S.D. Bhole

Microstructure and mechanical properties of dissimilar welded Mg-Al joints by ultrasonic spot welding technique

Abstract Dissimilar spot welds of magnesium–aluminium alloy were produced via a solid state welding process, i.e. ultrasonic spot welding, and a sound joint was obtained under most of the welding conditions. It was observed that a layer of intermetallic compound (IMC) consisting of Al12M17 formed at the weld centre where the hardness became higher. The lap shear strength and failure energy of the welds first increased and then decreased with increasing welding energy, with the maximum lap shear strength and failure energy occurring at ∼1250 J. This was a consequence of the competition between the increasing diffusion bonding arising from higher temperatures and the deterioration effect of the intermetallic layer of increasing thicknesses. Failure predominantly occurred in between the aluminium alloy and the intermetallic layer, which normally stayed at the magnesium side or from the cracks of the IMCs in the reaction layer.

Expulsion monitoring in spot welded advanced high strength automotive steels

Abstract Although there have been a number of investigations on monitoring and controlling the resistance spot welding (RSW) of low carbon galvanised steels, those of advanced high strength steels (AHSS) are limited. A data acquisition system was designed for monitoring weld expulsion via the measurement of voltage, current, electrode force and displacement and the calculation of resistance. The dynamic resistance, electrode force and tip displacement were characterised and correlated with the phenomenon of expulsion during RSW of dual phase (DP) steel using an ac welder. Two control strategies for DP600 spot welding were proposed on the basis of the rate of change in the dynamic resistance and the electrode force.

Improving weld strength of magnesium to aluminium dissimilar joints via tin interlayer during ultrasonic spot welding

Abstract Welding of magnesium to aluminium alloys is enormously challenging due to the formation of brittle Al12Mg17 intermetallic compounds (IMCs). This study was aimed at improving the strength of dissimilar joints of AZ31B-H24 magnesium alloy to 5754-O aluminium alloy by using a tin interlayer inserted in between the faying surfaces during ultrasonic spot welding. The addition of tin interlayer was observed to successfully eliminate the brittle Al12Mg17 IMCs, which were replaced by a layer of composite-like tin and Mg2Sn structure. Failure during the tensile lap shear tests occurred through the interior of the blended interlayer as revealed by X-ray diffraction and SEM observations. As a result, the addition of a tin interlayer resulted in a significant improvement in both joint strength and failure energy of magnesium to aluminium dissimilar joints and also led to an energy saving because the optimal welding energy required to achieve the highest strength decreased from ∼1250 to ∼1000 J.

Welding behaviour, microstructure and mechanical properties of dissimilar resistance spot welds between galvannealed HSLA350 and DP600 steels

Abstract Resistance spot welding experiments were conducted on dissimilar material combination of HSLA350/DP600 steels. The welds were characterised using optical and scanning electron microscopy. The fusion zone of the dissimilar material spot weld was predominantly martensitic with some bainite. Mechanical properties were also determined by tensile shear, cross tension and fatigue tests. The performance of dissimilar material spot weld was different from that of the similar ones in each of the HSLA350 and DP600 steels and exhibited different heat affected zone hardness. The DP600 weld properties played a dominating role in the microstructure and tensile properties of the dissimilar material spot welds. However, the fatigue performance of the dissimilar welds was similar to that of the HSLA350 welds. Fatigue tests on the dissimilar material spot welds showed that the 5·5 mm diameter nugget exhibited higher fatigue strength than the 7·5 mm diameter nugget.

