Interfacial characterization of resistance spot welded joint of steel and aluminum alloy

The dissimilar material resistance spot welding of galvanized high strength steel and aluminum alloy had been conducted. The welded joint exhibited a thin reaction layer composed of Fe2Al5 and Fe4Al13 phases at steel/aluminum interface. The welded joint presented a tensile shear load of 3.3 kN with an aluminum alloy nugget diameter of 5.7 mm. The interfacial failure mode was observed for the tensile shear specimen and fracture occurred at reaction layer and aluminum alloy fusion zone beside the interface. The reaction layer with compounds was the main reason for reduction of the welded joint mechanical property.

Cracking Phenomenon in Spot Welded Joints of Austenitic Stainless Steel

The spot welds nugget cracking of austenitic stainless steel at temperatures between 700°C - 1010°C was investigated. Traditionally, the cracks have been observed around the spot nugget in welded temperature. Actually, these cracks are developed due to incomplete melting and inappropriate electrode pressure, which causes an expulsion of molten metal. These cracks start to grow and cause either the interface or plug fracture according to the loading type. In this work, the micro-cracks in the weld nugget were indicated for this type of steel at elevated temperature. Cracks appear in a certain range of temperature;about 700°C - 750°C. The cracks like defect and cavitations were presented. According to the fracture mechanics point of view, these cracks reduce the mechanical strength. Therefore, these cracks have to be taken into account with a certain precaution. Moreover, considering the working temperature and reducing the element may develop ferrite particles.

Effect of tool plunge depth on the microstructure and fracture behavior of refill friction stir spot welded AZ91 magnesium alloy joints

We used refill friction stir spot welding(RFSSW)to join 2-mm-thick AZ91D-H24 magnesium alloy sheets,and we investigated in detail the effect of tool plunge depth on the microstructure and fracture behavior of the joints.A sound joint surface can be obtained using plunge depths of 2.0 and 2.5 mm.Plunge depth was found to significantly affect the height of the hook,with greater plunge depths corresponding to more severe upward bending of the hook,which compromised the tensile-shear properties of the joints.The hardness reached a minimum at the thermo-mechanically affected zone due to the precipitation phases of this zone as it dissolved into theα-matrix during the welding process.The fracture modes of RFSSW joints can be divided into three types:shear fracture,plug fracture,and shear–plug fracture.Of these,the joint with a shear–plug fracture exhibited the best tensile-shear load of 6400 N.

Effects of pin geometry on the material flow behavior of friction stir spot welded 2A12 aluminum alloy

A three-dimensional finite volume model was established by the ANSYS FLUENT software to simulate the material flow behavior during the friction stir spot welding （FSSW） process. Effects of the full-threaded pin and the reverse-threaded pin on the material flow behavior were mainly discussed. Results showed that the biggest material flow velocity appeared at the outer edge of the tool shoulder. The velocity value became smaller with the increase of the distance away from the tool surface. In general, material flows downwards along the pin thread when the full-threaded pin is used. Meanwhile, both the materials of the upper and the lower plates flow towards the lap interface along the pin thread when the reverse-threaded pin is used. The numerical simulation results were investigated by experiment, in which 2A12 aluminum alloy was used as the research object. The effective sheet thickness （EST） and stir zone （SZ） width of the joint by the reverse-threaded pin were much bigger than those by the full-threaded pin. Accordingly, cross tension failure load of the joint by the reverse-threaded pin is 23% bigger than the joint by the full-threaded pin.

