact, and due to the high impact force, a jetting is formed which cleans the surfaces and allows a close contact between these, promoting bonding. The high impact force breaks the surface oxide layer and pushes it out of the impact zone along with other debris and grease from the joint region. Therefore, this welding process requires minimum surface preparation prior to weld since, under adequate process conditions, the jetting is sufficient to ensure surface cleaning.

The main welding parameters controlled by the operator are the discharge energy, electronically set on the machine, and the stand-off distance and impact angle, imposed geometrically. The stand-off distance is the initial distance between the flyer and the target, allowing the flyer material to accelerate against the target before impact. The impact angle is imposed by the relationship between the stand-off distance and the overlap between the coil and the flyer and is set, so that, the jet formed at the interface can be expelled.

Due to the flyer conductivity and ductility requirement, this technology has been mostly applied to Aluminium, Copper and Magnesium alloys in both similar and dissimilar material combinations. Dissimilar combinations are limited due to the higher energy required to weld and weldability issues posed by the involved materials, namely new phase formation and intermetallics, while similar combinations are mostly limited to thin or small parts.

The existing equipments show limitations of: available range of energy to weld, control of process parameters, geometry and type of joints possible to weld and equipment costs.

These machines normally have large resistant coils and improve the electrical coupling by using a magnetic flux concentrator adapted to the joint dimensions. Despite improving the coil life time due to the larger resistant section and allowing the use of the same coil for several welds, it reduces the transferred energy to the materials to produce the weld. This is due to the current first travelling to the flux concentrator and after to the

(35)

3 flyer which results in energy losses. These coils also make the process more expensive since, to have a dedicated coil to improve the magnetic coupling, it must be tailor-made for a desired geometry to ensure the minimum distance between the flyer and the coil.

To improve the process flexibility and the energy transferred to promote weld a new equipment was envisaged to overcome the main identified drawbacks.

1.1. Motivation

The main motivation for this study was to develop a new prototype machine for Magnetic Pulse Welding which can potentially join any material combination, as long as the flyer material has a high electrical conductivity (above 20 %IACS) and ductility and, the target is able to withstand the impact force. In recent years the improvements of the technology mostly focused on the coils used, and on materials combinations. This study aimed at optimising the machine configuration, reducing the necessary discharge energy to weld, to allow joining dissimilar materials, which may be difficult to join by fusion welding techniques owing to weldability issues. The possibility to join light materials, typically used in aerospace applications, was a significant industrial incentive to perform this study. In fact, the present research was conducted with the financial support of EU and ESA (European Space Agency) funded projects at Omnidea, a small company in the space group, and Nova University through its Mechanical and Industrial Engineering Department.

1.2. Objectives

The main objectives of this study were:

• To develop a new concept of equipment for magnetic pulse welding requiring less discharge energy for joining and more flexible types of coils than the commercial available ones.

• To test and validate this prototype by producing joints in several materials combinations and characterize these.

Thus, the research work was organised in different stages as follows:

(36)

4

– Design, development and production of a functional prototype of a MPW equipment. For this, multiphysics numerical simulations were performed to understand the phenomena involved and the effect of major processing parameters on the electromagnetic interaction with the flyer, and the high-speed deformation process.

– Design the main components considering the results of the numerical simulations.

– Validate these simulations by comparing the deformation achieved both numerical and experimentally.

– Manufacture and assembly the prototype machine.

– Perform welds and their characterization, both microstructural and mechanical, correlating their properties with the welding conditions and the set of parameters.

– Compare the results of the welds produced with the prototype developed with samples produced with a commercial available machine.

1.3. Reading Guide

The structure of this document consists of the following main sections:

Chapter 2 describes the current state of art of the technology, including the process, equipment and existing knowledge on MPW joints and their characteristics.

Chapter 3 shows the numerical simulations performed to understand the process and optimise the equipment components.

Chapter 4 refer to the experimental procedure adopted to test and validate the equipment developed.

