Page 40 - MetalForming May 2016
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3D Metal Printing
 Additionally, the technique uses a wire feedstock that enables the use of a shielding gas, rather than the highly con- trolled oxygen-scrubbed environment required when using high-surface-area powders. Also, since drawing a wire is less energy intensive than atomization of material to produce well-controlled powder-particle size, the WAAM technique also is energy friendly and cost effective from a feedstock standpoint.
Printing Off of Preforms
To develop a cost-effective and rapid process to print dies, the Ricardo-UTRC team opted to use preforms—starting plates or extrusions—and then apply the WAAM process to 3D- print all of the necessary features onto the preforms. This saves significant time and money compared to 3D printing a die from scratch. The starting plates have a larger cross-section than the 3D-printing area in order to handle the stresses at the edge of the die during stamping. Further, they need sufficient thick- ness to handle the forces across the part.
To meet the required die dimensional tolerances, excess weld metal is deposited and then finish-machined. Researchers estimate that this excess metal should be 15 per- cent of the total volume needed—likely to be overly conser- vative for large dies with relatively flat surface geometries, but useful for determining the costs associated with AM of dies.
Further, in evaluating the cost of finish-machining the
3D-printed dies, it is assumed that the first 85 percent of the excess material can be removed quickly and at relatively low cost, and the final 15 percent finish-machined at a higher cost. Cost estimating also accounts for surface heattreat- ment and manual polishing of die surfaces.
A Case Study
Ricardo and UTRC estimated the cost savings realized by 3D printing dies for three Toyota Corolla parts (Table 1): a B- pillar panel, door outer skin and main floor pan. Manufac- turing the parts requires die sets for blanking, drawing and a trim/pierce operation; the cost comparison focused on the draw die set for each component.
The B-pillar panel is the smallest part, and has the small- est draw depth. The floor pan is the largest part, in cross- section and in draw depth. Table 2 presents the costs and the time required to produce the die sets via WAAM, including finish machining, polishing and heattreatment.
The results of this economic assessment show that the cost benefit of 3D printing the draw die depends on the part size and depth of draw. The draw die for the B-pillar panel requires less material deposition due to the smaller part size and rel- atively shallow draw depth, so it’s less expensive to manu- facture. However, as draw depth increases the cost savings achieved by using additive manufacturing diminish, due to the relatively slow deposition rate from the WAAM process.
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