3D Multi-Material Laser Powder Bed Fusion of 420 stainless steel-Cu parts for Plastic Injection Mold Inserts
Ângela Cunha, Ana Marques, Filipe Silva, Óscar Carvalho, Flávio Bartolomeu
CMEMS - Center for MicroElectroMechanical Systems, University of Minho, 4800-058 Guimarães, Portugal
LABBELS - Associate Laboratory, Braga, Guimarães, Portugal Michael Gasik
School of Chemical Engineering, Aalto University Foundation, Espoo, Finland Bruno Trindade
CEMPRE - Center for Mechanical Engineering, Materials, and Processes, University of Coimbra, 3030- 788 Coimbra, Portugal
Sónia Pereira, Paulo Alexandrino INCM - Imprensa Nacional Casa da Moeda
ABSTRACT
Plastic injection molding is one of the fastest-growing industries in the world. However, although it presents numerous advantages, the costs associated with the mold and machine are high and, therefore, this process is only profitable for mass production. Moreover, the reduction in the cycle time, more specifically the cooling time, has been a never-ending challenge since it has a direct influence on production costs. This study is focused on the production of 420 stainless steel-copper solutions by 3D multi-material laser powder bed fusion. This novel material’s design concept allows combining the high mechanical resistance of the steel alloy and the high thermal conductivity of the copper. The processing parameters and strategies as well as the transition zone between these materials are of the most
challenging and important aspects both from a mechanical and metallurgical point of view. The obtained results show that this approach is effective to produce inserts of copper in a 420 stainless steel capable of improving the in-service conditions of a plastic injection mold, enhancing its performance and life.
INTRODUCTION
Plastic injection molds are of major importance in the plastic injection molding industry because they are responsible for the aesthetic of the final plastic part [1,2]. One of the great challenges of this industry continues to be the cycle time, particularly the reduction of the cooling time (~ 70 % of the entire cycle) [3,4]. Therefore, several solutions have been studied and applied in this regard, namely the use of conformal cooling channels and high thermal conductive inserts [3,5,6]. Additive manufacturing
processes, particularly Laser Powder Bed Fusion (LPBF), have been used in the fabrication of tools with a high geometric complexity, challenging traditional design guidelines [5,7,8].
Steel alloys are usually used in plastic injection molds due to mechanical and corrosion resistance, hardness, wear resistance, and resistance to fatigue [9–11]. The 420 stainless steel (420SS) is one of the steels most used for the production of plastic injection molds due to the ability to combine the above- mentioned properties [10,12]. However, one of the main drawbacks of steel alloys is the low thermal
conductivity (~ 25 W/m.K), which makes difficult heat extraction of the mold after the injection cycle as well as reduces its energy efficiency [13]. Moreover, these alloys display the low thermal expansion in- service temperatures of these tools. Therefore, the choice of highly conductive material is preponderant to increase the heat transfer rate and so the thermal efficiency of the plastic injection mold [3]. Copper and its alloys have been used in mold inserts to solve this problem. However, although pure copper is an exceptional thermal conductor (~ 400 W/m.K) [14], it is a very soft and ductile material, and therefore, it is not a good solution for the production of the mold’s core and cavity [15]. In this sense, alloying elements, such as Cu-Be, Cu-Co-Ni-Be, Cu-Ni-Si-Cr, and Cu-Al-Ni-Fe, have been added to copper to improve its mechanical properties [15,16]. It is important to mention that its high reflectivity and thermal conductivity have been an obstacle when processing this material by LPBF [17,18]. Liu et al. [19]
reported the production of C18400 copper alloy by LPBF with a significant amount of porosity, with poor consolidation and low mechanical strength because of inadequate melting due to the high reflectivity and thermal conductivity of copper. Benedetti et al. [20] also stated the production of copper samples by this technology with more than 50 % of porosity. Nevertheless, Constantin et al. [21] produced high-density copper parts (densification of 95 %) with complex shapes by LPBF and attributed these excellent results to a high-quality laser beam and the suitability of the printing parameters. These authors also mentioned that the low-energy deposition led to a highly rough surface finish, and the excessive energy enhanced the balling effect.
