Sumário
5 CONCLUSÕES E SUGESTÕES PARA TRABALHOS FUTUROS
5.1 Conclusões
Dos resultados e discussões apresentados no capítulo anterior, pode-se concluir que:
1- Os lingotes das ligas Al-4%Cu e Al-4%Cu-4%Ag, solidificados em regime transitório de fluxo de calor, apresentaram crescimento colunar ao longo do comprimento longitudinal dos lingotes, comprovando a direcionalidade da solidificação, conforme macroestruturas apresentadas.
2- A análise da macrossegregação de ambas as ligas demonstrou que houve uma relativa homogeneidade da composição química das ligas ao longo dos lingotes, sem evidência de macrosegregação expressiva.
3- As ligas solidificadas no dispositivo de solidificação unidirecional vertical ascendente apresentaram microestruturas caracterizadas por uma morfologia dendrítica na quase totalidade do lingotes, sendo evidenciada uma estrutura em transição desde a interface metal/molde até P = 12 mm, na qual não houve definição da morfologia da microestrutura. A partir de P = 20 mm de ambas as ligas, os braços dendríticos terciários foram significativamente observados e seu crescimento permanece até o topo do lingote. Observou-se que os espaçamentos dendríticos aumentam com a posição a partir de P = 12 mm, mantendo sempre a tendência de crescimento representada por uma equação na forma de potência, com expoente positivo.
4- Os perfis dos parâmetros térmicos em função de P de ambas ligas foram muito próximos, sendo VL=f(P) praticamente coincidentes e Ṫ=f(P) da liga Al-4%Cu-4%Ag ligeiramente superior
em relação à liga Al-4%Cu. Ambas funções foram caracterizadas por uma equação na forma de potência, com expoente negativo, mantendo sempre a tendência de decaimento. O fato de os parâmetros térmicos serem praticamente coincidentes possibilitou ajustar, para as duas ligas, uma
lei experimental para λ1 = f(P), outra para λ2 = f(P) e outra para λ3 = f(P), representadas por uma
equação na forma de potência, sempre com expoente positivo.
5- As correlações que envolveram λ1 ou λ2 e as variáveis VL e Ṫ apresentaram as mesmas leis
experimentais para as duas ligas estudadas, na forma de uma função tipo potência com expoente negativo. Entretanto, as correlações entre λ3 e as variáveis VL e Ṫ foram diferentes para a liga Al-
4%Cu e para a liga Al-4%Cu-4%Ag, o que possibilita afirmar que o crescimento de λ3 é mais
sensível a pequenas mudanças de comportamento de parâmetros térmicos influenciadas pela adição de 4%Ag à liga Al-4%Cu, causando um aumento no valor da constante multiplicativa da função potência.
6- As comparações das leis de crescimento teóricas com os resultados experimentais, para a liga Al-4%Cu, permitiram concluir que as melhores previsões são dadas pelos modelos: (a) Hunt e Lu (1996), cujos limites inferior e superior abrangeram os dados de λ1 experimentais; (b) Okamoto e
Kishitake (1975) e Trivedi (1984), cujas previsões ficaram próximas aos dados de λ1 obtidos
experimentalmente; (c) Bouchard e Kirkaldy (1997), cujas previsões ficaram próximas aos dados de λ2 experimentais. Para a liga Al-4%Cu-4%Ag, o único modelo de previsão de λ2 encontrado
para ligas ternárias foi o de Rappaz e Boettinger (1999), que superestimou os pontos experimentais.
7- A liga binária Al-4%Cu não apresentou variação notável da microdureza em função de λ2 e de
λ3. Já os valores de microdureza da liga ternária Al-4%Cu-4%Ag aumentaram, mesmo com o
aumento dos espaçamentos entre os braços secundários e entre os terciários, e isto pode ser atribuído à prata dissolvida na fase γ-Al e presente nas fases intermetálicas µ-Ag3Al e ρ-Ag2Al
atuando juntamente com outros intermetálicos (θ-Al2Cu, Al-Cu, Al4Cu9 e Al6Fe) como bloqueio
de movimentação de discordâncias e à disseminação progressiva de braços terciários, que permitem uma distribuição cada vez mais homogênea dos intermetálicos pela microestrutura. Este resultado não vai de encontro com a proposição da equação de Hall-Petch, o que demonstra que uma análise mais aprofundada sobre a interação entre os microconstituintes dentro do contorno do grão, em particular os intermetálicos e os espaçamentos dendríticos, pode ser
8- Os resultados dos ensaios de resistência à tração, mostraram a influência do arranjo microestrutural nos valores de σu, σe e δ. Como o espaçamento dendrítico é dependente dos
parâmetros térmicos de solidificação, as expressões experimentais que o correlacionam com VL e
Ṫ, se devidamente acopladas com as expressões de espaçamento dendrítico em função das propriedades mecânicas podem ser utilizadas para se pré-programar o processo de fundição para a obtenção de propriedades mecânicas finais desejadas.
9- A adição da prata na liga binária resultou em perfis mais elevados de σu, σe e δ, melhorando as
propriedades mecânicas. Na liga ternária observou-se o aumento de σu sem detrimento de δ.
5.2 Sugestões para Trabalhos Futuros
São sugeridas as seguintes abordagens de pesquisa para a realização de trabalhos futuros, de acordo com os resultados obtidos no presente trabalho:
realizar medições de dureza do intermetálico AlAg3 em escala nanométrica para investigar
melhor o aumento de ductilidade observado na liga ternária;
avaliar alterações provocadas pela adição do terceiro elemento de liga (4%Ag), na liga Al- 4%Cu, com foco nas propriedades tribológicas e de corrosão decorrentes;
realizar refusão de superfície por laser em ambas ligas, Al-4%Cu e Al-4%Cu-4%Ag, e analisar após a modificação da estrutura da superfície da amostra, a microdureza e fases decorrentes da solidificação à altas Ṫ.
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