Effect of extrusion conditions on grain refinement and fatigue behaviour in magnesium alloys
Effect of extrusion conditions on grain refinement and fatigue behaviour in magnesium alloys
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DOI：
10.1016/j.msea.2006.06.117
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发表时间：
2006-10
期刊：
影响因子：
6.4
通讯作者：
Y. Uematsu;K. Tokaji;M. Kamakura;K. Uchida;H. Shibata;Norikatsu Bekku
中科院分区：
文献类型：
--
作者：
Y. Uematsu;K. Tokaji;M. Kamakura;K. Uchida;H. Shibata;Norikatsu Bekku
本文研究了AZ31B、AZ61A和AZ80三种镁合金的控制挤压细化晶粒和挤压材料的疲劳行为。首先，为了研究工作温度对AZ31B和AZ61A的挤压效果的影响，在三种不同的工作温度下，在挤压比为67的条件下对AZ31B和AZ61A进行挤压。结果表明，随着工作温度的降低，两种合金均实现了晶粒细化，且晶粒尺寸变小。采用光滑试件进行了旋转弯曲疲劳试验。在AZ31B中，疲劳强度随晶粒度的减小而增加，而在AZ61A中，疲劳强度与晶粒度无关。对AZ31B的疲劳裂纹萌生和随后的小裂纹扩展进行了研究。结果表明，细化处理提高了裂纹萌生抗力和小裂纹扩展抗力，从而提高了疲劳强度。结果表明，AZ31B的疲劳强度符合Hall-Petch关系式，而AZ61A的疲劳强度符合Hall-Petch关系。然后，为了考察挤压比对AZ61A和AZ80的影响，在控制的条件下对AZ61A和AZ80进行了三种不同挤压比的挤压试验。结果表明，两种合金的晶粒度均随挤压比的增大而减小，但力学性能受挤压比即晶粒度的影响不大，且不同合金疲劳强度的晶粒度依赖性不同。在AZ61A中，低挤压比(粗晶)挤压的材料疲劳强度较高，而在AZ80中，高挤压比(细晶)挤压的材料疲劳强度较高。疲劳裂纹萌生行为与疲劳强度的变化趋势一致，AZ61A型挤压材料在低挤压比时裂纹萌生延迟，而AZ80型挤压高挤压时裂纹萌生延迟。此外，根据挤压形成的织构和产生裂纹的夹杂物的存在，讨论了观察到的两种合金疲劳强度的晶粒度关系。
This paper describes the grain refinement due to controlled extrusion and the fatigue behaviour of the extruded materials in three magnesium alloys, AZ31B, AZ61A and AZ80. First, in order to investigate the effect of working temperature, billets of AZ31B and AZ61A were extruded at an extrusion ratio of 67 under controlled conditions with three different working temperatures. It was found that grain refinement was attained in both alloys whose grain size decreased with decreasing working temperature. Rotary bending fatigue tests were performed using smooth specimens. In AZ31B, fatigue strength increased with decreasing grain size, but in AZ61A, it did not depend on grain size. Fatigue crack initiation and subsequent small crack growth were examined in AZ31B. Consequently, grain refinement improved both crack initiation resistance and small crack growth resistance, resulting in the increase in fatigue strength. Furthermore, it was indicated that fatigue strength was expressed properly by the Hall–Petch relationship in AZ31B, but not in AZ61A. Then, in order to investigate the effect of extrusion ratio, billets of AZ61A and AZ80 were extruded at three different extrusion ratios under controlled conditions. It was found that grain size decreased with increasing extrusion ratio in both alloys, but mechanical properties were not affected significantly by extrusion ratio, i.e. grain size, and the grain size dependence of fatigue strength was different between alloys. In AZ61A, fatigue strength was higher in the material extruded at lower extrusion ratio (coarser grain), but in AZ80, in the material extruded at higher extrusion ratio (finer grain). Fatigue crack initiation behaviour was consistent with the tendency of fatigue strength, where crack initiation delayed in the material extruded at lower extrusion ratio in AZ61A and in the material at higher extrusion ratio in AZ80. Furthermore, the observed grain size dependence of fatigue strength in both alloys was discussed on the basis of a texture formed by extrusion and the presence of inclusions from which cracks generated.