One of the ways to overcome this issue is to design a multi-material component surface consisting of a steel matrix and pure copper that might present a good mechanical resistance induced by the steel and benefit from the high thermal conductivity characteristic of copper. Some studies about multi-material solutions produced by additive manufacturing can be found in the literature such as 420SS-TiN [22], 420SS-Inconel 718 [23], 420SS-300 maraging steel [24], H13 steel-copper [25], 316L stainless steel- C18400 alloy [19]. The main obstacles found in the production of multi-material components have been the residual stresses and defects on the interface [17,26,27]. When compared with conventional
manufacturing routes, laser-based technologies can be disruptive because by using optimized processing parameters and high solidification rates, the diffusion process can be minimized, avoiding the diffusion phenomenon once the materials experience high temperatures during less time, and consequently, there is no time for the formation of fragile and undesirable intermetallic phases [28].
The present work focuses on the use of a 3D Multi-Material Laser Powder Bed Fusion (3DMMLPBF) system to build a new multi-material approach by melting two distinct materials (420 stainless steel and copper) with unique and specific properties (high mechanical strength and thermal conductivity) in one single multi-functional component. This 3D multi-material solution can be employed in a highly energy- efficient plastic injection mold, as shown in Figure 1. The design presented in Figure 1 differs from the works available in the literature since the production of the part is in full 3D, in which both materials are deposited on the same layer, creating a continuous bonding between 420 stainless steel and copper. Other potential industrial areas where this multi-material approach may be used is, for example, the coin collection sector where different metals with different colors could be used for aesthetical purposes.
MATERIALS AND METHODS
Powder materials
Commercially available 420 stainless steel (420SS) powder from Carpenter Additive (United Kingdom) and 99.7 % purity copper powder from TLS Technik (Germany) with particle size between 15 and 45 µm were used for multi-material solution fabrication. The morphology of the two types of powders is presented in Figure 2, which shows the spherical shape of the powders. This is an important powder characteristic in LPBF due to its high flowability and high apparent density compared to irregularly shaped powders. The chemical compositions of the 420 stainless steel and copper powder particles are shown in Table 1.
Figure 1. Schematic representation of the design concept of the 3D multi-material 420 stainless steel-copper solution to be applied in the plastic injection molds.
(a) (b)
Figure 2. Morphology of the powders in the multi-material samples: (a) 420 stainless steel, and (b) copper.
Table 1. Chemical compositions of 420SS and copper powder particles (wt.%).
Materials // Elements C O Cr Mn N S P Fe Cu
420SS 0.22 0.02 13.5 0.29 0.09 0.004 0.015 Balance ---
Cu --- 0.3 --- --- --- --- --- --- 99.7
LPBF process and processing parameters
The 3D multi-material 420 stainless steel-copper samples (with approximately 2.5 × 4 × 1 mm) were produced using a homemade multi-material laser powder bed fusion system (MMLPBF) developed at Center for MicroElectroMechanical Systems (CMEMS) at the University of Minho which is equipped with an Nd: YAG pulsed laser as the heat source (maximum power of 80 W, laser wavelength of 1064 nm, frequency of 10 Hz, and pulse duration of 0.3 ms). Figure 3 shows a schematic representation of the LPBF process.
The 3DMMLPBF system includes two simultaneous and independent powder deposits (for 420 stainless steel and copper powders), two recoaters that are used to spread two feedstock materials alternatingly, and two vacuum cleaning systems to clean and replace the powder materials. As can be seen in Figure 3a, both 420 stainless steel and copper feedstock powders were deposited and processed on a 420 stainless steel build plate. Moreover, the 3D multi-material samples were produced under an argon atmosphere (gas pressure of 2.5 bar) in order to control the level of oxygen since this latter is one of the main problems of processing copper alloys by this technology [29]. Figure 3b shows that the powders were alternately deposited on the build plate, first the steel alloy and then the copper, allowing to fabricate 3D multi- material samples and layer by layer, make available to have different materials in each single layer.
The 3D multi-material samples were designed with nine 420SS single scan tracks (~ 82 %) intercepted by two of Cu (~ 18 %), and each single track is separated by 200 µm (scan spacing) as shown in Figure 4.
The samples were produced with a bidirectional scan strategy, without rotation between each layer. The processing parameters are presented in Table 2. The laser power for copper was higher than for 420 stainless steel powder, given that the copper has significantly higher thermal conductivity (~ 400 W/m.K at room temperature) than this steel alloy (~ 25 W/m.K) [13,15]. Wessel et al. [30] reported that if the processing parameters were optimized for copper only, the liquid melt pool temperatures of the 420 stainless steel would likely be higher than the suitable for processing, leading to poor final properties.