Chapter 5 presents the results achieved, as well as, a detailed discussion considering the existing knowledge for metallic similar and dissimilar combinations while the metal to non-metal joint are presented in chapter 6.

Finally, in chapter 7 conclusions and proposals for future work are provided.

(37)

5

CHAPTER 2 STATE OF ART

The constant technological drive to reduce weight and improve components performance in sensitive conditions in the design, force for efficient processes development for joining difficult-to-weld materials. This is even more significant for the space and aeronautics industries where dissimilar combinations using advanced materials are often required and are only possible to produce with solid state technologies like Magnetic Pulse Welding (MPW).

This process has the advantage of being clean and energy efficient when compared to arc welding processes and even to friction stir welding. Furthermore, since it is a solid state one, there are minimum thermal effects potentially degrading the properties of the base material adjacent to the weld [1, 2], so it is expected that the welds have similar or even better performances than the base materials.

2.1. Technology Principles

Magnetic pulse welding (MPW) emerged in the beginning of the 1970’s as a spinoff technology of nuclear energy programs [3]. The automotive industry followed aiming to increase productivity and reduce costs [4, 5].

(38)

6

This is a cold welding process [4, 6] since there is no melting of the materials and the metallurgical bond is produced without fusion. Therefore, mechanical and chemical properties of the materials do not undergo solid-liquid-solid transformations [7, 8].

The driving force is the Lorentz force which results from the interaction force of the magnetic field generated in the coil and the induced electrical current, thus it is also influenced by the flyer material conductivity as will be explained in detail on chapter 3.

So, the interacting forces F [N] are given by equation 2.1:

F=_2πd^μ⁰ I₁I₂ (2.1)

Where I1 [A]and I2 [A] are the two referred currents (the primary and the induced current), d [m] the distance between both conductors (the coil and the flyer) e µ0 [-] is the vacuum permeability.

The energy required to join two parts is produced by discharge of the capacitor, charged with high voltage, which generates a high intensity current pulse in a very short period of time.

The process consists of a high intensity current discharged through a coil that produces an eddy current, contrary to the primary, in one of parts to join originating two opposing magnetic fields. The Lorentz force generated by the interaction between the generated magnetic field and the electric charges accelerates the flying part to collide with the fixed one at a very high velocity, between 250 and 500 m/s [9]. Due to the impact force, a jet is formed at the interface which acts as a cleaning agent removing the oxides and debris at the interface leaving the faying surfaces of the raw materials in intimate contact promoting the bond. Since this technology is based on establishing a close contact due to a high speed impact , it is limited to overlap joints and investigations have focused on both planar and tubular geometries [2].

The nature of this jet is not yet fully understood but it is clearly a condition for a good bond to be achieved. In fact, at such high impact speed the solid material behaves as a liquid in a transient regime [9].

In impact-based technologies the material pushed to impact the other is called the flyer material and the other the target. The target should be able to withstand the impact forces without being damaged or deformed while the flyer material, for the MPW process

(39)

7 needs to have a good electrical conductivity, since the impact force is proportional to the intensity of electrical current and the magnetic field.

This welding technology presents good capabilities to joint several material combinations. However, as any process, it has some disadvantages. These include: the part which undergoes plastic deformation, must have a good electrical conductivity to rise the induced current, thus increasing the magnetic forces; the inner material must have sufficient structural strength to withstand the impact without deformation, due to the high impact velocity and pressure, [3] and the security level must be kept high [10] due to the high current intensity and voltage.

Due to the deformation speed, the flyer undergoes high strain deformation. This severe plastic deformation imposed to the material, like in other high strain rate deformation processes such as Friction Stir Welding (FSW) or Explosion Welding (EW), results in continuous grain fragmentation due to micro-shear or kink bands introduced by the deformation. The deformation with the reduced heat input at the interface make the material undergo continuous dynamic recrystallization [11].

