Fundamental Study On Weld Bead Modelling And Forming Behaviour For Laser Welded Blanks

Tailor welded blanks (TWBs) are increasingly used to meet the stringent requirements imposed by the automotive industry in terms of weight reduction and energy saving, etc. In the application of TWB technology, one of the main challenges faced by the automotive designers is the drop in formability due to the existence of welds. In other words, an earlier failure often occurs during biaxial stretching of a TWB compared to monolithic sheets. Accordingly, formability and failure analyses of TWBs have become hot topics with high scientific and practical significances. The aims of this dissertation are to probe into (1) the factors influencing TWB formability and failure onset modes,(2) the approaches to enhancing accuracy in predicting the failure behaviour of TWBs, and (3) the methods of improving the formability of TWBs. The main contents of this dissertation are listed as follows.(1) Influences of different yield models and hardening models on the prediction accuracy in finite element (FE) simulation of sheet metal forming process were analysed.(2) Average weld properties of TWBs were determined by uniaxial tension tests of subsize TWB specimens. Inhomogeneous weld properties of TWBs with different combination types were determined by using an analytical method as well as a nano-indentation test. Also, influences of weld property inhomogeneity on TWB formability were evaluated by comparing the results based on homogeneous constructive models with those based on inhomogeneous constructive models.(3) Influences of stamping process parameters (i.e., punching velocity, friction coefficients between contact surfaces of blank and tools, blank holder type, blank holding force and the angle between welding and rolling directions of base sheets) on the formability of TWBs with different combination types were systematically analysed. The optimized stamping process parameters were then provided.(4) Influences of strength and thickness ratios of base sheets on the formability of TWBs were systematically analysed. A method was proposed to assess the formability of a TWB made from base sheets with dissimilar strengths (or thicknesses) based on the strength (or thickness) ratio and the formability of a TWB with identical base strength and thickness, assuming that they all undergo a similar stamping process. Meanwhile, influences of weld plastic parameters (i.e., strength coefficients, strain hardening exponent and elongation) on formability of TWBs were studied. (5) Failure initiation modes of TWBs with different initial weld locations during biaxial stretching were studied based on FE simulations and experimental tests. The failure behaviours of base and weld metals in transition from one failure mode to another were examined and a calculation model was proposed to evaluate the failure onset site for a TWB with identical base strength and thickness. Moreover, mechanism of different failure onset modes of TWBs was discussed by analysing stress and strain condition as well as thickness distribution.