(a)
(b)
Figure 3. 3D Multi-Material Laser Powder Bed Fusion: (a) schematic representation of the process, and (b) sequence on the deposition of two feedstock materials alternatingly.
Table 2. LPBF processing parameters for 420 stainless steel and copper powders.
Processing parameters 420 stainless steel Copper
Laser power (W) 10 12
Scanning speed (mm/min) 120 120
Layer thickness (µm) 30 30
Scan spacing (µm) 200 200
Characterization techniques
Both melt pool and polished surfaces (in top and cross-section) were analyzed through scanning electron microscopy (SEM) (JEOL JSM-6010LV) equipped with electron dispersive spectroscopy (EDS) (EDAX - Pegasus X4M) for further assessment of the chemical composition at the 420 stainless steel, interface, and copper zones.
The Vickers hardness was assessed through a micro-hardness indenter from EMCO-TEST - DuraScan using a diamond indenter. The measurements were performed in three distinct zones of each material considering three load values (10, 50, and 100 gf) due to ductility discrepancy between two materials.
RESULTS AND DISCUSSION
Morphological characterization
T
he metallurgical bonding of 420 stainless steel and copper powders in 3DMMLPBF technology was achieved through a local fusion of the powders and later consolidation (Figure 5). Although literature has reported undesirable results on copper solidification due to its high reflectivity and thermal conductivity [17–19,25], after adjusting 3DMMLPBF process parameters to compensate the energy loss (namely by increasing the energy density), the copper powder has been reasonable solidified and bonded to the 420 stainless steel melted powder as well as steel substrate. The porosity of the sample is approximately 1.098± 0.025 %. This value was determined by image analysis (ImageJ) based on surface and cross-section images of the samples
Figure 4. Schematic representation of the design and laser scanning strategy (adapted from [31]).
Figure 5. Morphological characterization of the top and cross-section surface of the 3D multi-material 420 stainless steel-copper produced by LPBF (adapted from [31]).
The design of multi-material structures has been a strategy adopted in additive manufacturing since they allow to obtain customizable properties, including physical, chemical, mechanical, thermal properties, among others, in predetermined locations of components, namely in inserts of plastic injection molds [28,32]. Particularly in materials such as steel and copper, the processing and joint of materials are problematic due to their metallurgical incompatibility and the wide difference in physical properties [17,27]. Moreover, and considering the characteristics of each of the materials, 420 stainless steel absorbs more energy from the laser when compared to copper, so the complete melting of the first material is significantly easier and the required energy density is substantially less. Therefore, the processing parameters were optimized individually for each material, using a higher laser power for the sintering copper (12 W) than for the 420 stainless steel (10 W). This is an innovative study since both materials are solidified in the same layer, creating a 3D multi-material concept. The copper powder was deposited and melted over the 420 stainless steel. This strategy allows the difficulty of sintering copper to be mitigated since the low thermal conductivity of the steel alloy helps to retain heat in the melt pool when the copper lines are being solidified.
The morphology of the surface and their cross-section were evaluated (Figure 5) to assess the possibility of severe keyhole formation or lack of fusion with the underlying substrate. The 420 stainless steel zone is almost completely consolidated and free of defects as it is visible at the top surface and cross-section of the SEM images (Figure 5). Some occasionally irregular and circular-shaped pores (sub-micrometer size) are perceptible, which may have originated by gas entrapping during the solidified process [33–35].
Therefore, these results allowed us to conclude that the applied laser energy density was enough to melt the 420 stainless steel material without lack of fusion. On the other hand and although the processing of copper is still a challenge for researchers, processing parameters used in the present study allowed us to obtain well-defined copper heat conductive channels with approximately 200 µm. Residual irregular pores (with tens of micrometers) were found in copper melted powder (as can be observed in Figure 5) due to lack of fusion. Circular pores observed in copper melted zones were attributed to the high cooling rate of the 3DMMLPBF technology since the gases do not have time to escape, being entrapped into the
solidified metal.