Therefore, Magnetic Pulse Welding is expected to produce joints with Ultimate Strength similar to the strength of the weakest base material since there is no thermal affected zone when the optimized parameters are used. Also, due to the high strain rate deformation, grain refinement has also been reported which, depending on the refinement, can account for the slight thickness reduction due to material flow during deformation.

2.2. Equipment Development

A common machine architecture for MPW is schematically shown in Figure 2.1.

It basically consists of a high voltage power source, a capacitor and a transmission line through which the high intensity current flows into the coil. The discharge is initiated by a high voltage switch, normally a spark gap.

(40)

8

Figure 2.1 – Schematic of Magnetic Pulse Welding machine architecture

One of the advantages of MPW is that the required equipment is relatively simple.

They are designed to discharge very high currents in very short time intervals in order to produce high force to drive the flyer to collide against the target [12].

The result, as observed in Table 2.1, are highly different discharge energies, even when similar material combinations are used. Also, studies often highlight the joint properties and interface behaviour disregarding machine configurations or optimizations.

Approximately 60 patents have been granted over the past years mostly focusing on real product production methods for different industries, such as the automotive, which applied the technology for driver shafts [13, 14] and vehicle frame parts [15-17]. Other applications have been, or are under development, where Dana Corporation (USA) and Pulsar (Israel) are the leading firms designing applications for MPW for the automotive industry [18].

However, machine developments have just focused in one part of the machine:

the coil, aiming at its durability or to develop a specific geometry to weld a dedicated part. Aizawa et al. [19] studied the life time of a one-turn E-shaped flat coil made from

Cr–Cu alloy with discharge energies ranging from 0.6 to 4 kJ. The life time of the coil was studied according to the width and the thickness of the middle part of the coil which varied from 3 to 5 mm. Two setups were tested, one with one specimen on each side of the coil and the other with just one specimen on one side. Higher discharge energies produced larger deformations on the coil especially in the setup with just one specimen and thinner sections deformed more [19]. Most published studies present new coil geometries for tubular [20, 21], planar [22] or designs where pre-heating is an option [23].

Weddeling et al. [24] developed a coil for planar geometries in which the induced current closes the circuit through the workpiece, as depicted in Figure 2.2, to produce the deformation. The coil produced homogeneous magnetic field and has proven to be able

(41)

9 to produce similar welds of aluminium alloy EN AW-5005A with 1.5 mm thick with only 1.5 kJ of discharge energy proving the good efficiency of the coil.

Figure 2.2 – Uniform pressure actuator schematic for planar geometries [25]

A Bitter coil was also developed by Zaitov et al. [26] for tubular geometries. The coil was dimensioned according to the thermal and load-bearing boundaries for discharge energies up to 50 kJ. The coil proved to focus the field and to be able to produce sufficient magnetic field.

Besides specific applications in the industry there are no published developments regarding the machines, but just on the coils.

The difference between most machines are on the capacity of the capacitors bank or in the type of commercial discharge switch selected.

No significant optimizations were made to the machines in the past years which lead to wide ranges of parameters combination, even for the same materials combinations.

Most developments were focused on the coil, aiming at designing specific configurations to dedicated industrial applications or to increase the capacitors bank capacity without improving the energy transfer efficiency to the parts to weld.

2.3. Processing Parameters

Like all impact-based welding technologies the parameters need to be tuned to impose the desired impact conditions prone to weld. As it is known for all impact based joining processes, the main welding parameters are:

(42)

10

• The impact velocity;

• The stand-off distance;

• The impact angle.

For magnetic pulse welding, the impact velocity is controlled by the discharge energy which is controlled electronically in the machine while the stand-off distance and the impact angle are imposed geometrically.

The impact velocity derives from the magnetic force generated from the interaction of the generated magnetic field from high current discharge with the induced current in the flyer thus, it is a function of the discharge energy. The flyer electrical conductivity of the flyer also plays a role since the induced current depends on it [2]. In the simulations the magnetic force is considered as a magnetic pressure to simplify the calculations since, due to the high frequency discharge and the coil shape the area of the force application is the overlap region between the coil and the flyer.

