Study On Resistance Spot Welding Of Dissimilar Materials Of Aluminum Alloy And High Strength Steel

With the increasing use of high strength steel and aluminum alloy in automotive manufacturing industry, it has became an indispensible problem to be solved to study the joining technology of dissimilar materials of aluminum alloy and high strength steel. Due to the great discrepancies concerning the physical and chemical properties between aluminum alloy and high strength steel, and the extremely low solid solubility of Fe in Al, the weldability of dissimilar materials of aluminum alloy and high strength steel is poor. Plenty of defects suck as shrinkage cavities, cracks as well as intermetallic compound layer are readily to be formed in the welded joint, and they will lower mechanical properties of the welded joint greatly, which thereby turns out to be one of the main technical bottlenecks in further extensive use of aluminum alloy and high strength steel in the domain of vehicle manufacturing.The study on resistance spot welding of dissimilar materials of aluminum alloy and high strength steel was carried out in the paper. Microstructures and mechanical properties of dissimilar material resistance spot welded joints were studied systematically firstly. The results showed that the resistance spot welded joint of dissimilar materials of aluminum alloy and high strength steel was composed of aluminum nugget and heat affected zone, which belonged to an especial welding-brazing joint in essential. An evident interface was observed between aluminum nugget and high strength steel, and intermetallic compound layers with unequal thicknesses were formed in the aluminum/steel interface, which were composed of Fe₂Al₅ on high strength steel side and Fe₄Al₁₃ on aluminum alloy side. It was not uniform in the nanohardness distribution at the interfacial zone, and the value of nanohardness of the intermetallic compound layer was higher than that of other regions. Interfacial failure mode was obtained during the tensile shear testing, with aluminum nugget detaching from the high strength steel at the interfacial zone. The cracking tended to initiate at the inetrmetallic compound layer as well as the interface between the intermetallic compound layer and high strength steel, and then propagate through the intermetallic compound layer. Besides, the interfacial reaction mechanism was studied, and the growth process of the intermetallic compound layer was studied by means of thermal dynamics and kinetic analysis. The results showed that the growth kinetic coefficient for Fe₂Al₅ was higher than those of Fe₄Al₁₃ and FeAl, which resulted in the faster growth for the former phase.Effects of resistance spot welding parameters (welding current, welding time and electrode force) on microstructures and mechanical properties of dissimilar material resistance spot welded joints were studied. The results indicated that the tensile shear load of the welded joint was mainly decided by welding current, welding time and electrode force. The nugget diameter of the welded joint and the thickness of the brittle intermetallic compound layer, however, were direct factors which regulated the tensile shear load of the welded joint. When the nugget diameter varied from 2.4mm to 5.7mm, it became the major factor which decided the tensile shear load of the welded joint, while the limiting of the interfacial intermetallic compound layer on the tensile shear load began to act immediately its thickness exceeded 5.6μm. Resistance spot welding of dissimilar materials of aluminum alloy and high strength steel with electrode plate was carried out, and microstructures of welded joints were studied. Besides, effects of welding parameters on microstructures and mechanical properties of the welded joints with electrode plate were carried out. The results showed that the indentation ratio of the welded joint was shallow enough, and the surface forming quality was acceptable since no expulsion and alloying process were observed. Compared with the nugget diameter and tensile shear load of the welded joint with F type electrode, the nugget diameter and tensile shear load of the welded joint were improved by 60% and 30%, respectively. Furthermore, the thickness of of interfacial intermetallic compound layer also decreased to a certain extent.The morphology of electrode tip was optimized, and the optimum electrode morphologies were drawn as follows: the electrode tip diameter on high strength steel side was 10mm, and the spherical electrode radius was 35mm on aluminum alloy side. Based on the optimum electrode morphologies, welding parameters were optimized. Under the optimum welding parameters (electrode force of 3.5kN, welding current of 22kA, welding time of 300ms), the nugget diameter (10.1mm) and tensile shear load (5.4kN) were improved by 77% and 69%, respectively, compared to those of the welded joint obtained with F type electrode. Meanwhile, the indentation ratio (16.8%) decreased by 43%. The tensile shear specimen of the welded joint fractured in the nugget pullout failure mode, which was improved dramatically as compared to that with F type electrode.Effects of alloy elements (Cu, Si) on joint microstructures and properties were studied by the method of adding interlayer between high strength steel and aluminum alloy during resistance spot welding. The results showed that both Cu and Si had great effects on joint microstructures and properties. With the addition of 100μm thick Cu interlayer, the single-layer structure of the intermetallic compounds with thickness of 1.1μm was formed in the aluminum/steel interface at the joint center, which contained 5.36wt.% Cu, and the phase compositions were mainly (Fe,Cu)₄Al₁₃. Cu additions led to the suppression in the interfacial reaction between aluminum alloy and high strength steel to a certain extent. The tensile shear test results showed that CuAl₂ near the aluminum/steel interface turned out to be the main reason to restrict the joint mechanical properties. With the addition of 300μm thick 4047 AlSi12, the intermetallic compound layer exhibited serrated feature, with the thickness of 0.9mm and containing 4.26wt.% Si. The joint tensile shear load reached the highest value (6.2kN), increasing by 15% compared with the welded joint without interlayer. Tensile-shear specimen of the welded joint displayed a nugget pullout failure mode.An axisymmetrical finite element model by ANSYS finite element simulation software was developed for studying the process of resistance spot welding of dissimilar materials of aluminum alloy and high strength steel with both F type electrodes and optimized electrodes. The elastic-plastic mechanics analysis results of the pre-pressure contact behavior showed that the distribution of the contact pressure on the interfaces was un-uniform when using F type electrode, and that the distribution of the contact pressure on the interfaces improved to a certain extent when optimized electrodes were used. The multi-physics coupling analysis of heat, electricity and force on thermal process during resistance spot welding under both electrodes conditions showed that the distribution of the current density in workpieces and electrodes was un-uniform when using F type electrodes, and that when the resistance spot welding parameters was (electrode force of 2.5kN, welding current of 9kA, welding time of 250ms), the current density appeared a peak value of 3.7×10³A/mm² on the region near the edge of electrode tip, and the temperature of the aluminum/steel interface at joint center reached the highest value (973℃). The distribution of the current density in workpieces and electrodes was improved greatly by using optimized electrodes, and when the resistance spot welding parameters were (electrode force of 3.5kN, welding current of 22kA, welding time of 300ms), the current density appeared a peak value of 2.7×10³A/mm² on the region near the edge of the electrode tip, and the temperature of the aluminum/steel interface at joint center reached the highest temperature (925℃). The nugget sizes (5.9mm,9.7mm) obtained by means of after numerical simulation under the two types of electrodes were consistent with experimental results.