Effects of nugget alloying on microstructures and properties of resistance spot welded joints of aluminum and steel

The resistance spot welding of 6063-T6 aluminum alloy and 16Mn steel was studied by nugget alloying. The results indicated that the Al-steel joint had characteristics of welding-brazing. The nugget zone consisted mainly of α-Al solid solution with dislocations and fine Mg2Si particles. The interface zone had a double-layer structure： Fe2Al5 layer at steel side and Fe4Al13 layer at Al nugget side. The nugget alloying has a significant effect on the joint properties by changing phase composition and refinement of grains. When alloy elements Cu, Zn, Ti and Ni were added, the tensile shear load of Al-steel joints reached 2 780 N, 2 910 N, 2 915 N and 2 929 N respectively, which increased by 24. 1%, 29.9%, 30. 1% and 30. 7% respectively compared with that （2 241 N） of joint without nugget alloying. Therefore, it is an effective way for improving mechanical properties of resistance spot welded Al-steel joints.

Relationship between microstructures and mechanical properties of resistance spot welded joint of spring steel

Based on mechanical properties tests and microstructures investigations, the relationship between microstructures and mechanical properties of resistance spot welded joints of spring steel is studied. The agglomeration of precipitated carbide is the sign that indicates spot welded joints structure of spring steel is fully tempered. Spot welded joints microstructure of nickelplaied spring steel is fully tempered in random tempering heat treatment process, and mechanical properties of the welded joints are advanced. This research results showed a new approach for increasing spot welded joints quality of spring steel and hardened steel.

Bonding interface morphology of keyholeless friction stir spot welded joint of AZ31B Mg alloy and DP600 galvanized steel

Because the bonding interface of dissimilar metal joint between AZ31 B Mg alloy and DP600 galvanized steel by keyholeless friction stir spot welding(KFSSW)is permanent bonding,the interface morphology cannot be directly observed.If the joint is separated by external force,the original features of bonding interface of joint will be destroyed,which has influence on the accuracy for observation and analysis of the result.In this paper,the coordinates of the key point at the interface of every cross-section at intervals of 0.2 mm were measured and connected into an outline.The outline of all interfaces makes up the three-dimensional morphologies of bonding interface between AZ31 B Mg alloy and DP600 steel by KFSSW,which was constructed by Solidworks software to restore the real mechanical bonding state of joint.Combined with the microhardness analysis of cross-section and results of in-situ tensile test,the unique bonding state and morphology of Mg and steel in the welded joint were confirmed.

Effects of Cover Sheets on the Nugget Growth and Fracture Behavior of Resistance Spot Welded Q&P980 Steel Joints

The resistance spot weldability of galvanized ultra-high-strength steels is not satisfed,the joints are prone to interfacial fracture and the weldable current range is narrow.To solve the problems,a novel method called resistance spot welding with double-sided cover sheets was introduced to weld a galvanized Q&P980 steel with the thickness of 1.2 mm.Two thin SPCC mild steel sheets were chosen as cover sheets and were placed symmetrically at both sides between the Q&P980 steels and the electrodes,then the RSW process was carried out.Compared with the traditional RSW method,the joints obtained by using the novel method achieved larger tensile shear strength and energy absorption,which increased by 26.9%and 52.6%,respectively.With increasing the welding current,the failure mode transferred from interfacial fracture to nugget pull-out fracture or base metal tearing fracture.By contrast,the joints always showed interfacial fracture without cover sheets.The improvement of the joint performance was mainly attributed to the enlargement of the nugget.With the help of fnite element simulation,it was found that the cover sheets helped increase the contact area and reduced the current density during welding,which postponed the expulsion,and a larger area could be evenly heated.The application of the novel method can be easily extended to the resistance spot welding of other ultra-high-strength steels with various thicknesses.

Investigation of failure mechanism and mechanical properties of resistance spot welded magnesium alloy joints

Resistance .spot u,e/ded magnesium alloy joints can fail in two markedly different failure modes （interfiscialfitilure and button pullout failure） under tensile shear loading conditions. For button pullottt failure, the crack first propagates along cellular dendritic structure of the nugget circumference, and then passes through heat-affected zone （HAZ） and base metal in sequence. The tensile shear load has smaller values under the interracial failure occurring in a small weld nugget as compared to the button pullout failure appearing in a large weld nugget. The tensile shear load increases with the increasing nugget diameter for expulsion free joints. However, for joints which experienced expulsion, the tensile shear load decreases in spite of nugget diameter increasing. Under the equivalent nugget diameter （5. g mm）, the tensile shear load of joints with 9 × 10^-4 g KBF4 addition was increased by around 20% as compared to that of joints without KBF4 addition.