The stand-off distance, which is the distance between the flyer and the target, prior to weld also influences the impact velocity since this is the distance the material has to accelerate before impact [2]. Although there is a relative consensus on the stand-off distance range of variation, which is commonly between 1 and 3 mm, the same does not apply to either the discharge energy or the impact angle. The impact angle needs to be adjusted accordingly with the discharge energy while the necessary discharge energy varies both with the discharge conditions of the machine (electrical properties which influence the discharge frequency), since these rule the current peak, and the electric coupling between the coil and the flyer [27]. In this work, the relation between the discharge energy and the energy transferred to the flyer to produce the weld is presented as the process efficiency.

Although, there is a relatively wide window defined in the literature for both parameters, which is between 5 and 35º for the impact angle while the impact velocity varies between 250 and 500 m/s [1, 2], the welding window defined in this range, as shown in Figure 2.3, is narrow thus, the correct selection of the joining parameters is critical.

(43)

11

Figure 2.3 – Welding window representation [1]

Concerning the joint, the impact velocity and the stand-off distance control the impact force which is responsible to ensure the intimate contact between both parts to establish a bond and to expel the jetting which is responsible for cleaning the impact surfaces. The impact angle has a minimum to guarantee an unobstructed path for the jetting to be expelled [2, 27].

In the developed prototype machine, the discharge energy is controlled by the charging voltage, the stand-off distance is imposed by the difference between the target outer diameter and the flyer inner diameter and the impact angle is controlled by the geometric relationship between the stand-off distance and the coil overlapping, as further explained in section 4.3.

2.4. Interface characteristics

The jetting formation is mandatory to weld in MPW since it is the surface cleaning agent necessary to prepare the materials for joining. The jetting formation is due to the combined effect of the impact force and the impact angle [1] and remove all debris and oxides to establish an intimate contact between the raw materials to weld.

On the impact initial point, the jetting was not yet formed, thus the surface is not prepared to be welded. In addition, the impact is at 0º which gives no path for the jetting to be expelled thus, the central zone of the weld, both in tubular and in planar geometries, often presents a central region without welding. The weld is formed on the adjacent zones on both sides due to the progressive nature of the impact from the centre to both sides [28]. The weld is characterised by a non-welded central region since the impact angle is

(44)

12

0º at this point, and two parallel lines on each side of this region are observed, corresponding to the weld zone.

This directionality help expelling the surface oxides and debris nevertheless, if the impact force is not sufficiently high the oxide layer and the debris may not be completely expelled, or no jetting is formed, preventing the weld to be produced [1, 28].

The impact welding processes are known to produce a wavy interface which is generated by pushing the flyer against the target, colliding at a given point (Figure 2.4 a).

At this point the heat and the pressure are at their maximum and initiates the shock wave.

The shock waves propagation inside the materials gives rise to a Kelvin-Helmholtz instability creating the waves periodically at the interface Figure 2.4 c) and d) [29].

The impact velocity reduces as the welding progresses and new collision points appear causing the changes in wavelength, as in Figure 2.4 f), where the wave propagation is ahead of the impact point new waves cannot be created Figure 2.4 g) and a non-effective joint is obtained [29].

Figure 2.4 – Wavy interface formation mechanism [29]

These interface phenomena are difficult to numerically simulate due to the high strain rate deformation imposed, during which the material behaves like a non-Newtonian fluid for a short period of time [30].

This high impact force breaks the oxide layers and, along with the wave propagation generate the jetting. Thus, it is considered as a clean process where the jet formed in the high velocity impact between both materials do not emit harmful fumes,

(45)

13 gases or radiation. The oxide layers and debris are expelled from the welded region and deposited on the adjacent sections [3, 4, 31, 32].