The copper areas were consolidated after the 420 stainless steel zones and therefore the interface region is done with the processing parameters associated with copper. The use of higher energy density for
processing the interface (12 W) leads to the adjacent 420 stainless steel surface also undergoes melting and mixing with the as-deposited copper. Wits et al. [30] have reported the development of graded structures in LPBF and stated that it is advantageous to stretch the diffusion zone to create a gradual transition between the two materials. This improves the overall performance of the component as it lowers the level of residual stresses between the materials involved due to the gradual transition in terms of physical, metallurgical, and mechanical properties. The interface is the most prominent and critical zone regarding the existence of pores and cracks (Figure 5). The existence of pores at the interface can be due to different factors, highlighting the use of an insufficient overlap to create a bond between 420 stainless steel and copper, the difference in the physical properties between the two materials, and the use of inappropriate processing parameters for effective melting of materials. On the other hand, the existence of cracks at the interface has been a discussed topic and can have several causes. Liu et al. [19] developed a multi-material solution based on 316 stainless steel-copper alloy C18400 to study the metallurgical diffusion and indicated that the existence of cracks at the interface may be associated with the
incompatibility of physical properties of the two materials, but also with the infiltration of copper in the grain boundaries of the steel. The significant difference in the values of the thermal expansion coefficient and thermal conductivity between the two materials leads to large deformations and residual stresses in the joint [17,36]. The crack formation also can be originated by the amount of copper in the melt pool during the process. The low amount of copper in the melt pool leads to the formation of a dilute solution with iron, without significant stresses, and cracks in the melting zone. On the other hand, a high amount of copper in this zone leads to a greater tendency for the formation of cracks in the steel due to the thermal incompatibility between the two materials [19].
Melt pool analysis
Multiple tracks of 420 stainless steel and copper were analysed and represented in Figure 6. The analysis of the geometry and size of the melt pool is a very important aspect of the LPBF process because it is an evidence of interactions between processing parameters and intrinsic materials properties. Thus, it is considered an essential criterion for optimizing processing conditions [37,38]. Both materials presented uniform melt tracks with no signals of pores at the surface and directional solidification resultant from the high heat gradient imposed by the Nd: YAG laser. No balling effect was observed at laser tracks of which means that the processing parameters considered were suitable for producing a continuous melt pool for both materials. Regarding 420 stainless steel, it can be stated that the consolidated laser track has a small variation of the width along the length (Figure 6) (420 stainless steel tracks having a width of
approximately 246.42 ± 14.75 μm). The melt pool boundaries of 420 stainless steel showed a mirror effect along the fusion track due to the scanning strategy used (bidirectional scan strategy). On the other hand, although the copper tracks presented some irregularities in the melt pool, the processing parameters used were considered suitable, as there is no evidence that these defects influence the solidification and densification of the powders.
The size and shape of the melt pool are strongly influenced by the amount of energy absorbed by the powder bed. The scan spacing should be defined as it allows to form an adequate melt pool overlap, avoiding defects between successive tracks. The 420 stainless steel and copper melt pools have a width of 246.42 ± 14.75 and 251.15 ± 9.13 μm with overlapping of 12 and 13 %, respectively. Moreover, the 420 stainless steel and copper melt pools have an overlap of 60.51 ± 4.94 μm, which corresponds to 25 % of overlapping.
Chemical characterization
EDS analysis was performed along a line, including different regions of the sample surface (420 stainless steel and copper). The results are shown in Figure 7. The values of the percentage of carbon (an essential element of 420 stainless steel alloy) are not present, as the EDS technique has a high associated error in the quantitative determination of elements of low atomic number. The EDS results show a narrow interface region of ~ 10 µm wide (position 60-70 µm) between the steel and copper regions. It is known that the cooling rates associated with the LPBF process are very high leading to the melt pool undergoing supercooling. Furthermore, analyzing the Fe-Cu phase diagram, it is known that iron and copper elements are immiscible and that there are no intermediate phases. However, in the liquid state these elements become miscible and therefore the two liquids undergo diffusion within the melt pool during the sintering of the interface. However, as previously mentioned, the cooling rates associated with this process are very high and therefore the diffusion of these elements is minimized. Small variations in the detection of Fe and Cu can be observed in the zones related to each material individually, suggesting some mixing of powders and incomplete aspiration between the deposition of each material.