Static Performance and Fracture Numerical Analysis of 301L/Q235B Dissimilar Steel Resistance Spot Welded Joints

This paper presents an experimental study on the static tensile test of resistance spot welding between 2.0 mm thick dissimilar 301L⁃DLT and Q235B in tensile⁃shear specimens with different welding nugget diameters.Force⁃displacement curves of the specimens were compared with simulation curves by the finite element software.The stress⁃strain distribution and fracture evolution process during the tensile process were analyzed.The hardness of the nugget was higher than that of the base material and the heat affected zone.Under static tensile load,the stress and strain in the spot welded joints increased exponentially with the increase of displacement,and the maximum stress was located at the nugget edge of the 301L plate loading side.The static tensile strength and plastic deformation of the spot welded joint failure by the nugget pulled⁃out fracture mode was better than that by the interface fracture mode.The critical nugget diameter of Q235B for the transition from nugget interfacial fracture to the pull⁃out fracture was 7.12 mm,and that of 301L was 7.81 mm,which was about 5√t.

Microstructure and mechanical property of resistance spot welded joint of aluminum alloy to high strength steel with especial electrodes

Dissimilar material joining of 6008 aluminum alloy to H220 YD galvanized high strength steel was performed by resistance spot welding with especial electrodes that were a flat tip electrode against the steel surface and a domed tip electrode upon the aluminum alloy surface. An intermetallic compound layer composed of Fe2Al5 and FeAl3 was formed at the steel/ aluminum interface in the welded joint. The thickness of the intermetallic compound layer increased with increasing welding current and welding time, and the maximum thickness being 7. 0 μm was obtained at 25 kA and 300 ms. The weld nugget diameter and tensile shear load of the welded joint had increased tendencies first with increasing welding current （ 18 -22 kA） and welding time （ 50 - 300 ms）, then changed little with further increasing welding current （ 22 - 25 kA） and welding time （300 -400 ms）. The maximum tensile shear load reached 5.4 kN at 22 kA and 300 ms. The welded joint fractured through brittle intermetallic compound layer and aluminum alloy nugget.

Microstructure and mechanical optimization of probeless friction stir spot welded joint of an Al-Li alloy

In this work, a third generation AI-Li alloy has been successfully spot welded with probeless friction stir spot welding （P-FSSW）, which is a variant of conventional friction stir welding. The Box-Behnken exper-imental design in response surface methodology （RSM） was applied to optimize the P-FSSW parameters to attain maximum tensile/shear strength of the spot joints. Results show that an optimal failure load of 7.83 kN was obtained under a dwell time of 7.2 s, rotation speed of 950 rpm and plunge rate of 30 mm/rain. Sufficient dwell time is essential for heat conduction, material flow and expansion of the stir zone to form a sound joint. Two fracture modes were observed, which were significantly affected by hook defect. In addition to mechanical testing, electron backscattering diffraction （EBSD） and differential scanning calorimetry （DSC） were used for microstructure evolution and property analysis. The precipitation of GP zone and AI3Li as well as the ultrafine grains were responsible for the high microhardness in the stir zone.

