Influence of paint baking conditions on the mechanical performance of thin aluminum-steel blanks

Effect of paint baking on the halo ring and mechanical behavior of 30MnB5 hot-stamped steel resistance spot welding joints

Effects of Cu content and preaging treatments on precipitation sequence and artificial aging response in aluminum alloy 6022 were investigated using transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and hardness tests. It was found that Cu induces the formation of Q and its precursor metastable phases and has a beneficial effect on the kinetics of artificial aging. For the alloy with 0.07 wt pct Cu, the precipitation sequence is GP zones → needlelike β″ → rodlike β′ + lathlike Q′ → β + Si. On the other hand, the precipitation sequence in the alloy with 0.91 wt pct Cu is GP zones → needlelike β′ → lathlike Q′→Q+Si. For the artificial aging condition of 20 minutes at 175 °C, which is the typical automotive paint bake condition, suitable preaging treatments were found to significantly reduce the detrimental effect of the natural aging on artificial aging response.

In order to fabricated high strength aluminum sheets with reasonable cost, several Al-Mg and Al-Zn-Mg-Cu alloy strips were successfully fabricated by the twin roll casting with a proper operating condition. The strips had good workability during subsequent warm/cold rolling processes due to the fine cast structure. The final annealed sheets have good mechanical properties at ambient temperature. The tensile strength and elongation of annealed Al-10wt%Mg sheet are about 400MPa and 30%, respectively. The tensile strength and yield strength of Al-Zn-Mg-Cu alloy sheet after the aging at paint baking condition (180oC × 30min) are 480MPa and 350MPa, respectively. The developed aluminum alloy sheets have a superior strength to commercial aluminum alloy sheets used for automotive body structures.

The strain aging behaviour of an ultralow carbon steel partially stabilised by additions of titanium and niobium was studied as a function of prestrain, aging temperature, and aging time. The increase in strength owing to bake hardening occurred in two stages. The first was independent of prestrain and reached a maximum of 30 MPa after 100 min aging at 100°C. This increase is attributed to the pinning of dislocations by solute carbon atoms. The second hardening stage reached a maximum of 40 MPa after aging at 200°C for 100 min. A dependence on prestrain was observed in this stage, with maximum values being obtained in material prestrained by 1%. It thus appears that the second stage is caused by the formation of fine precipitates on dislocations. Evaluation of the information gained shows that the same bake hardening increment can be achieved under less severe paint baking conditions than those normally encountered. With the increasing emphasis on environmental issues, this will allow automobile manufacturers to reduce energy consumption without compromising part performance.

Cluster hardening in Al-3Mg triggered by small Cu additions

In order to fabricated high strength aluminum sheets with reasonable cost, several Al-Mg and Al-Zn-Mg-Cu alloy strips were successfully fabricated by the twin roll casting with a proper operating condition. The strips had good workability during subsequent warm/cold rolling processes due to the fine cast structure. The final annealed sheets have good mechanical properties at ambient temperature. The tensile strength and elongation of annealed Al-10wt%Mg sheet are about 400MPa and 30%, respectively. The tensile strength and yield strength of Al-Zn-Mg-Cu alloy sheet after the aging at paint baking condition (180oC × 30min) are 480MPa and 350MPa, respectively. The developed aluminum alloy sheets have a superior strength to commercial aluminum alloy sheets used for automotive body structures.

The effect of pre-aging on microstructure and tensile properties of Al-Mg-Si alloys

<div class="section abstract"><div class="htmlview paragraph">Temperature effects on the deformation and fracture of a commercially produced transformation-induced plasticity (TRIP) steel subject to a two-step quenching and partitioning (Q&P) heat treatment are investigated. Strain field evolution at room temperature is quantified in this 980 MPa grade Q&P steel with a stereo digital image correlation (DIC) technique from quasi-static tensile tests of specimens with 0°, 45°, and 90° orientations. Baseline tensile properties along with the variation of the instantaneous hardening index with strain were computed. Variations of the bake-hardening index were explored under simulated paint bake conditions. Tensile properties were measured at selected temperatures between -100°C and 200°C and the TRIP effect was found to be temperature-dependent due to stress-induced martensitic transformation at lower temperatures versus strain-induced transformation at higher temperatures. Electron back-scatter diffraction (EBSD) and X-ray diffraction (XRD) of regions close to the fracture area were used to explain the observed temperature variations of the ultimate tensile strength (UTS), 0.2% yield strength (0.2%YS), uniform elongation (UE) and total elongation (TE).</div></div>

Corrigendum to “Mitigation of distortion of Al/steel part under simulated paint baking condition: Experiment and numerical model studies” [Journal of Manufacturing Processes, volume 132 (2024) 6070

The potential for weight reduction, ease of manufacturing and improved crashworthiness makes advanced and ultra high strength steels attractive for automotive applications. Resistance spot welding is by far the most widely used joining method in the automotive industry due to the high operating speeds, the reliability of the process and the suitability for automation. Safe microstructures in resistance spot welds in AHSS and UHSS have to be assured to promote acceptance of these steels in the automotive industry. However, the higher alloying contents of AHSS/UHSS steels limit their weldability and unfavourable modes of weld failure are frequently observed. The main aim of this research is to identify and understand the unfavourable failure of the AHSS welds and to modify the microstructure and thus the mechanical response of the welds. In this PhD thesis the results of alternative welding schedules to modify the microstructure and mechanical performance of the AHSS resistance spot welds are reported. The effects of a paint bake cycle on the microstructure of the welds have also been investigated and the predominant mechanisms involved were studied. The residual stress within these welds were measured and simulated to facilitate the residual stress prediction before welding. Double pulse resistance spot welding with different second pulse current levels was applied to improve the microstructure of the weld edge. The second current pulse equal to the first pulse anneals the weld edge and modifies the weld edge microstructure. Microstructural analysis was performed using optical microscopy, scanning electron microscope, electron probe microanalysis (EPMA) and electron back scattered diffraction (EBSD). The double pulse weld showed a reduction in segregation of alloying elements such as phosphorous and a change in grain morphology from dendritic to a more equi-axed shape and smaller grain size. The results obtained from the mechanical testing i.e. cross tension strength test (CTS) and tensile shear strength test (TSS) showed enhanced cross-tension strength and energy absorption capability of the weld for the double pulse welds.

While the automotive industry is striving for the reduction of car body weight to increase the fuel efficiency and to reduce CO2 emission without compromising the safety and crashworthiness of vehicles, a new generation of advanced high strength steels (AHSS) have emerged as excellent candidates to meet these requirements. However, their integration into the car body structure is associated with welding-related problems. This research utilizes a novel approach to establish a fundamental correlation between welding parameters, microstructure and mechanical performance of AHSS resistance spot welds. In-situ micro-cantilever bending experiments are executed and analyzed in a quantitative manner to evaluate the effect of texture and post-welding heat treatment on the local fracture toughness of spot welds. A striking finding is that, through a switch from single to double pulse weld scheme, the texture of martensite formed in the fusion zone becomes responsible for a significantly higher fracture toughness of the area in front of the pre-crack. In addition, it is found that paint baking heat treatment also results in a much enhanced fracture toughness through tempering of the martensitic microstructure. A quantitative correlation is made between the micro-scale fracture toughness and macro-scale mechanical performance of advanced high strength steel welds.

Effect of alloy composition and heat treatment on mechanical performance of 6xxx aluminum alloys