내부 변수를 이용한 분무 주조 과공정 Al-Si계 합금의 Creep 변형 거동 해석

Abstract

Hypereutectic Al-Si based alloys have been regarded attractive for automotive and aerospace application, due to high specific strength, good wear resistance, high thermal stability, low thermal expansion coefficient and good creep resistance. In this study, the high temperature deformation behaviors of a spray casting hypereutectic Al-25Si-2Cu-1Mg alloy and Al-25Si-2Cu-1Mg-2Mn alloy have been investigated by applying the internal variable theory proposed by Chang et al. A series of load relaxation test and constant load creep tests were performed to analyze the high temperature deformation mechanisms of these alloys at temperature ranging from 300℃ to 500℃.The flow stress-strain curves obtained from load relaxation tests were found to shift toward the faster strain rate region and the lower stress region with increasing temperature due to the enhanced thermal activation processes. The flow stress-strain curves were also found to shift toward the lower strain rate region and the higher stress region due to the addition of Mn contents. The transition of flow behavior was occurred below a certain strain rate. The flow stress-strain rate curves can be resolved into dislocation glide and dislocation creep mode above 300℃. The activation energy of a hypereutectic Al-25Si-2Cu-1Mg alloy for dislocation glide and dislocation creep was obtained as 93 KJ/mol and 145 KJ/mol, respectively. The activation energy of a hypereutectic Al-25Si-2Cu-1Mg-2Mn alloy for dislocation glide and dislocation creep was obtained as 102 KJ/mol and 153 KJ/mol as well.The creep behavior of two alloys has been investigated at temperature ranging from 300℃ to 500℃. The minimum creep rate as a function of applied stress by using power-law equation suggested the existence of two different regimes: a low-stress regime controlled by dislocation creep having a stress exponent close to 5 and a high-stress regime controlled by dislocation glide having a stress exponent close to 3. The creep resistance is enhanced at low stress region and the minimum creep rate is increased with the addition of Mn content. The creep activation energy a hypereutectic Al-25Si-2Cu-1Mg alloy for dislocation glide and dislocation creep was obtained as 85 KJ/mol and 137 KJ/mol, respectively. The creep activation energy of a hypereutectic Al-25Si-2Cu-1Mg-2Mn alloy for dislocation glide and dislocation creep obtained as 107 KJ/mol and 163 KJ/mol was higher than the reported activation energy for grain boundary diffusion (90 KJ/mol) and for lattice self-diffusion (140 KJ/mol) due to the addition of Mn contents. The creep test results are consistent with the load relaxation test results roughly, indicating that load relaxation test is a reliable method to investigate creep behavior.The minimum creep rate is increased with the accumulation of pre-strain. The increment of creep rate at low stress region was higher than that at high stress regime. The creep resistance of a spray casting hypereutectic Al-25Si-2Cu-1Mg alloy can be enhanced considerably by the accumulation of pre-strain at low stress region. The accumulation of damage such as a cracked primary Si particles and matrix/particles de-bonding acts as the pinning action blocking the passage of dislocation on a soften Al-matrix. The volume fraction and size of inclusions can be also affected the creep resistance of this alloy.