Research On The Effect Of Different Heat Treatment On Variable Polarity TIG Welded2219Aluminium Alloy Joints Properties

A new method of solving aluminium alloy welded joint softening problem wasproposed in this project.2219-T6aluminium which is widely used as the rocket liquidpropellant tanks materials was researched in this study. This method was put forwardfrom the perspective of heat treatment conditions of the base metal and the joints.Before VPTIG welding, the base metal needed to be treated by solution. Artificialaging treatment was conducted on the welded joints at last. The strength of this kindof joint was compared with the conventional welded joint which is obtained from theprocess of solution, artificial aging and welding. Microstructure, hardness and tensileproperties were tested to analyse the difference of joints. Stress corrosion cracking(SCC) test was performed to investigate the effects of heat treatment on the jointspropeties.The experiment results indicated that the precipitated phases in solution treatedbase metal were obviously less than that in the original base metal. After welding andartificial aging, the number and the size of the precipitated phases significantlyincreased. For welded joints, fine and equiaxed grain was present in weld zone andcoarse equiaxed grain was observed in HAZ. Hardness of the original base metal is112HV. The lowest hardness value of77HV was present at weld zone. In HAZ, thehardness was unevenly distributed and78HV was tested in the middle area. After postweld artificial aging, the hardness increased by17%in weld zone and5%in HAZ,respectively. In the joints welded from solution treated metal, there was an overallhardness reduction. After artificial aging, hardness increased greatly by12%in weldzone and11%in base metal. However, only less than4%increase in hardness wasoccurred in HAZ. The tensile test shows that for the original metal, joint strengthdecreased from425MPa to235MPa and the joint coefficient was only55.3%. For thesolution treated metal, joint strength was only225MPa and after artificial aging itreached to260MPa with the joint coefficient of61.2%. In other words, the jointstrength was improved by10.6%when the new method was adopted.The slow strain rate tension (SSRT) test was applied to investigate the effect ofheat treatment on SCC resistance of2219aluminium alloy welded joints.Experimental results shows base metal got the highest resistance against stress corrosion, much better than that of as welded and heat treated joints. In addition, theexperimental data of as welded joints was very scattered due to the non-uniformmicrostructure. Compared with the as welded joints, the heat treated joints slightlydecreased in corrosion resistance but got an even data distribution. In conclusion, aserious decline in SCC resistance of welded joints was not caused by heat treatment.