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This study is carried out to investigate the laser spot welds (LSWs) of advanced high strength steel sheets of 1·0 mm thickness for application to the automotive industry. Mechanical properties and failure behaviour of LSWs for transformation induced plasticity steel sheets (TRIP800) subjected to monotonic coach–peel loading are investigated by experimental and finite element (FE) simulation methods. Microstucture of LSW and fracture surfaces of the welded specimens were examined by scanning electron microscopy (SEM) to describe the microstructural features and to clarify the crack initiation mechanism respectively. Based on simulation solutions equivalent plastic strain was obtained to describe local deformation of LSW joints. Experimental results revealed a 'plug type' of failure mode of the circular LSW joints under 'peel–coach' loading condition. This type of ductile failure is the most common failure mode for spot welds used in the automotive industry. Numerical simulation of damage process w...

Defect-free joints of TiAl/Ti6242 have been produced by TLP bonding using Ti, Cu foils as insert metals. The influence of the bonding parameters such as bonding pressure, bonding temperature, dwell time and the stacking sequence of the parent materials on the microstructure of the joined zone has been investigated. The results showed that bonding pressure played an important role even though there was a liquid phase during bonding. The stacking sequence of the parent materials also had a major effect on joint formation. Elimination of defects at the interface of the TiAl/ interlayer in order to obtain sound joints was a major problem. Similar interface structures were observed in all cases except that the thickness of the joined zone was different. The results of the study revealed that the surface oxide layer on TiAl had a major effect on the interface diffusion process. Based on the interface diffusion and interface structure, the joint formation process has been investigated and a mechanism for...

TiAl intermetallic alloys have been joined using the diffusion brazing technique with Ti insert foil metal combined with Cu, Ni or Fe foils. Defect-free joints can be fabricated successfully, no matter which kind of foil combination is used as insert metal. The interface structures of the joints were also investigated and results showed various interface structures in the joined zones. A mechanism for the development of the interface structure is proposed. It is suggested that the formation process of the liquid phase was responsible for the resulting interface structures. Post-bond heat treatment (PBHT) had no obvious effect on the interface structure of the joints because of the low diffusion coefficient of Cu, Ni and Fe in the parent material. Microhardness results showed a slight decrease in the joint area as a result of PBHT.

Laser beam welding is currently used in the welding of steels, aluminium alloys, thin sheets, and dissimilar materials. This high power density welding process has unique advantages of cost effectiveness, deep penetration, narrow bead and heat affected zone (HAZ) widths, and low distortion compared to other conventional welding processes. However the metallurgical and mechanical properties of laser welds and the response of conventional materials to this new process are not yet fully established. The welding process may lead to drastic changes in the microstructure with accompanying effects on the mechanical properties and hence, on the performance of the joint. The thermal cycles associated with laser beam welding are generally much faster than those involved in the conventional are welding processes. This leads to the formation of a rather small weld zone that exhibits locally a high hardness in the case of C-Mn structural steels owing to the formation of martensite. It is currently difficult to determine the tensile properties (full stress-strain curves) of the laser welded joint area owing to the small size (similar to 2-3 mm) of the fusion zone. Complete information on the tensile and fracture toughness properties of the fusion zone is essential for prequalification and complete understanding of the joint performance in service as well as for conducting a defect assessment procedure on such welded joints. Therefore, an experimental investigation into the mechanical properties of laser welded joints was carried out to establish a testing procedure using flat microtensile specimens (0.5 mm in thickness, 2 mm in width) for determination of the tensile properties of the weld metal and HAZ of the laser beam welds. Three similar joints, namely St 37-St 37, St 52-St 52, and austenitic-austenitic, and two dissimilar ferritic-austenitic joints were produced by CO2 laser using 6 mm thickness plates. The mechanical properties have been examined by microhardness survey and testing of conventional transverse tensile, round tensile, and flat microtensile specimens. The results for the microtensile specimens were compared with those for standard round tensile specimens and this clearly showed the suitability of the microtensile specimen technique for such joints.