局部强冷作用下厚板铝合金/镁合金搅拌摩擦焊界面金属间化合物的析出行为

Precipitation Behavior of Intermetallic Compounds at the Interface of Thick Plate Friction Stir Welded Al Alloy/Mg Alloy Joints Under Local Strong Cooling

厚板铝合金/镁合金在搅拌摩擦焊(FSW)时接头界面沿板厚方向存在严重的温度分布差异问题,导致接头界面组织分布极不均匀,接头成形较为困难。减小接头界面沿板厚方向温差成为改善厚板铝合金/镁合金FSW接头成形的关键之一。本工作选取15 mm厚的5A06-H112铝合金和AZ31B-O镁合金板材进行异种材料FSW,采用液氮喷洒于焊缝上表面进行局部强冷,利用EBSD和TEM获得焊接接头物相分布和晶粒取向散布,研究了铝合金/镁合金接头搅拌区(SZ)两侧界面处金属间化合物(IMCs)的析出行为。结果表明,在空冷和液氮冷却2种条件下SZ的铝合金侧主要析出Al_(3)Mg_(2)相;镁合金侧主要析出Al_(12)Mg_(17)相,且在上部界面处与Mg发生了共晶反应;焊缝表面局部强冷,降低了接头各位置的峰值温度和高温停留时间,有效抑制了IMCs的析出和低熔点共晶的形成。不施加表面冷却时,铝合金侧界面上部和中部附近的SZ中主要析出Al_(3)Mg_(2)相,底部SZ中则为细小的等轴铝合金晶粒;镁合金侧界面上部的SZ中则主要析出了Al_(12)Mg_(17)相,并与Mg形成低熔点共晶,且在共晶区与SZ中的铝合金之间析出Al_(3)Mg_(2)薄层,中部和下部SZ与镁合金之间析出了界线分明、由Al_(3)Mg_(2)层和Al_(12)Mg_(17)层组成的IMCs叠层,且中部的IMCs叠层总厚度远大于峰值温度较低的底部处IMCs叠层厚度,其中Al_(3)Mg_(2)层厚度减薄更为明显。当在焊缝表面施加液氮冷却时,铝合金侧界面上部的SZ中除了析出Al_(3)Mg_(2)相外,还有少部分Al_(12)Mg_(17)和铝合金晶粒,而界面中部和底部SZ中均为等轴状的铝合金晶粒;在镁合金侧界面处,各部位IMCs析出行为与不施加表面冷却时相似,但界面上部析出的Mg+Al_(12)Mg_(17)共晶层和Al_(3)Mg_(2)层、中部和下部SZ界面IMCs叠层总厚度明显减小,Al_(3)Mg_(2)薄层厚度降低更为显著。应变速率对IMCs的析出有显著影响,表现为界面层实际厚度远大于由扩散定律计算的理论厚度。

Al alloy and Mg alloy are not only the lightest metal structural materials,but also have the advantages of high specific strength and damping performance,which are very attractive for automobile,high-speed rail and aerospace.To meet the requirements of structural lightweight and different service environments,it is usually necessary to join Al alloy and Mg alloy into a complete structure.As a new solidstate joining method,friction stir welding(FSW)has obvious advantages in the field of Al alloy/Mg alloy hybrid structure because its welding temperature is lower than the melting point of base metal.However,the formation temperature of Al alloy/Mg alloy intermetallic compounds(IMCs)is lower than the melting point of Al and Mg,if the thickness of base metal exceeds 10 mm,the Al alloy/Mg alloy FSW is still very difficult because of the formation of IMCs in the weld.To obtain some control methods of the formation of IMCs,5A06-H112 Al alloy and AZ31B-O Mg alloy plates with a thickness of 15 mm were used for the Al alloy/Mg alloy FSW.Liquid nitrogen was sprayed near the rear of the stirring head to locally cool the upper surface of the weld.EBSD and TEM instrument were used to obtain the phase distribution and grain orientation spread at different positions of the welded joint.The precipitation behavior of IMCs at the interfaces on both sides of the stirring zone(SZ)of Al alloy/Mg alloy joints under ambient temperatures and liquid nitrogen cooling conditions was studied.The results indicate that Al_(3)Mg_(2) mainly precipitates on the Al alloy side of SZ;the main precipitation on the Mg alloy side is Al_(12)Mg_(17),and it was generated eutectic reactions with Mg at the upper interface;liquid nitrogen cooling on the surface can reduce the peak temperature and high-temperature residence time at various positions of the joint,and has a significant inhibitory effect on the precipitation of IMCs and the formation of low melting point eutectic.When surface cooling is not applied,almost only Al_(3)Mg_(2) phase is precipitated in the SZ at the upper and middle parts of near the interface of Al alloy side,and at the bottom,only fine equiaxed Al grains are observed in the SZ.At the side of magnesium alloy,Al_(12)Mg_(17) phase is mainly precipitated at the upper interface of SZ and it forms low melting point eutectic with Mg,and at the same time,a thin layer of Al_(3)Mg_(2) is precipitated between the eutectic zone and the Al alloy in SZ.Two layers of Al_(3)Mg_(2) and Al_(12)Mg_(17),which are sticked close to each other and there is a distinct boundary layer between them,is precipitated between SZ and the Mg alloy at the middle and lower interfaces,and the total thickness of the IMCs layer at the middle interface is much greater than the IMCs layer thickness at the bottom interface,at here the peak temperature is lower,the thickness of the Al_(3)Mg_(2) layer is decreased more significantly.When liquid nitrogen cooling is applied to the weld surface,in addition to Al_(3)Mg_(2) phase precipitation,there are also a small number of Al_(12)Mg_(17) and Al grains in the SZ at the upper part of the interface of Al alloy side,while equiaxed Al grains are present in the SZ at the middle and bottom parts of the interface.At interface close to the Mg alloy side,the precipitation behavior of IMCs in each parts is similar to that without surface cooling,but the total thickness of the Mg+Al_(12)Mg_(17) eutectic layer and Al_(3)Mg_(2) layer precipitated at the upper part of the interface,and the thickness of the IMCs layer at the middle and lower SZ interfaces are significantly decreased,and the thickness of the Al_(3)Mg_(2) thin layer decreases more significantly.The strain rate has a significant impact on the precipitation of IMCs,and it is confirmed by that,the actual thickness of the interface layer is much greater than the theoretical thickness calculated by the diffusion law.