Microstructure and mechanical properties of Mg-to-Al dissimilar welded joints with an Ag interlayer using ultrasonic spot welding

Lightweight ZEK100-0 Mg alloy and A16022-T43 Al alloy with an Ag interlayer were joined via ultrasonic spot welding(USW),focusing on the microstructural change and tensile lap shear strength of the welded joints in relation to welding energy.Mg/Al interface was superseded by Mg/Ag and Al/Ag interfaces,and unfavorable Mg门A-intermetallic compound was eliminated.Ag foil was observed to be intact in the nugget center,while it was broken or dissolved at the nugget edge at high welding energy levels.The diffusion layer at the Mg/Ag interface consisted of two distinctive sub-layers:Mg3Ag intermetallic compound adjoining Ag foil,and Mg3Ag-l-Mg eutectic structure adjacent to Mg.Only a thin diffusion layer consisting mainly of Ag3Al occurred al lhe Al/Ag interface.The tensile lap shear strength first increased,reached its peak value,and then decreased with increasing welding energy.The shear strength achieved in the present study was〜31%higher than that of the joint without interlayer.Interfacial failure occurred at all energy levels,with Ag foil particles or fragments being stuck on both Mg and Al sides due to its intense interaction with Mg and Al via accelerated diffusion during USW.The results obtained pave the way for the challenging dissimilar welding between Mg and Al alloys.

Influencing mechanism of pre-existing nanoscale Al5Fe2 phase on Mg-Fe interface in friction stir spot welded Al-free ZK60-Q235 joint

Al-free ZK60 magnesium (Mg) alloy sheet was selected as substrate material of Mg-steel pinless friction stir spot welding (FSSW), avoiding the effect of the Al element in the substrate on the alloying reaction of Mg-iron (Fe) interface. The sound FSSW joint of ZK60 Mg alloy and Q235 steel with a hot-dipped aluminum (Al)-containing zinc (Zn) coating was successfully realized. The detailed microstructural examinations proved that Al5Fe2 phase at the Mg-Fe interface came from the pre-existing Al5Fe2 phase in the coating and acted as the transition layer for promoting the metallurgical bonding of Mg and Fe. The interfaces with well-matched lattice sites among Fe, Al5Fe2 and Mg were formed during FSSW. A low energy interface with good match of lattice sites ((002)Al5Fe2//(110)Fe, [110]Al5Fe2//[113]Fe) between Al5Fe2 and Fe was identified. For the interface between Al5Fe2 and Mg, an orientation relationship of (622)Al5Fe2//(3112)Mgand[158]Al5Fe2//[2423]Mg was observed. The tensile-shear load of the ZK60-steel joint could reach 4.6 kN. Moreover, the joint fracture occurred at the interface between the Al5Fe2 layer and the Mg alloy substrate, suggesting the brittle fracture characteristic.

Mechanical Behavior and Microstructure of Ultrasonic-Spot-Welded Al/Cu Dissimilar Joints with Zn Interlayer

The effect of Zn interlayer on the microstructural evolution and mechanical behavior of dissimilar ultrasonic-spot-welded Al/Cu joints was investigated.The tensile lap shear strength in relation to welding energy was analyzed.The experimental results show that two intermetallic compounds,Cu5Zn8 and Al2Cu,were generated at the interface of the ultrasonic-spotwelded Al/Cu joint with a Zn interlayer.The primary joining mechanisms of the joint included the intermetallic compound bonding and metallic bonding caused by solid shear plastic deformation.Meanwhile,with increasing welding energy,the plastic deformation of the material became more substantial.With increasing welding energy,the tensile lap shear strength of the joints first increased and then decreased for the ultrasonic-spot-welded Al/Cu joints with and without Zn interlayers.Under the energy input of 700 J,the bearing load capacity of the ultrasonic-spot-welded Al/Cu joints with a Zn interlayer improved signifi cantly due to the observed intermetallic compound(Cu5Zn8).

Mechanical Properties of Resistance Spot Welded Components of High Strength Austenitic Stainless Steel

Resistance multi-spot welding （MSW） in column, triangle and tetragonal symmetry arrangements was prepared using cold-rolled 301L high-strength sheets, and their static and fatigue properties were in- vestigated. The effects of spot welds on the fracture strengths and fatigue limits were discussed. The results show that the static strengths can be estimated using an inherent linear relationship formula of the load versus the welding area. It was based on the 28%-33% volume fraction of α′ martensite induced at the interfacial spot weld fracture because of the failure deformation. The fatigue limits of the MSW nonlin- early increase with the number of spot welds. The arrangement of spot welds in the MSW significantly affects the average fatigue limit of each spot weld; its 26% maximum reduction occurred in the triangle, and the interaction stress between spot welds led to its 18% reduction in the tetragonal symmetry. The calculated fatigue stress of all MSW loadings with their mean fatigue limits was 230-270 MPa.