An energy threshold exists where, below which, unsuccessful bonding is achieved and above which tearing and cracking occurs [4]. Welding between dissimilar materials is possible since joining occurs by plastic deformation without melting. According to Kore et al. [33, 34] insipient local fusion can be observed because the thin layer of metal at the interface behaves like a liquid in a fraction of time, however, there is still some discussion about interfacial phenomena in MPW. These pockets of fused material are due to excessive discharge energy used, which in turn produce larger impact forces and faster deformations and thus, larger heat input due to internal friction [35].

The characteristic wavy interface is not mandatory to achieve weld however, it is commonly observed especially when high discharge energies are used. The wave formation mechanisms are also responsible for the grain refinement process.

2.5. Materials Combinations

2.5.1. Similar Joining

The technology has been applied to similar combinations, in tubular and in sheet configuration, mostly of aluminium alloys, but copper and magnesium alloys have also been investigated.

Similar joining with AA6061-T6, both in plate to plate and tube to tube joining, have been tested with reported grain refinement near the interface, from around 25 μm in the base material to close to 1 μm on the processed region, due to the high strain rate deformation typical of the process [36]. Similar refinement was also found in copper joining by the same author proving the technique capability to produce resistant joints [37].

Investigations were conducted on planar geometry using AA1050 and an E- shaped coil to produce the welds. The joint present a slightly different geometry since two parallel seam welds are created with a non-bonded central region. This is due to the impact propagation which is different depending on the joint and coil geometries. The author also reported changes in the wavy interface wave length with the discharge energy [31].

(46)

14

Using 0.5 mm thick plates of AA5052-O and with discharge energies ranging from 9 to 22 kJ weld was also achieved. The welds had and elliptical shape which the width and length varied with the discharge energy, being more pronounced for higher energies as presented in Figure 2.5. The welds were characterized by a central non bonded region with weld only at the ellipse edge with a wavy interface [38].

Figure 2.5 – Visual aspect of welds produced with: a) low discharge energy (9.6 kJ); b) high discharge energy (21.6 kJ) [38]

For similar combinations, such as Aluminium to Aluminium in planar sheet overlap configuration investigated by M. N. Kazeev et al. [39], is common to achieve joint resistances strength equal to the base material. However, for tubular geometries, the energy needs to be carefully controlled since, for tube to tube joints, if excessive energy is used, it can damage the target as spalling of the tube wall has been reported [36].

Nevertheless, this can be avoided either by using a rod as a target or, in planar configurations since the target is fixed against a support to both increase the target impact resistance and to avoid such problems.

Investigations were made with AA1050 tube to rod for interface study where the joining phenomenon dispersed the Al3Fe precipitates present in the base material. This homogenization leaded to a hardness increase on the welded region [40].

A study, also in tubular configuration, but expanding the driver instead of collapsing, was carried out by Mishra et al. [41] with AA6061 in tube to flange configuration. In this study the effect of the flyer thickness and the stand-off distance with tubes with larger thicknesses needing higher discharge energies to be deformed and accelerated to the same impact velocity as thinner ones. Therefore, for constant discharge energy the thinner tube accelerates longer reaching peak velocity with the 3 mm of stand- off distance while the thicker tube starts to decelerate after the 2.5 mm of stand-off distance. Wavy interface and jetting formation was also reported.

(47)

15 Joining Copper 110 alloy by MPW was investigated by Zhang et al. [37] with improvements on the mechanical properties due to grain refinement after the high strain rate deformation imposed by the process.

Berlin et al. [42], joined AZ31 magnesium plates with MPW and achieved ultrafine grains of about 300 nm compared with 10 µm of the base material and improvements in the mechanical properties of the joint when compared to the base materials characteristics. The nanometre sized grain structure achieved is the reason for the superior mechanical properties obtained.

These investigations were made in planar geometry and the known non-bonded region in the centre of the coil was also seen for this material combination. This is due to the impact angle being dynamic, starting with 0º at the initial impact point and varying with the impact propagation [42].

