高强镁合金薄板激光焊接接头热裂机制及控制技术研究

Aimed at the current difficulties in weld forming of high-strength wrought magnesium alloy sheets especially the severe weld hot cracking tendency, the applicants propose the thoughts that the prohibition of liquid cracking can be achieved by microstructure refinement and modification of the size and the distribution of the precipitates and the prevention of solidification cracking can be realized by the reduced solidification temperature range and the refined as-solidified microstructure associated with micro-alloying of the welding pot with the RE element. The high hot-crack susceptible wrought magnesium alloy sheets of the high Zn Mg-Zn-Zr series and the high Al Mg-Al-Zn series are selected for the .present study. The influences of the welding techical parameters and the microstructure features of the alloy matrix such as grain size, size and distribution of precipitation on the formation of liquid cracking in the half-fused zone of the laser-welded joints will be studied, and thus the correlations between the formation mechanism of liquid cracking and the microstructure features under the optimal welding parameters will be clarified. The effects of the solidification of the weld pot on the liquidification recation and the formation in the half-fused zone under will be explored. The effects of the micro-alloying of the RE elements of the weld pot on the as-solidified microstructure features and the formation of solidification cracking will be examined. Furthermore, the mechanism of solidification cracking will be explicited. On the basis of above, the detailed means for the prevention of hot cracking of the welds will be developed. A novel method for the prevention of hot cracking in high-alloyed magnesium alloy welds will be explored, which will .offer the scientific guide for the welding of the high hot-cracking suseptible alloys.

针对高强变形镁合金板材焊接成形困难特别是焊接热裂严重的现状，申请人提出通过细化合金微观组织和调控析出相尺寸与分散性等以抑制液化裂纹形成和通过焊接熔池稀土微合金化缩小凝固温度区间和细化凝固组织以抑制结晶裂纹形成的思想。以焊接热裂倾向严重的高Zn含量Mg-Zn-Zr系和高Al含量Mg-Al-Zn系变形镁合金薄板为研究对象，系统研究工艺参数和母材微观组织特征（母材晶粒尺寸、第二相尺寸及分布特征等）对镁合金激光焊接接头半熔化区液化裂纹形成的影响规律，探明在优化的工艺条件下半熔化区液化裂纹的形成与母材微观组织特征之间的关系，明确焊接熔池凝固过程对半熔化区液化反应机制和液化裂纹形成的影响，考察焊接熔池稀土微合金化对焊缝凝固组织特征和结晶裂纹形成的影响，明确焊缝区结晶裂纹的形成机制，提出抑制焊接热裂的具体措施。藉此探索一条抑制高合金含量镁合金焊接热裂倾向的新途径，为高热裂倾向合金材料的焊接提供理论指导。

针对高合金元素含量的高强变形镁合金板材焊接成形困难特别是焊接热裂严重的现状，本课题采取了细化合金板材的微观组织和调控析出相特征的手段来抑制液化裂纹的形成、通过对焊接熔池进行微合金化处理来缩小凝固温度区间和细化凝固组织来抑制结晶裂纹形成的研究思路。系统研究并阐明了板材的微观组织特征和焊接工艺参数对焊接接头半熔化区液化行为及液化裂纹形成的影响。研究发现，微米级的大尺寸MgZn相、Mg17Al12相在熔合线附近区域形成的液化熔池体积大，容易导致液化裂纹；板材的晶粒尺寸越大，第二相在晶界上分布的越密集，越容易发生液化反应而形成连续液膜，液化裂纹越容易形成；焊接热输入越大，越容易诱发熔合线附近液化裂纹的形成；大尺寸第二相颗粒的液化机制有与基体-Mg发生共晶反应而液化、直接熔化而液化；亚微米以下的沉淀相液化反应现象不明显，不会导致液化裂纹的形成。. 揭示了焊缝区结晶裂纹的形成机制。在焊接熔池凝固结晶后期，低熔点共晶组织或脆性第二相沿晶界连续连续分布时，在熔池区凝固收缩和热收缩产生的拉伸应力作用下沿晶界开裂，形成结晶裂纹。. 采用填带焊的方法对焊接熔池中进行了微合金化处理，阐明了元素种类及加入量对焊缝凝固组织的影响机制，通过产生显著的成分过冷效应和异质形核效应而细化了焊缝中心的晶粒组织、抑制了熔合线附近区域柱状晶区的范围。. 提出了抑制焊接热裂纹形成的有效措施，抑制液化裂纹的措施为消除板材中微米级大尺寸第二相、细化晶粒组织、提高沉淀相的分布均匀性；抑制结晶裂纹的措施为采用成分合适的带材进行填充焊、缩小熔体的凝固温度区间，采用合适的焊接热输入量等。. 本项目的研究成果对高热裂倾向合金材料焊接热裂问题的研究和解决具有重要的理论指导意义和实际参考价值。