| Aluminum alloy(Al)/steel composite tailor-welded blanks(TWBs)is one of the effective approaches to achieve lightweight of automobile,which can make full use of the traits of materials of double kinds that the excellent mechanical properties of steel and the low density,good corrosion resistance of Al alloy.The Al/steel composite TWBs are produced by welding Al alloy plate to steel and then subjecting to a forming process.So there is a higher requirement on the formability of welded joint.However,the brittle Fe-Al intermetallic compounds(IMCs)formed in the brazed interface of welded joint generally exhibits poor effect on the formability of Al/steel composite structure,which hinders the industrial application of the Al/steel composite TWBs.In addition,for Al/steel composite structure,its adaptability to environment and security were characterized by the corrosion resistance of welded joint.In the present study,the formability and corrosion resistance of MIG arc brazed-fusion welded joint of Al/steel at different welding heat inputs were tested and analyzed.Moreover,the microstructures of the brazed interface in welded joint as well as its relationship with the properties of welded joint were also discussed in detail.The object of the study is to provide basic data and theoretical support to realize engineering application of Al/steel composite TWBs produced by MIG arc brazing-fusion welding of Al alloy to steel plate.In the welding heat inputs range of 592-1141 J/cm,all joints show good weld appearance and the mechanical resistance is equivalent to to that of the fusion welded joint of Al alloy base metal.With the increase of welding heat input,the thickness of IMCs layer which consists of Fe2Al5,FeAl3,Fe-Al-Si ternary intermetallic compounds and Fe2Al5Zn0.4 increases.Meanwhile,a sharper fluctuation exists and voids are even formed on steel side of the brazed interface.The formability can be preliminary characterized by the bending property of lap welded joint,the three-point longitudinal face bending test of welded joints was designed to study on the formability of MIG arc brazed-fusion welded joints of Al/steel,and the bending angle were adopted as an evaluation index to identify the bending property of welded joints.The results showed that the correlation between welding heat input and bending property of joint is linear and negative.And the max bending angle reaches up to 108° within the scope of bending test.Based on the crack propagation path in the brazed interface when the workpiece was bended to 180°,the fracture mode in bending tests can be classified to 3 types.One is that only several fine cracks occur near the weld toe,and microscopic cracks perpendicular to IMCs layer go through the whole brittle IMCs layer,which initiate at the tip of protuberance in the wavy brazed interface.One is cracks go through the whole weld seam end in heat affect zone,and microscopic cracks initiating at the interface of Fe2Al5/FeAl3 in central zone of brazed interface,propagate along this interface till weld toe and root with some cracks going through the FeAl3 phase.The other is that cracks go through the whole weld seam end in A1 base metal,and microscopic cracks propagate to the inner part of Fe2Al5 and till the interface between Fe2Al5 and FeAl3,derive from the interface of steel and IMCs layer in the central zone of brazed interface.As the welding heat input being increased,the thickness of IMCs layer increases,voids form and the content of Fe2Al5Zn0.4 decreases in transition zone of brazed interface,which lead to the change of crack propagation path in the brazed interface,and affects the bending property of joints.Cupping test was used to characterize the formability of MIG arc brazed-fusion welded joints by using the bulging drawing property of joint as the evaluation index.By cupping test,the cupping test value,initial position of surface cracks and the strain distribution in surface of welded joints were tested and analyzed for the MIG arc brazed-fusion welded joints at the same welding heat input with joints which have excellent bending property.The results showed that the cupping test value of joint reaches up to 25.5 mm and its initial position of surface crack occurs in heat affected zone of aluminum side at the welding heat input of 592 J/cm,which shows that the bulging drawing property of the joint is equivalent to that of Al alloy base metal.With the welding heat input being increased,the IMCs layer in the brazed interface becomes thicker,which results in the deterioration of joints,then the initial position of surface cracks transfers to weld seam of A1 alloy,the cupping test value decreases and the bulging drawing property of joint decrease obviously.The corrosion resistance of joint at different welding heat inputs were studied by using current-potential polarization test and immersion corrosion test.The results showed that with the increase of welding heat input,the corrosion tendencies of joints are similar,but corrosion resistances of joints are gradually decreased.Especially,at the welding heat input of 652 J/cm,corrosion grooves appear in weld seam near the IMCs layer,which makes the corrosion resistance of joint decreased.The main causes of corrosion grooves were analyzed that the accelerated dissolution of weld metal adjacent to the IMCs layer,which is due to the strong galvanic corrosion among IMCs at brazed interface,steel base metal and weld metal of Al alloy.Moreover,as the thickness of IMCs layer being increased and Fe-Al-Si ternary IMCs formed,the accelerated dissolution of weld metal with IMCs is more serious. |
PDF Full Text Request