When the weldability of high strength steels is analyzed, it is the softening in the heat-affected zone (HAZ) that is mostly investigated, and the reduction of toughness properties is generally less considered. The outstanding toughness properties of quenched and tempered high strength steels cannot be adequately preserved during the welding due to the unfavorable microstructural changes in the HAZ. Relevant technological variants (t_8/5 = 2.5–100 s) for arc welding technologies were applied during the HAZ simulation of S960QL steel (EN 10025-6) in a Gleeble 3500 physical simulator, and the effect of cooling time on the critical HAZ areas of single and multipass welded joints was analyzed. Thermal cycles were determined according to the Rykalin 3D model. The properties of the selected coarse-grained (CGHAZ), intercritical (ICHAZ) and intercritically reheated coarse-grained (ICCGHAZ) zones were investigated by scanning electron microscope, macro and micro hardness tests and instrumented Charpy V-notch pendulum impact tests. The examined HAZ subzones indicated higher sensitivity to the welding heat input compared to conventional structural steels. Due to the observed brittle behavior of all subzones in the whole t_8/5 range, the possible lowest welding heat input should be applied in order to minimize the volume of HAZ that does not put fulfillment of the allowed maximal (450 HV10) hardness at risk and does not lead to the formation of cold cracks.

Among the available high-strength steels, there is growing demand for dual phase (DP) steels for wide application in the automotive industry owing to their good combination of high strength, ductility and formability. Also, the use of innovative welding technologies like laser beam welding (LBW) has growing importance in the field of high-strength steel because of its excellence in providing high-quality welds, high welding speed, high power density, low heat input, a narrow heat-affected zone and low heat distortions as compared to the conventional gas metal arc welding process. However, the hardening and softening in the heat-affected zone is a major issue when welding high-strength steel, i.e. DP steel grades, greatly affecting the strength, formability and plasticity of the whole-welded joint and thus affecting service performance and reliability. Based on preliminary experiments, the optimal welding condition was a nominal laser power of 1.0 kW and a welding speed of 8 mm/s. The aim of this work is to analyse and compare the weld and heat-affected zone characteristics, microstructure and mechanical properties of DP steels with 1-mm thick butt joints of DP800 and DP1200 high-strength steel (HSS) by diode laser beam welding. The effects of post-weld heat treatment (PWHT) on the strengthening of the laser-welded joints were evaluated by microstructural examinations under optical microscope and scanning electron microscope, and mechanical properties were examined by microhardness test, three-point bending tests and tensile tests.

With continuous improvements, structural steels are available in even higher strength grades above 1000 MPa yield strength. As the great majority of these steels are used in welded structures, their weldability needs to be taken into account. Several factors can cause difficulties during welding of these steels, but in this paper the softening behavior and the toughness characteristics of the heat-affected zone (HAZ) are examined. As the critical parts of the HAZ in a real welded joint are relatively small, their investigating ability is limited. However, the physical simulation provides a way of evaluating specimens made from a given material to produce the specified HAZ areas in a suitable size range for subsequent testing. In this research work, three different strength categories of high-strength structural steels (with yield strength of 960 MPa, 1100 MPa, and 1300 MPa) are investigated by physical simulation. In the case of different technological variants of gas metal arc welding (GMAW) process, the effect of the cooling time t_8/5 is investigated in different HAZ subzones considered to be critical. The thermal cycles were determined according to the Rykalin 3D model. The investigated cooling times were t_8/5 = 5 s, 15 s, and 30 s. The properties of the selected coarse-grained, intercritical and intercritically reheated coarse-grained zones are analyzed by laser scanning microscope, scanning electron microscope, hardness test, and instrumented Charpy V-notch impact toughness test. Furthermore, additional investigation like JMatPro calculations, electron backscatter diffraction measurements, and prior austenite grain size calculation were carried out. As a result of the tests, the investigated heat-affected subzones indicated higher sensitivity to the welding heat input compared to conventional structural steels. Overall, the results of the tests show that the application of shorter t_8/5 cooling time can be beneficial for the investigated high-strength steel grades, since significant toughness reduction and the risk of softening occur in the whole cooling time range.

The aluminium alloy studied in this paper is the heat treatable AA7075 which has zinc as the main alloying element. 7xxx aluminium alloys have the best strength performance among all commercial series and AA7075 has tensile strength above 500 MPa. The outstanding strength properties open the possibility to use this alloy in automotive industry as a possible alternative material for car body elements instead of steel. However, their limited formability properties mean obstacle when a complicated shape car body panel is formed, since the elongation is limited only between 5 and 11%. In order to successfully form these sheets into the demanded geometry, hot forming should be used, and thus the body panels should be formed when the AA7075 is in a solution heat treated condition, such as using the HFQ® process. Then, after assembling the body elements, when resistance spot welding is among the most common joining methods, the AA7075 body parts should get the artificial ageing. It means that welding is followed by artificial ageing as a kind of post weld heat treating which can partially compensate the softening of this alloy during welding. Softening is considered among the most challenging weldability issues similarly to the high melting point oxide layer at the surface which also reduces the load bearing capacity of RSW joints. During the performed experimental program, three routes were investigated. The RSW experiments are performed on 1-mm-thick AA7075 sheets. The spot-welded joints were examined by macroscopic testing, tensile shear and hardness tests.

In recent years, ultra-high-strength structural (UHSS) steel in quenched and tempered (Q+T) conditions, for example, S960QL has been found in wider application areas such as structures, cranes, and trucks due to its extraordinary material properties and acceptable weldability. The motivation of the study is to investigate the unique capabilities of electron beam welding (EBW) compared to conventional gas metal arc welding (GMAW) for a deep, narrow weld with a small heat-affected zone (HAZ) and minimum thermal distortion of the welded joint without significantly affecting the mechanical properties. In this study, S960QL base material (BM) specimens with a thickness of 15 mm were butt-welded without filler material at a welding speed of 10 mm/s using the high-vacuum (2 × 10⁻⁴ mbar) EBW process. Microstructural characteristics were analyzed using an optical microscope (OM), a scanning electron microscope (SEM), fractography, and an electron backscatter diffraction (EBSD) analysis. The macro hardness, tensile strength, and instrumented Charpy-V impact test were performed to evaluate the mechanical properties. Further, the results of these tests of the EBW joints were compared with the GMAW joints of the same steel grade and thickness. Higher hardness is observed in the fusion zone (FZ) and the HAZ compared to the BM but under the limit of qualifying the hardness value (450 HV10) of Q+T steels according to the ISO 15614-11 specifications. The tensile strength of the EBW-welded joint (1044 MPa) reached the level of the BM as the specimens fractured in the BM. The FZ microstructure consists of fine dendritic martensite and the HAZ predominantly consists of martensite. Instrumented impact testing was performed on Charpy-V specimens at −40 °C, which showed the brittle behavior of both the FZ and HAZ but to a significantly lower extent compared to GMAW. The measured average impact toughness of the BM is 162 J and the average impact toughness value of the HAZ and FZ are 45 ± 11 J and 44 ± 20 J, respectively.