Concluding, light alloys and non-ferrous materials have also been studied, with special focus on copper, aluminium and magnesium mainly due to the good electrical conductivity they exhibit and the poor weldability when fusion-based processes are used.

The energy balance for all material combinations will be presented in the next section. However, for similar combinations the discharge energy has a high variation from test to test, even with similar materials, showing the large range of machine efficiencies for the commercial machines.

2.5.2. Dissimilar Joining

Although other similar combinations of Aluminium, Copper and Magnesium alloys were tested, this technology have attracted more interest for dissimilar joining where the absence of a melting phase is an asset in the joint quality.

The increasing need of components with increased functionally, together with the requirements for lighter structures motivates the demand for dissimilar joining especially when light alloys and Metal Matrix Composites (MMC) are concerned [12, 27, 43].

Though the technology has proven to be highly applicable to joint both similar and dissimilar material combinations in tubular and planar overlap geometries, excessive discharge energies have seen to produce melted phases at the joint interface which can originate the presence of intermetallic phases (IMP) due to the involved materials having distinct melting temperatures, chemistry and thermo-physical properties.

(48)

16

This affects the heat flow and maximum temperature in each material, distortions and formation of new phases including intermetallics. As such, solid state processes are a good alternative to join dissimilar materials considering that the mechanical resistance required for the final application is achieved [44-46]. However, even in these cases, IMP have not seen to significantly degrade the mechanical properties as these are randomly distributed along the interface and have very small dimensions.

Nevertheless, dissimilar metal joining tend to present Intermetallic Phases (IMP) at the joining interface, which are normally brittle, thus weakening the joint. This is commonly observed, for example, in Aluminium to Titanium joining, especially with fusion based welding technologies, where due to the thermal cycle (maximum temperature and heating/cooling rates) these materials tend to form intermetallic layers of TiAl3 which are brittle and hard thus severely reducing the joint strength [47-49].

Addition of alloying elements can control the intermetallics size, such as Silicon (Si) which is more prone to form more ductile intermetallics, with the aluminium due to the higher affinity between these materials. Minimizing the heat input may also prevent its formation as observed in solid state welding processes like MPW. The maximum temperature at the interface, as well as, the time at high temperature, limits the formation of intermetallic phases. Since no heat is introduced, there is no heat affected zone and no thermal distortions [50-52].

A process variation, in planar geometry was investigated by A. P. Monogaran et al. [53] where Magnetic Pulse Spot Welding (MPSW) was used to join AA 1199 to EN355 steel alloy. For this variation, localized deformation is induced in the Aluminium sheet to create the stand-off distance by means of a “hump”. Sound welds were produced and tensile–shear tests were made with pull-out failure on the Aluminium side at 0º along the solicitation direction.

Dissimilar tubular combinations were also reported when joining of Al 1050 to AZ31 magnesium alloy and pockets of Mg17Al12 intermetallics were found due to excessive energy. The high impact velocity produced melted zones which were evidenced by the presence of porosity and intermetallics, nevertheless, joining was successfully achieved [40].

Aluminium 6060-T6 to commercially pure copper was joined by MPW in tubular geometry with the aluminium tube as the flyer and copper as the target. Bonding was successfully achieved but the need to control the discharge energy was evidenced due to

Developments in Magnetic Pulse Welding

Diogo Jorge de Oliveira Andrade Pereira

Developments in Magnetic Pulse Welding

Developments in Magnetic Pulse Welding

RESUMO

ABSTRACT

PALAVRAS-CHAVE

KEYWORDS

AGRADECIMENTOS

ACKNOWLEDGEMENTS

CONTENTS

TABLES INDEX

FIGURES INDEX

ABBREVIATIONS

CHAPTER 1 INTRODUCTION

1.1. Motivation

1.2. Objectives

1.3. Reading Guide

CHAPTER 2 STATE OF ART

2.1. Technology Principles

2.2. Equipment Development

2.3. Processing Parameters

2.4. Interface characteristics

2.5. Materials Combinations