Local melting mechanism and its effects on mechanical properties of friction spot welded joint for Al-Zn-Mg-Cu alloy

Local melting and the eutectic film and liquation crack formation mechanisms during friction spot weld- ing （FSpW） of Al-Zn-Mg-Cu alloy were studied by both experiment and finite element simulation. Their effects on mechanical properties of the joint were examined. When the welding heat input was high, the peak temperature in the stir zone was higher than the incipient melting temperature of the Al-Zn-Mg-Cu alloy. This resulted in local melting along the grain boundaries in this zone. In the retreating stage of the welding process, the formed liquid phase was driven by the flowing plastic material and redistributed as a ＂U-shaped＂ line in the stir zone. In the following cooling stage, this liquid phase transformed into eutectic films and liquation cracks. As a result, a new characteristic of＂U＂ line that consisted of eutectic films and liquation cracks is formed in the FSpWjoin. This ＂U＂ line was located in the high stress region when the FSpW joint was loaded, thus it was adverse to the mechanical properties of the FSpW joint. During tensile shear tests, the ＂U＂ line became a preferred crack propagation path, resulting in the occurrence of brittle fracture.

Effect of refilling time on microstructure and mechanical properties of friction spot welded LY12 aluminum alloy

Friction spot welding （FSpW） was successfully used to produce joints of LY12 aluminum alloy. The effects of refilling time on microstructure and mechanical properties of FSpW joints were systematically studied. Results show that the cross-section of FSpW joint presents a basin-like morphology. A white bonding ligament exists in the center of the joint. The stir zone can be clarified into sleeve affected zone and pin affected zone based on different grain sizes. With increasing the refilling time from 2. 0 s to 3.5 s, grains in the stir zone become coarser, microhardness of the joint decreases and tensile shear failure load of the joint firstly increases and then decreases. The maximum tensile shear failure load of 8 130 N is attained when the refilling time is 3.0 s. Shear-plug fracture mode and shear fracture mode can be observed in the tensile shear tests. The maximum hardness of 169. 7 HV is attained in the joint center when the refilling time is 2. 0 s.

Effect of process parameters on SPRC35 spot welded automotive panels

This paper presents an experimental study on the resistance spot weldability of SPRC35（steel plate re-phosphorised cold rolled） sheets.The sheets were joined by using resistance spot welding as lap joint.The weld nugget diameter and tensile shear force were investigated.Tensile-shear tests were applied to the welded specimens to understand the tensile shear force that the joint can withstand.Empirical relationships were developed for both nugget diameter and tensile shear force.Use of automatic mode, will increase the dependence on the use of equations to predict the nugget diameter.The developed models have been checked for their adequacy and significance by the F test and t test respectively.The results obtained from the empirical relationships have been optimized and also tested using conformity test runs.

Design of Fully Automatic Specification Selection System for Resistance Welding Equipment

A system for fully automatic selection of welding specifications in resistance welding equipment has been developed to address the problem of workers frequently choosing the wrong specifications during manual welding of multiple parts on a single machine in automobile factories. The system incorporates an automatic recognition system for different workpiece materials using the added machine fixture,visual detection system for nuts and bolts,and secondary graphical confirmation to ensure the correctness of specification calling. This system achieves reliable,fully automatic selection of welding specifications in resistance welding equipment and has shown significant effects in improving welding quality for massproduced workpieces,while solving the problem of specification calling errors that can occur with traditional methods involving process charts and code adjustments. This system is particularly suitable for promoting applications in manual welding of multiple parts on a single machine in automobile factories,ensuring correct specification calling and welding quality.