利用热模拟试样模拟TiNi合金激光焊接接头焊缝热影响区, 对焊接接头、焊缝金属、模拟热影响区和母材的相变温度进行差热分析, 研究了焊接接头与其它三者逆相变温度的关系, 并利用OM, SEM和XRD对母材、热影响区和焊缝金属的组织、析出相的分布以及晶体结构进行了研究. 结果表明,TiNi合金激光焊接接头的形状恢复率与母材无明显差别, 但形状恢复温度区间与母材差异较大, 其形状恢复开始温度比母材低40 ℃; 焊缝金属和热影响区逆相变开始温度(As)和结束温度(Af)均不同于母材,主要原因是焊缝金属经历了熔化-凝固过程, 失去了母材中原有的晶体择优取向,且析出相尺寸小、分布不均; 而热影响区As和Af降低,可能是细小析出相重新固溶于基体所致. 整个接头的逆相变温度区间与焊缝金属近似,调控焊缝金属的相变温度是控制TiNi合金激光焊接接头形状记忆功能的关键.
In order to control the shape memory function of TiNi alloy weld joint, it is necessary to clarify the effect of the three different parts (weld metal, heat-affected zone (HAZ) and base metal) on the shape recovery temperatures of the whole weld joint, but few reports are available on this aspect. In this work, the microstructure in the HAZ was studied by Gleeble thermal-simulation test. Phase transformation temperatures of weld joint, weld metal, HAZ and base metal were measured by differential thermal analysis. The inverse phase transformation temperature was analyzed. The microstructure, distribution of precipitation and crystal structure were investigated by using OM, SEM and XRD. The weld joint shows the similar shape recovery ratio to the base metal, but the shape recovery temperature range is significantly different. The start recovery temperature of the weld joint is lower about 40 ℃ than that of the base metal. Both of the austenite start temperature (As) and finish temperature (Af) of the weld metal and HAZ vary much compared with the base metal. The change in the weld metal is attributed to the fusion-solidification process, in which the preferred crystal orientation is lost. The newly formed precipitation phases show a small size and an uneven distribution. The change in the HAZ refers to the drop of As and Af, which is possibly caused by the solution of minor precipitation phase in the matrix. The Asand Af of the laser weld joint are quite the same as those of the weld metal for TiNi shape memory alloy, which indicates that the key to guarantee the shape memory function lies in controlling the phase transformation temperatures of the weld metal.
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