•Al-Zn-Mg alloy suffers severer corrosion with T4 condition than with T5 condition.•The corrosion resistance of various zones after welding is different.•The heat affected zone can be further divided into two zones.•The difference of corrosion potential results in severe galvanic corrosion.The 7N01-T5 alloy (AT5) and 7N01-T4 alloy (AT4) were butt welded by metal inert gas (MIG) welding with ER5356 filler wire, and the mechanical properties and corrosion resistance of welded joint were investigated. The mechanical properties of welded joint are inferior to those of base metals (BMs). In the welded joint, the difference in composition of Zn, Cu and the amount of precipitates between AT5 side and AT4 side induces the variance of corrosion potential, resulting in the appearance of galvanic corrosion, then the highest corrosion susceptibility occurred on the AT4 side of the region adjacent to the weld zone.Download high-res image (239KB)Download full-size image

The corrosion resistance of Al–Zn–Mg alloy subjected to different times in flame rectification was investigated based on the exfoliation corrosion test. The results indicate that the flame rectification deteriorates the exfoliation corrosion resistance of Al–Zn–Mg alloy. The corrosion resistance of Al–Zn–Mg alloy is ranked in the following order: base metal>two times>three times>one time of flame rectification. The exfoliation corrosion behavior was discussed based on the transformation of precipitates at grain boundaries and matrix. With increasing the number of times in flame rectification, the precipitate-free zones disappeared and the precipitates experienced dissolution and re-precipitation. The sample was seriously corroded after one time of flame rectification, because the precipitates at grain boundaries are more continuous than those in other samples.

•Mechanical property of AlZnMg alloy after flame rectification was studied.•Microstructure evolution of alloy after flame rectification was investigated.•Property of alloy reached optimal after flame rectification at 350 °C for two times.The mechanical properties of AlZnMg alloy after flame rectification at 300 °C and 350 °C for different numbers of times were investigated, and the mechanism of precipitation transformation in AlZnMg alloy was interpreted. The results show that the matrix precipitates (MPs) coarsened or diminished with increasing the number of times in flame rectification at 300 °C, and meanwhile the grain boundary precipitates (GBPs) dissolved. However, the matrix precipitates largely dissolved into the matrix and the grain boundary precipitates changed complicatedly with increasing the number of times in flame rectification at 350 °C. The difference of microstructure evolution after flame rectification at two temperatures led to significant differences in mechanical properties of AlZnMg alloy. To conclude, the mechanical properties of AlZnMg alloy achieved optimal after flame rectification at 350 °C for two times.

Lap joining of 1-mm-thick Novelist AC 170 PX aluminum alloy to 1.2-mm-thick ST06 Z galvanized steel sheets for automotive applications was conducted by cold metal transfer advanced welding process with ER4043 and ER4047 filler wires. Under the optimized welding parameters with ER4043 filler wire, the tensile shear strength of joint was 189 MPa, reaching 89% of the aluminum alloy base metal. Microstructure and elemental distribution were characterized by optical metalloscope and electron probe microanalysis. The lap joints with ER4043 filler wire had smaller wetting angle and longer bonded line length with better wettability than with ER4047 filler wire during welding with same parameters. The needle-like Al-Fe-Si intermetallic compounds (IMCs) were spalled into the weld and brought negative effect to the tensile strength of joints. With increasing welding current, the needle-like IMCs grew longer and spread further into the weld, which would deteriorate the tensile shear strength.

Vacuum brazing of TiAl alloy to 40Cr steel sheets was conducted with newly developed CuTiNiZrV amorphous foils. It was found that a diffusion layer, filler metal and reaction layer existed in the brazed seam. The diffusion layer in the joint brazed with Cu43.75Ti37.5Ni6.25Zr6.25V6.25 (at.%) foil was flat and thin, containing Ti19Al6 and Ti2Cu intermetallic compounds; however, the diffusion layer brazed with Cu37.5Ti25Ni12.5Zr12.5V12.5 foil was uneven with bulges, consisting of essentially Ti-based solute solution. The foil with 12.5 at.% V showed inferior spreadability compared to that with 6.25 at.% V at brazing temperature. However, fracture happened along the diffusion layer with 6.25 at.% V foil due to the formation of brittle intermetallic phases, but the joints brazed with 12.5 at.% V foil failed through the TiAl substrate. These results show that designing amorphous alloy with less Ti and more V for brazing TiAl alloy to steel is appropriate.

Dissimilar metal joining of Ti–6Al–4V (TC4) titanium alloy to as-rolled 40Cr steel rods was conducted with friction welding, and the effect of post-weld heat treatment (PWHT) on the microstructure and mechanical properties of the resultant joints was investigated. The average tensile strength of the as-welded joints reached 766 MPa and failure occurred in 40Cr steel base metal. However, after PWHT at 600 °C for 0.5, 1, 2 and 3 h, the tensile strength of the joints decreased and fracture happened through the interface with quasi-cleavage features. The bending angle of specimens was improved from 9.6° in as-welded state to 32.5° after PWHT for 2 h. The tensile strength of the joint was enhanced by martensitic transformation near the interface in as-welded state. Sorbite formed near the interface in PWHT state and improved the bending ductility of the joint. TiC brittle phase formed at the interface after PWHT for 0.5 h and deteriorated the tensile strength and bending ductility of the joint. After PWHT for 2 h, no TiC phase was detected at the interface. The microhardness on the interface in as-welded state was higher than that after PWHT, indicating that the decrease of microhardness around the interface could be accompanied by degradation of tensile strength but improvement of bending ductility of the joints.