FORMING BEHAVIOUR OF TAILOR (LASER) WELDED BLANKS OF AUTOMOTIVE STEEL SHEET

Abstract The drive towards weight reduction in the automotive industry has led to the use of tailor welded blanks (TWB) for structural applications. This work is aimed at evaluating the forming behaviour of interstitial free and low carbon steel TWBs with different thickness combinations and compositions. Forming tests were carried out to determine the forming limit diagrams (FLD) of the TWBs and compared with the theoretical FLDs of the individual steel sheets. The results showed that the FLD of the TWBs lies in-between those of the individually formed steel sheets that comprise the TWB. A semiempirical relation based on the mean values of the strain hardening exponents (n values) and of the thickness of the base metals was developed to calculate the forming limit value in plane strain (FLD0) of the TWBs. The calculated FLD0s were found to be in good agreement with the experimentally determined values. La tendance vers la réduction de poids de l’industrie de l’automobile a conduit à l’utilisation de découpes soudées sur mesure (TWB) pour applications structurelles. Ce travail vise à évaluer le comportement de formage de TWBs en acier sans élément interstitiel et à faible carbone avec différentes combinaisons d’épaisseur et de compositions. On a effectué des épreuves de formage pour déterminer les diagrammes des déformations-limites (FLD) des TWBs et on les a comparés avec les FLDs théoriques des tôles d’acier individuelles. Les résultats ont montré que le FLD des TWDs se situe entre ceux des tôles d’acier individuellement formées qui comprennent le TWB. On a développé une relation semiempirique basée sur les valeurs moyennes des exposants d’écrouissage (valeurs n) et de l’épaisseur des métaux de base, pour calculer la valeur des déformations-limites en contrainte plane (FLD0) des TWBs. On a trouvé que les FLD0s calculés étaient en acccord avec les valeurs déterminées expérimentalement.

Resistance Spot Welding Characteristics and Mechanical Properties of Galvannealed HSLA 350 Steel

Abstract Galvannealed high strength low alloy steel 350 (GA HSLA 350), because of its high strength-to-weight ratio, is particularly attractive in transportation applications. The GA HSLA 350 steel was spot welded at different welding currents to identify a suitable welding current giving good spot welds. Failure loads in the tensile shear increased with increasing welding current. There was a concomitant increase in the nugget diameter and the failure mode changed from interfacial fracture to button pull-out failure. Cross tension and microhardness tests were also carried out. The fusion zone microstructure was martensitic with a microhardness that was twice that of the base metal.

Fracture behavior in a pressure vessel steel weld

The fracture behavior of pressure vessel steel weld, with notch pre-crack tip completely located in the heat-affected zone (HAZ) is investigated. It is shown that the ductile fracture occurs by the redirection of the crack propagation from the HAZ to the weld metal. Analysis by optical microcopy, SEM and energy-dispersive X-ray spectrometry has revealed that the improvement in the toughness, and thus the higher resistance to crack propagation in the HAZ, is due to the presence of a large proportion of fine acicular ferrite. The size and distribution of inclusions seems to influence the formation of fine acicular ferrite. The effect of the hardness distribution on the fracture behavior of the weld is also analyzed.

Dissimilar ultrasonic spot welding of Mg-Al and Mg-high strength low alloy steel

Sound dissimilar lap joints were achieved via ultrasonic spot welding (USW), which is a solid-state joining technique. The addition of Sn interlayer during USW effectively blocked the formation of brittle al 12 Mg 17 intermetallic compound in the Mg-Al dissimilar joints without interlayer, and led to the presence of a distinctive composite-like Sn and Mg 2Sn eutectic structure in both Mg-Al and Mg-high strength low alloy (HSLA) steel joints. The lap shear strength of both types of dissimilar joints with a Sn interlayer was significantly higher than that of the corresponding dissimilar joints without interlayer. Failure during the tensile lap shear tests occurred mainly in the mode of cohesive failure in the Mg-Al dissimilar joints and in the mode of partial cohesive failure and partial nugget pull-out in the Mg-HSLA steel dissimilar joints.

Tensile and Fatigue Properties of Single and Multiple Dissimilar Welded Joints of DP980 and HSLA

The present study focused on single and multiple dissimilar joints between DP980 and high-strength low-alloy (HSLA) galvanized steels. The tensile properties of the dissimilar joint between the strong DP980 and the relatively soft HSLA reflected only the properties of HSLA with plastic deformation, and final fracture took place entirely in HSLA. The fatigue properties of the dissimilar joints were more intriguing, with the strong DP980 outperforming at high stress amplitude and the ductile HSLA outperforming at low stress amplitude. For different load amplitudes, fatigue failure occurred in different materials and at different locations. The fatigue strength of DP980 was more negatively impaired by weld defects than that of HSLA.