Hardness characterization
Vickers microhardness of the multi-material samples was measured in different regions (left - zone 1, middle - zone 2, and right - zone 3) of the 420 stainless steel and copper, all over the top surface to study how the strategy used (bidirectional scan strategy without rotation between each layer) influences the hardness results. Moreover, three load values (100, 50, and 10 gf) were considered for the measurement since the two materials exhibit very different ductility behavior. Regarding 420 stainless steel, the results obtained proved that there was no significant hardness discrepancy between distinct regions, which proves the isotropy of the material along its length (Figure 8a for 50 gf). The values obtained were dependent on the applied load (the higher load the lower the hardness) due to the indentation load/size effect (Figure 8b) [39]. The hardness range values (482.0 to 532.8 HV) were coherent with those reported in the literature for as-built LPBFed 420 stainless steel parts (500-550 HV)
[17,40,41] or wrought + quenched and tempered (567 HV) ones [42]. On the other hand, the values obtained are higher than the ones reported for the same material in the annealed condition (247 HV) [34] or processed by metal injection molding (490 HV) [43]. The cooling rates of these two processes are low, leading to the formation of equilibrium phases with high grain sizes and low hardness. In agreement with the multi-material 420 Figure 6. Schematic representation of the melt pool and surface morphology before polishing (top surface and cross-section surfaces).
stainless steel-copper multi-material solution chemical composition (which transit from 100 % of 420 stainless steel to almost 100 % of copper), the hardness values jumped from approximately 533 to 116 HV. As expected, the hardness results of the copper zone were significantly lower than those of 420 stainless steel. The hardness values range between 99.3 and 115.7 HV and as in the zone associated with the steel alloy, the values were consistent along the length of the sample (Figure 8a). The obtained Vickers’ hardness values are higher than the ones reported for the annealed condition (57 HV) [44] or LBBF parts (65 HV) [25], but they are in accordance with those reported in the literature for the conventional processing routes, cold worked (100 HV) [44,45] and wrought (107 HV) [44]. Yu et al. [44]
stated that the difference observed between samples in the annealed and wrought conditions can be explained by the difference in grain sizes and the number of dislocations present in the two samples. The high hardness values obtained in this work are the result of the low porosity obtained. Furthermore, it is known that an increase in oxygen in the copper lattice causes an increase in strength and therefore, although a protective atmosphere (argon) was used, there is a possibility of incorporation of oxygen into the copper during the production process [46].
Figure 7. EDS mapping of the surface of the multi-material 420 stainless steel-copper sample (top surface) (adapted from [31]).
Zone 1 Zone 2 Zone 3
Copper 108.3 ± 2.5 HV 103.7 ± 5.5 HV 112.0 ± 4.0 HV
420 stainless steel 495.5 ± 42.5 HV 522.7 ± 3.3 HV 494.7 ± 11.4 HV (a)
Copper 420 stainless steel
0 100 200 300 400 500 600
103.8
486.2
108.0
504.4
111.4
528.4
100 gf 50 gf 10 gf
Material
Hardness (HV)
Figure 8. Vickers microhardness: (a) results obtained for 50 gf measured in three different zones, and (b) average results obtained for each of the materials considering three different test loads (based on [31]).
The results obtained indicate strong evidence that this new 420 stainless steel-copper multi-material 3DMMLPBF concept is successful mainly due to the presence of a well-processed copper region. The high cooling rate of this technology avoids the generation of large diffusion areas in the interface zones, thus preventing the existence of undesirable fragile zones and intermetallic phases.
CONCLUSIONS
The first approach to producing 3D multi-material 420 stainless steel-copper samples by laser powder bed fusion here presented was considered successful. The 420 stainless steel zones are almost free of defects and show hardness values ranging from the values reported in the literature. On the other hand, the copper lines present a well-defined geometry with about 200 µm width and occasional defects. A thin diffusion zone of about 10 µm is present in the transition between 420 stainless steel and copper. In this sense, the samples produced by this new 3DMMLPBF system presented a metallurgical bonding at the most critical
(b)
zone (interface) as well as a desirable metallurgical bonding between these two distinct materials. Also, there is no evidence of the formation of intermetallic compounds. Future works should focus on studying the microstructural features, mechanical, corrosion, tribological, and thermal analysis under plastic injection mold operation conditions.
ACKNOWLEDGEMENTS
This work is supported by FCT (Fundação para a Ciência e a Tecnologia) through the grant SFRH/BD/147460/2019 and the reference project UIDB/04436/2020 and UIDP/04436/2020, and UIDB/00285/2020. Additionally, this work is co-financed by FEDER, through the Competitiveness and Internationalization Operational Program (POCI), in the projects Add.Additive and MoedINOV, with the references POCI-01-0247-FEDER-024533 and POCI-01-0247-FEDER-033361, respectively.
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