Phase transformations and control of residual stresses in thick spray-formed steel shells
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INTRODUCTION
TOOLS and dies are required by a spectrum of manufacturing processes including the stamping of automotive body panels, the laying-up of composite aerofoil sections, and polymer injection molding. While prototyping and shortrun tooling may be required to produce tens of components, volume capable tooling for mass production may be expected to produce upward of 1 million components, within tight design tolerances. Most volume capable tooling is manufactured by computer numerically controlled (CNC) machining of either gray iron/Al castings or steel billet material, which in the case of large press tools can easily weigh several tons. Provision of tooling represents a significant fraction of the overall investment required to introduce a new product line and is frequently the longest lead-time item. For example, in the development of a new mass production automotive vehicle, the large tooling sets used to press steel or aluminum sheet to form large body panels, such as bonnets and doors, may take 6 to 9 months to manufacture. Consequently, designs must be finalized far from the product launch date with little opportunity for modifications. There has been sustained effort to develop rapid tooling technologies to reduce lead times, but many of the processes in development are limited to prototyping applications only, because in general the tools cannot withstand the rigors of volume production.[1,2] Electric arc spraying is a relatively simple process that has been used successfully to manufacture soft Zn tooling for short-run prototyping,[3] and offers the potential for reduced lead-time tools. Shells are formed by spraying Zn onto a shaped, low-cost substrate or pattern. However, the electric arc spraying of thick (10-mm) free-standing three-dimensional “shells” in engineering materials (typically steels) suitable T. RAYMENT, Enterprise Fellow, S. HOILE, Research Assistant, and P.S. GRANT, Cookson Professor of Materials, are with the Department of Materials, Oxford University, Oxford OX1 3PH, United Kingdom. Contact e-mail [email protected] Manuscript submitted October 14, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS B
for large volume or high impact loading applications[4] has proved difficult, because significant residual stresses develop during the spraying process, resulting in shell peeling, warping, and loss of dimensional accuracy.[5,6] These residual stresses arise because impingement of molten metal droplets upon a surface, whether it be a substrate or previously deposited layer of material, results in a flattening of the droplets followed by rapid solidification and cooling to the bulk temperature. Thermal contraction of the flattened droplet “splats” is restrained by cooler underlying material, leading to tensile stress generation, known as the “quenching stress.”[7] Soft Zn tool manufacture by spray-based techniques is possible because the Zn yield stress is sufficiently low to accommodate the quench stresses. When thermal residual stresses can be controlled to minimize steel
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