Effects Of Welding Parameters And Lap Configurations On Microstructures And Mechanical Properties Of6061Welds Produced By Tungsten Inert Gas Welding And Friction Stir Spot Welding

In this paper, wrought aluminum alloy6061-T6was selected to explore the effectsof heat input, post-welding aging treatment and pulse frequency on themacro/micro-structures and mechanical properties of tungsten inert gas welding （TIG）and pulse tungsten inert gas welding （P-TIG） joints by microstructure observation,micro-hardness analysis and tensile tests, etc. Additionally, in order to investigate theweldability of dissimilar base materials6061-T6and AZ31B, friction stir spot weldingtechnology was adopted, and the influences of pre-heat treatment and lap configurationon the evolution of structure and tensile shear strength were systematically discussedusing optical microscope, tensile machine, X-ray spectroscopy and scanning electronmicroscope, etc. Based on experimental basis and combined with theoretical analysis,the main conclusions may be derived as follows:Effects of aging treatment and heat input on the microstructures and mechanicalproperties of TIG-welded6061-T6alloy joints were discussed. Results show that withan increase in heat input, the width in heat affected zone （HAZ） increasing from315μmto690μm and the average grain size in fusion zone （FZ） increasing from43μm to51μm. However, no appreciable changes in grain size had been noticed after post-weldaging treatment. The hardness of the FZ was lower than those of the BM and HAZ.With an increase in heat input, the hardness values of the HAZ decreased whereas thehardness values of the FZ decreased initially and then increased slightly. Post-weldaging treatment was beneficial to improve the hardness of the TIG-welded aluminumalloy joints. Partial penetration and pores presented at low heat input and resulted in lowUTS value of the welded joints. Post-weld aging treatment was found to increase thetensile strength of the TIG-welded6061-T6joints because of the formation ofprecipitates. Fracture surfaces of welded joints were characterized by ductile fracture.Effects of pulse frequency on macro/micro-structures and mechanical properties ofP-TIG welded6061-T6alloy joints were studied. Results show that at lower pulsefrequency （10Hz and20Hz） welds presented a bad performance, with a small amountof oxide appeared on the welds surface. At40Hz, desired welds were obtained,characterized by a typical stick weld feature. After that, higher pulse frequency resultedbad performance, with the increasing in the width of welds. The average grain size inFZ decreased initially and then increased slightly. More precipitates formed at lower or higher pulse frequency. The relationship of microhardness value was base material（BM）>FZ>HAZ, microhardness in both of FZ and HAZ increased and then decreasedslightly. Similar trend were also detected when studying with evolution of ultimatetensile strength and the highest value was255MPa which was gained at40Hz. Thefracture mode for BM was ductile fracture while the welds fracture in a mix-fracturemanner.Effects of different lap configurations and pre-heat treatment on microstructuresand tensile shear strength of Al and Mg welded joints produced by friction stir spotwelding were investigated. Results show that sound welds containing a free defectsbonded region could be achieved using this simple rotational tool when meeting withthe similar base materials. However, for the dissimilar base materials, two differentcross-section morphologies were acquired, in the Mg-Al welds the interface betweenthe upper and lower sheet was cut off by pin, and in the Al-Mg welds the pin failed topenetrate into the lower sheet and the interface of the two sheets just curved along thedirection of tool penetration. In Mg-Al welds, eutectic reaction taken place in mixedmaterials extruded by the base of pin, with a small amount of IMC-Al₁₂Mg₁₇generatingthrough local melting. While, in Al-Mg welds, eutectic reaction occurred in the wholeinterface of the two sheets underneath the shoulder, with a large amount of island phase(IMC-Al₁₂Mg₁₇) and lamellar phase (IMC-Al₁₂Mg₁₇plus-Mg) generating throughconstitutional liquation. Higher pre-heat temperature resulted higher tensile shearstrength in all similar base materials welds and Mg-Al welds. Inversely, it was notconducive to improving the strength of Al-Mg welds by increasing in pre-heattemperature because of the formation of thicker eutectic layer. Nugget pull-out fractureobserved in welds with faying and large bonded area. Insufficient stirring or not fullydeveloped nugget easily resulted shear fracture. No actual nugget formed in Al-Mgwelds, interfacial fracture occurred.