Modeling the heat flow in spray formed steel shells for tooling applications
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INTRODUCTION
IN the development of a new mass production vehicle, the provision of the tooling is frequently the longest leadtime item. The large press tooling sets that press steel or aluminum sheet to form large body panels and other automobile parts may take 9 months to manufacture. Consequently, vehicle designs must be finalized far in advance of the product launch date, and few design iterations are possible. Large tool sets also represent a significant fraction of the overall investment required to introduce a new model. Volume capable tooling for mass production is manufactured by the computer numeric controlled machining of large gray iron castings or steel billets. There has been sustained effort to develop rapid tooling technologies to reduce lead times,[1,2,3] but most have been limited to prototyping applications because in general rapid tools cannot withstand the rigors of volume production. This article concerns the spray form tool process based on a metal spraying approach[4] that may allow the manufacture of production tooling at reduced time and cost.[4,5] II.
SPRAY FORM TOOL PROCESS
Steel droplets are sprayed onto a freeze-cast substrate[6] by electric are spraying, forming a thick shell that replicates accurately the surface geometry of the substrate. The electric arc spray guns create a droplet spray from a DC arc that melts the tips of consumable Fe-0.8 wt pct C wires and a high pressure N2 gas flow that atomizes the liquid at the wire tips and then projects the arising droplets to the substrate.[7] Figure 1 shows the spray cell used in this study featuring a six-axis industrial robot and four electric arc spray guns,[8] which are moved over the surface of the shaped freeze-cast substrate following a predetermined pat-
T. RAYMENT, formerly D. Phil student in the Department of Materials, Oxford University, is Software Engineer, Instron Limited, High Wycombe HP1 2TJ, United Kingdom. P.S. GRANT, Cookson Professor of Materials, is with Department of Materials, Oxford University, Oxford OX1 3PH, United Kingdom. Contact e-mail: [email protected] Manuscript submitted April 12, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
tern termed the ‘‘path plan.’’[9]. Not all the spray adheres to the substrate during manufacture because of overspray at the substrate edges and droplet splashing.[10,11] The shell is removed from the substrate, trimmed, and backed before entering production service. Until recently, the spray form tool process has been limited to the manufacture of thin Zn alloy shells used to produce ‘‘soft’’ tooling for prototyping or short production runs.[1,12] Creep of the relatively soft Zn shell during spraying relieves the thermal stresses that normally lead to distortion in these relatively thick thermally sprayed coatings.[13] More recently, careful thermal control has been used to eliminate thermal residual stresses and distortion in thick-sprayed steel shells by manipulation of the expansive austenite to bainite transformation in the shell during manufacture.[14,15] This
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