Direct rapid tooling for die forging – A new challenge for Layer-Based Technologies

Selective Laser Melting technologies have given a new boost to Rapid Tooling applications of Layer-Based Technologies, especially in plastic injection molding for prototype and pre-series tooling, but also for Conformal Cooling in production moulds. So far only very few and basic applications have become known for forming tooling like sheet metal forming, stamping or die forging. Market demands for “real forgings in a one week time” have lead to a case study with the goal to successfully produce forgings using a layer manufactured die. This paper presents this case study and its first results, compared to conventional tooling. Derived from the experimental work this paper gives a prospective on expedient future applications of direct Rapid Tooling for die forging, using Laser Melting technologies. fused to an almost 100 % dense microstructure. Now it has become possible to use laser melting technologies to manufacture full series tooling for mass production without tool life limitations compared to conventional tool making by machining or EDM. We can now see a rather wide acceptance of laser melted mold inserts in full production injection molding tools, making use of the advantageous Conformal Cooling possibilities to place cooling channels of virtually any complexity in a very close and constant distance to the cavity surface to assure a faster and more uniform mold cooling in the injection molding cycle, bringing down cycle times and improving part quality in shrinkage-sensible areas (Dimitrov et al. 2008). First successful applications have also been reported in aluminum die casting (N.N. 2007). Nevertheless the dimensional accuracy and surface quality that can be reached today in Laser Melting processes still require a finish machining operation of the mold cavity. Metal forming processes have been investigated scarcely for their applicability of layer manufactured tooling yet (Levy et al. 2003, Schell 2004, Siegert et al. 2005, Stanchev 2006) but the successful application in injection molding and light metal die casting suggest a deeper look into other fields of application in Rapid Tooling. 3 THE INNOVATIVE APPROACH So why die forging? Industry demands for real forging prototypes in a very short time could be recognized by the authors from their wide range of industrial partners. The ambitious goal to deliver “real forgings in a one week time” like addressed in this overstatement from industry officials gave the kickoff to the project presented in this paper. The die forging process sets similar requirements to the tooling like die casting regarding temperature and wear resistance. By contrast there is no need for die cooling, geometries are simpler due to the lower natural strain in forging compared to casting, surface quality and dimensional accuracy demands are not as high. While it can be considered a downturn that the added value of Conformal Cooling is of no use in forging, the lower surface and dimensional demands give the chance to avoid a finish machining of the die cavity surface which gives a significant time saving in the Rapid Tooling process, not only for the ceased machining operation itself but also for all preparation steps like tool path generation, NC programming, cutting tool selection etc. The projects aim was to prove the applicability of laser melted tooling in die forging to manufacture forged prototypes, based on a real life reference part of rather complex geometry, using a standard production forging press. A first assessment of this innovative process chain was to be done compared to the conventional process by means of timing, quality and cost. 4 TOOLING DESIGN AND MANUFACTURING 4.1 Selection of reference part The reference part for this project got selected according to the following criteria: − typical forging part − moderate complexity − deep cavity − cup-shape − including mandrel These criteria were supposed to assure generalization of results for future real life applications. The choice was made for a segment of a car crankshaft, not of a conventional one-piece forged crankshaft but of a segmented light-weight version, joined from single crank segments (one segment per cylinder including counterweight – see figure 1). Figure 1. Conventional one-piece crankshaft (top) and segments for light weight version with hollow low end (bottom left: with counterweight, bottom right: without counterweight). A special challenge with this part was the two asymmetrically dislocated mandrels for shaping the hollow low end with asymmetric inner shape, giving a tricky die filling and respective material flow with the consequence of extra shear force to the tooling in the forging process (see figure 2). Figure 2. Geometry of reference part. Crankshaft segment. 4.2 Feasibility study The project’s second step was a feasibility study to evaluate the chances of a laser melted forging die to be successfully used in the forging process. Typical mechanical property demands for forging dies are at between 1300 and 1500 MPa tensile strength, based on forming forces of 5 to 7 t/cm2 taking effect on the tooling in the forging process. Laser melted test specimen have reached a tensile strength of 1730 up to 1950 MPa after heat treatment, meeting the demands safely. Size limitations of laser melted parts are today to be found with 250 × 250 × 250 mm3 with a typical machine size, while the largest available laser melting machine currently on the market can deliver parts of up to 300 × 350 × 300 mm3, even larger machines are under development at some machine manufacturers. The selected reference part has a size of 141 × 105 × 72 mm3, the first draft for the forging die cavity inserts showed dimensions of Ø 190 × 67/61 mm3 (upper/lower die half). That meant that the available build envelope of the laser melting machine allowed the fabrication of both die inserts. By downsizing the cavity inserts during the tooling design process to dimensions of Ø 170 × 54/47 mm3 (upper/lower die half) it even became possible to manufacture both die halves in one build job of the large laser melting machine (see figure 3). Standard tooling material for conventional tooling in die forging is hot-work tool steel like 1.2343 or the more expensive 1.2709 which can as well be processed in the laser melting process, making the laser melted tooling cavity inserts out of full serieslike, standard material. The forging process got simulated with a standard forming simulation software (Forge 2008, cluster version). The installed computing power (8 cores) allows elastic tooling calculation and detection of stress maxima. Optimization of part and tooling geometry to reduce die load can be done in several iterations with short computing time (depending on the model from several hours up to two days). Concerning the focus of the application presented in this paper another simulation feature bears high potential – the optimization of tooling size (in terms of volume and mass) can further bring down manufacturing times in the laser melting process significantly (other than in machining the lead time in laser melting is mainly determined by build volume). Figure 3. Both die halves placed in build envelope of laser