Gas tungsten arc welding of HSLA steel was conducted with different welding heat inputs, and the influences of welding heat input on the microstructure, Vickers hardness, and impact toughness of heat-affected zone (HAZ) in prepared joints were systematically investigated. The microstructure in HAZ with low welding heat input mainly consisted of martensite, and the microhardness of coarse grain HAZ was measured higher than that of fine grain zone. The results show that increasing the welding heat input could suppress the formation of martensite and reduce the microhardness of HAZ. However, the impact toughness of HAZ was not monotonously improved with the increase of welding heat input. It is deemed that the coarsened grain, the formation of upper bainite, and the non-uniform distribution of carbides and inclusions in HAZ could degrade the impact toughness. Tests demonstrate that the optimum comprehensive properties of HAZ were obtained when the welding heat input was 0.67 kJ/mm.

Dissimilar metal vacuum brazing between TiAl alloy and 40Cr steel at 900 °C for 5 min, 10 min and 15 min was conducted with Ti-based alloy foil as the filler. Five distinct regions were detected in the brazed seam from each joint during microstructure examination. The hardness in the brazed seam was measured much higher than those in the two substrates, and the hardness in the filler metal layer was higher than that in the reaction layer. However, brazing for longer time can reduce the hardness of the brazed seam. The average shear strength of the joint was 26 MPa when the joint was brazed for 5 min, and it increased to 32 MPa when brazed for 10 min and 15 min. The specimen brazed for 5 min fractured through the interface between 40Cr steel base metal and filler metal layer, but the specimen brazed for 10 min and 15 min fractured through the filler metal layer in the brazed seam. It was found that the compositions of the filler material and sufficient interdiffusion between the filler and substrates are important to improve the joint strength, and the mechanical properties of the filler metal also control the strength of the resultant joint.

Butt joining of 5A02 aluminum alloy to 304 stainless steel sheets was conducted using gas tungsten arc welding process with Al-12%Si (wt.%, the same below) and Zn-15%Al flux-cored filler wires. The effects of gap width and groove in steel side on the microstructure and tensile strength of the resultant joints were investigated. For the joint made with 0 mm-wide gap and without groove in steel side, severe incomplete brazing zone occurred along the steel side and bottom surfaces, and consequently seriously deteriorated the joint strength. However, presetting 1.5 mm-wide gap or with groove in steel side could promote the wetting of molten filler metal on the faying surfaces, and then significantly enhance the resultant joint strength. Moreover, post-weld heat treatment could further improve the tensile strength of the joints. During tensile testing, the specimens from the joints made with Al-12%Si flux-cored filler wire fractured through the weld or interfacial layer, but those from the heat-treated joints made with Zn-15%Al flux-cored filler wire fractured in the aluminum base metal.

Aluminum alloy sheets were lap joined to galvanized steel sheets by gas tungsten arc welding (GTAW) with Al–5% Si, Al–12% Si, Al–6% Cu, Al–10% Si–4% Cu and Zn–15% Al filler wires. Different amounts of Si, Cu and Zn were introduced into the weld through different filler wires. The effects of alloying elements on the microstructure in the weld and tensile strength of the resultant joint were investigated. It was found that the thickness of the intermetallic compound (IMC) layer decreased and the tensile strength of the joint increased with the increase of Si content in the weld. The thickness of the IMC layer could be controlled as thin as about 2 μm and the tensile strength of the dissimilar metal joint reached 136 MPa with Al–12% Si filler wire. Al–Si–Cu filler wire could result in thinner interfacial layer than Al–Cu filler wire, and fracture during tensile testing occurred in the weld for the former filler wire but through the intermetallic compound layer for the latter one. A Zn-rich phase formed in the weld made with Zn–15% Al filler wire. Moreover, the Zn–Al filler wire also generated thick interfacial layer containing a great amount of intermetallic compounds and coarse dendrites in the weld, which led to a weak joint.Highlights► Aluminum alloy sheets were gas tungsten arc welded to galvanized steel sheets. ► Al–Si, Al–Cu, Al–Si–Cu and Zn–Al alloys were used as the filler wires during welding. ► The effect of Si, Cu and Zn on the microstructure and tensile strength was analyzed. ► Al–Si filler wire resulted in thinnest interfacial layer and strongest Al-steel joint. ► Zn–Al filler wire generated thick interfacial layer and coarse dendrites in the weld.

Gas-tungsten arc welding (GTAW) with controlled heat input could avoid the formation of massive brittle Al–Fe intermetallics during solidification in dissimilar-metal joining of aluminum to steel. In the first part of the present study an Al–Mg alloy was joined to galvanized steel by GTAW with an Al–Si filler metal. The weld solidification microstructure was explained based on the Al–Mg–Si phase diagram. Ultrasonic vibration during GTAW increased the joint strength by 27%. Grain refinement, decreased Fe–Al intermetallics, and increased microhardness in both the heat-affected zone and the weld were observed, thus explaining the improved joint strength. In the second part, an Al–Mg alloy was joined to 304 stainless steel using a Zn-15Al filler metal with a noncorrosive flux core. The weld solidification microstructure was explained based on the Al–Zn–Mg phase diagram. Postweld heat treatment (PWHT) at 280 °C for 30 min doubled or even tripled the joint strength depending on the Al-sheet thickness. Dissolution of coarse Zn-rich particles at the weld/steel interface was observed after PWHT, thus eliminating these weak particles from the interface. The formation of Zn-rich particles near the interface and their dissolution by PWHT were explained based on the Al–Fe–Zn phase diagram.Highlights► Al alloy was joined to galvanized steel with assistance of ultrasonic vibration. ► Ultrasonic vibration during GTAW increased the joint strength by 27%. ► Al alloy was joined to stainless steel with Zn-15Al flux-cored filler wire. ► Postweld heat treatment doubled or even tripled the joint strength. ► Weld solidification microstructure was explained based on ternary phase diagrams.