HIP Processing of Improved Tooling Materials for High-Productivity Hot Metal Forming Processes

Abstract

Much work has been carried out in the last decade on the development of high performance alloys to reduce vehicle weight. These alloys are often characterized by low roomtemperature formability. A variety of hot forming processes (hot stamping, hot extrusion and high-pressure die casting) are thus being used or adapted for these alloys. The final mechanical properties, shape complexity and production cost of parts made using these processes will be closely related to mold/die thermal and mechanical performance. Hot work tool steels generally have the required mechanical properties and durability to meet hot-processing requirements but have low thermal conductivity. The stringent low processing cost and high-volume production requirements of the automotive industry compel part producers to find ways to shorten unit production times at equivalent product quality. In order to meet the processing requirements of advanced alloys and transfer heat more rapidly, the tooling should thus have a higher thermal conductivity than the standard tool steel dies currently in use. The aim of this work is to optimize die properties to improve heat transfer kinetics during part shaping, thus providing an increase in efficiency and productivity for automotive metal part manufacturing. Hot Isostatic Pressing (HIP) has been used to clad a conformal-cooled copper core with a layer of either hot-work tool steel or High-Thermal Conductivity (HTC) composite material designed at NRC. Properties and performance of these systems are compared with those of standard tool materials to demonstrate the practical potential for future development and optimization of advanced tooling. Introduction. A promising way to manufacture structural automotive components using high strength AA7xxx aluminum alloy sheets is through the hot stamping process. The process itself is not new and is currently used in production with boron steel sheets. Rana et al. [1] provides different process details with boron steel and the average processing time is about 7 min for each part. For AA7xxx aluminum alloy sheets, the hot stamping processing sequence can be summarized as follows: 1) a blank sheet is heat treated to put the alloying elements into solid solution, 2) the blank is rapidly transferred to the press, 3) the punch is partially closed to shape the part and 4) the punch is completely closed to quench the shaped part and prepare the alloy for precipitation 5) the part is removed from the press and artificially aged to reach high mechanical strength. The hot stamping process with aluminum is similar to steel except that the solution heat treatment step is longer and aluminum's thermal conductivity is higher. During the last few years, aluminum alloys for hot stamping have attracted the interest of many scientists. Quenching rate effects for AA7xxx alloys have been analyzed by Keci et al. [2] and Kumar et al. [3]. High temperature mechanical behavior and high temperature formability analysis and modeling have been analyzed by Mohamed [4] and Elfakir [5]. Harrison et al. [6] have also produced real AA7xxx aluminum alloy pillars using hot stamping. Due to the major investments required for future part production, another vital process parameter to consider is the hot stamping cycle time. During blank quenching, heat is transferred to the punch, the latter being cooled either by flowing water or oil via internal cooling channels. Conventional tool steels are used for the punches, yet their thermal conductivity is low, which increases the cycle time. As other authors have also realized [7, 8], the development of HTC tool steels would thus contribute to the improvement of hot working efficiency and productivity, which would be beneficial for the automotive industry where large production volumes require low processing times and costs. Experimental. HIP processing and characterization of reference tool steel and HTC composite : Spherical powders of D2 tool steel powder (-150+45 m, Sandvik – see composition in Table 1) and of pure copper (grade 153A, 2%max+100m / bal+45m / 10%max-45m, ACuPowder) were used to process the materials required for this study. D2 is not generally used for hot working, but it was chosen as it has been a reference for different sheet forming studies at NRC during the last few years, so comparisons with earlier work could easily be made (smaller-scale preliminary work based on H13 tool steel gave similar results [9]). For the production of the D2 reference and the development of the HTC tool steel, cylindrical 304L stainless steel canisters (190 mm-high, 138mm OD, 1.59 mm wall thickness) were filled with either D2 powder or a D2+30vol.% Cu blend and tapped to tap density. A cover plate featuring a tube for gas evacuation was welded on top of each of the canisters, which were then submitted to a vacuum degassing treatment (14h @ 150C, 4h @ 550C under mechanical vacuum (7x10 Torr)). After mechanical crimping of the vacuum tubes and sealing by TIG welding, each canister was then HIPed in a model AIP10-30H hot isostatic press from American Isostatic Presses, Inc. The HIP parameters chosen for the pure D2 material were the following: 4h @ 1100C and 15000 PSI (103 MPa). In the case of the D2+30vol.% Cu blend, the HIP plateau temperature was decreased to 1000C to avoid formation of a liquid Cu phase. No additional heat treatment was applied to the resulting HIPed materials. Table 1: Chemical Composition (wt. %) of D2 Powder Fe Cr C Mo V Mn Si Ni P S Cu Bal. 12.8 1.41 0.95 0.72 0.6 0.27 0.19 0.02 0.01 0.01 After HIPing, coupons were machined out of the D2 and D2+30vol. % Cu billets by wire Electro-Discharge Machining (w-EDM). These specimens were used for evaluation of Heat Capacity (Cp) by Differential Scanning Calorimetry (NETZSCH DSC 404F3), Thermal Diffusivity () by Laser Flash Analysis (NETZSCH LFA 457 Microflash) and Hardness (Instron Series B2000). Thermal Conductivity (k) was calculated using the measured Cp and  values by means of the following relationship, where  is the density of the material: