7xxx系铝合金凝固过程中收缩行为的测定

Study of Determination of Contraction Behavior of 7xxx Aluminum Alloys during Solidification

应用自主设计的测量装置对7xxx系铝合金凝固过程中的收缩行为进行了系统的测量。主要研究了3种7xxx系铝合金的收缩行为和微观组织对热裂敏感性的影响。研究发现,在凝固过程中,3种7xxx系铝合金表现出了非常不同的收缩行为。7050和7075铝合金的热膨胀系数曲线从收缩起始点(固相率fs分别为0.69和0.73)开始陡然增加至峰值,之后随着温度的降低减小至一个常数值;7022铝合金的热膨胀系数曲线与以上两种不同,从收缩起始点(固相率为0.8)开始先出现一个波动上升,其后随着温度降低数值单调递增。微观组织显示,收缩过程的起始点是枝晶搭接的开始点。凝固过程中,7050合金的热膨胀系数大于7022合金,可以认为凝固过程的收缩行为能作为热裂敏感性的一种判断准则。

The paper studied the effects of contraction behavior and microstructure of three 7xxx series aluminum alloys on the hot cracking susceptibility by using self-developed device for systematically measuring contraction behavior of the 7xxx alloys during solidification.The results showed that the three alloys performed extremely different contraction behavior.The linear expansion coefficient（LEC） of 7050 and 7075 alloys（with solid fraction of 0.69 and 0.73 separately） steeply increased from contraction starting to peak value and then dropped to a constant value with the decrease of temperature; The LEC of 7022 alloy（with solid phase of 0.8）,differing from that of the two alloys,increased from contraction starting with little fluctuation and then it monotonously increased with the decrease of temperature.Starting point of contraction was observed to be that of dendrite contact in the microstructure.In light of thermal expansion coefficient larger for 7050 than 7022 during solidification,contraction behavior of the alloy during solidification could be considered to be criteria for evaluation of hot cracking susceptibility.