Year

2001

Degree Name

Doctor of Philosophy

Department

Department of Materials Engineering - Faculty of Engineering

Abstract

Zinc coated steel is an attractive material for the manufacture of automotive body panels due to its excellent corrosion resistance and good weldability. It does however pose problems during sheet metal forming operations due to zinc pick-up on the die surfaces which changes the original forming characteristics of the part and increases maintenance of the die. The hypothesis presented here is that the severity of the problem may be reduced through surface engineering of the critical areas of the die so as to modify the frictional and chemical properties of the die surface. The critical area of the die to be investigated is the drawbead region. Drawbeads are added to deep drawing sheet metal forming dies to control the flow of metal into the die to eliminate wrinkling. The surface engineering involved coating experimental drawbeads with thin, hard coatings of TiN, TiCN and TiC using filtered arc physical vapour deposition. Uncoated Steel drawbeads of varying hardness were also investigated. Frictional forces of the drawbeads were determined using a Drawbead Simulation Test. Zinc pick-up on the drawbeads was assessed through ICP analysis. All results were correlated with hardness, roughness and chemistry of the drawbead surface. Hardening and polishing the surface of the uncoated steel drawbead reduces the average coefficient of friction between the sheet and the bead. Over time however as more sheets are drawn, a steady increase in friction coefficient occurs. Changing the surface chemistry of the drawbead by coating not only reduces the average coefficient of friction but also acts to reduce friction as more sheets are drawn. Thus, the minimisation of zinc pick-up when using galvanneal steel depends on a complex interaction between increasing die surface hardness, decreasing die surface roughness and changing die surface chemistry in the regions of high forming forces (i.e. the drawbead). Thin, hard, smooth ceramic-like coatings of alloy carbides (in particular) and alloy nitrides deposited onto the drawbead section of sheet metal forming dies effectively reduced the coefficient of friction between the die and galvanneal sheet. The reduction in the friction contribution of the forming forces in the drawbead region assists in maintaining forming characteristics of the die and may also assist in reducing the powder/flaking of the zinc-alloy coated sheet. The hardness of the coated drawbead together with the reduced affinity between the carbides and nitrides in the drawbead coating and the Fe in the zinc-alloy coating also reduced the amount of zinc-alloy pick-up.

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