Formability analysis of tailor-welded blanks of steel sheets

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Formability analysis of tailor-welded blanks of steel sheets

 

Author: Chan, Sui-man
Title: Formability analysis of tailor-welded blanks of steel sheets
Degree: M.Phil.
Year: 2003
Subject: Hong Kong Polytechnic University -- Dissertations
Metal stamping
Department: Dept. of Industrial and Systems Engineering
Pages: xxi, 216 leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1714683
URI: http://theses.lib.polyu.edu.hk/handle/200/2073
Abstract: Tailor-welded blanks (TWBs) are produced by welding together flat sheets that may differ in material, thickness, strength, surface finishing, or coating to form single sheets for subsequent stamping. Thicker and stronger materials are used at critical parts of the sheet metal blank so as to increase local stiffness. In automobile production, this approach simplifies the manufacturing process, produces lighter car-body panels and lowers the weight and requirements for processing tooling, labor and cost. It also improves safety, quality and reliability of the automobiles. This project analyzes the formability of TWBs made from cold rolled steel sheets (SPCC) of different thicknesses. Steel sheets ranging from 0.5mm, 0.6mm, 0.7mm, 0.8mm, 0.9mm and 1.0mm in thickness were used to produce TWBs of different thickness combinations. For each thickness combination, a series of specimens varying in width, height and radii of cut-off were used for formability analysis. Nd:YAG laser was used to butt-weld different tailor-made blank specimens for uniaxial tensile tests and Swift tests. Circular grid analysis was used to construct the Forming Limit Diagram (FLD). The principal deformation of major strain e1 and minor strain e2 that occurs in the plane of the TWBs was measured and plotted. The experimental results of uniaxial tensile tests clearly showed that there were no significant differences between tensile strengths of TWBs and those of base metals. There is little difference in the tensile strength between the TWBs of different thickness ratios. After the Swift tests, formability of TWBs was analyzed in terms of three measures: Limiting Dome Height (LDH), Forming Limit Diagram, and minimum major strain. The experimental results showed that TWBs of different dimensions and radii of cut-offs yield different major and minor strain values of FLDs and different LDHs. TWBs of a thickness ratio closer to one, yields a higher LDH value. The results also showed all the TWBs have lower formability than the base metals. The effects of different thickness ratio on the FLDs, were studied using three sets of TWBs. Each set was made from materials of 1.0mm, 0.7mm or 0.5mm thickness in one part and materials of varying thickness in the other. It was found that the higher the thickness ratio, the lower the FLC level. In addition, TWBs of similar thickness ratios and different thickness combinations, did not yield the same FLD. However, TWBs with similar thickness combinations did yield similar FLD. The smaller the thickness combination differences between the two parts of a TWB, the higher the formability. Furthermore, minimum major strain was a good measure for comparing the formability of TWBs of different thickness ratios. There was an inversely proportional relationship between thickness ratio and minimum major strain. TWBs of a thickness ratio closer to one were found to have a closer minimum major strain to those of the base metals. Microstructural study and microhardness measurement were carried out on the TWBs. The microstructural analysis showed a larger difference of grain size at the Heat Affected Zone (HAZ) for a TWB of a higher thickness ratio. The grain size in the thinner region was larger than that in the thicker region. This is due to the fact that thinner part having a longer recrystallization time than the thicker region during cooling. Microhardness measurement was carried out at the weld bead and across the base metals. The findings of the test indicated that the fusion zone was harder than the base metal by 60%, while the hardness of the HAZ of TWBs of different thickness ratios could be either harder or softer than the base metal.

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