May 2023 Volume 5

FORGING RESEARCH

At the conclusion of this project, one unresolved question was the performance of claddings applied to the surface of 718 nickel alloy tooling using the laser hot- wire process developed and implemented by the Lincoln Electric Company. In principle, the hot-wire process has three main advantages over arc- or cold-wire laser welding. The first is that the hot wire effectively degasses prior to reaching the weld pool reducing the likelihood of gas porosity in the deposit. Secondly, the net deposition rate is much higher than arc processes since the degree of superheat is much lower. (Typically, the temperature of the arc is far above the melting point of the filler metal in order to ensure arc stability.) Thirdly, the degree of dilution (melt-back of the base metal) is significantly lower – again, because of reduced net heat input. All these, in principle, would be advantages to forging die rejuvenation. A particular Presrite Corporation part, the B208 Gear Die, was selected for the additional work to be carried out under FIERF support. The formed product and overall geometry of the die are illustrated in Fig. 1.

This permitted an analysis of the economics of the proposed rejuvenation. The first step of which was to determine the minimum contour that would remove all the damage. This hypothetical contour was modified through consultation with Presrite engineers to determine an optimized contour that was practically achievable. An illustration of contour is shown in Fig. 2. It is nominally the removal of 3 mm , but more is locally removed from the tips of the teeth in the die to avoid the creation of a knife-edge tip that would be inappropriate for subsequent cladding.

Fig. 2: Contours representing the original contour of the Presrite B208 die and that after a “skim cut.” This “skim cut” approach is very promising in that the volume fraction of this particular tool that is then required to be replaced is less than 7% of the total. This is significant as the economics are then favorable to resurface a 718 tool. Prior experiments using a composite approach in which the cladding deposit is a different alloy that the original tool had been unsuccessful for this die due to metal movement during attempts to use wire EDM to recut the contour. The presumption was that residual stress due to differential thermal expansion led to a high stress state. The technical staff at Lincoln Electric Automation had developed a processing strategy for a nuanced set of toolpaths designed suggested by CWRU, with the objective of minimizing residual stress in the deposit.The underlying premise was to use axial paths parallel to the central axis of the cylindrical tool. The sequence of metal deposition toolpaths included independent build-ups on individual die teeth that were linked with subsequent passes. A schematic of the build up sequence is shown in Fig. 3. The deposit had been created, but not characterized at the start of the FIERF supported project. Furthermore, the wire EDM was yet to be carried out.

Fig. 1: Illustration of the part formed using the B208 die (left) and the dimensions of the cross section of the die (right). The working surface of the die is the interior surface that forms the gear teeth. This is of constant cross section. Specifically, work was carried out to explore the ability to rejuvenate end-of-life 718 dies. The first step was to characterize the accumulated damage during die service. An exhaustive analysis of the surface of a tool that had produced some 1,500 parts revealed characteristic damage that was dependent on the base material used. Dies made from H13 had locally reduced hardness, mushrooming (plastic deformation at the apices of the tool), and cracking on the flanks of the teeth in the tool. In contrast, tools produced in 718 showed evidence of hardening of the near-surface region of the working face, sharp edge retention, and localized cracking at the roots between teeth in the tool. Importantly, serial sectioning and fluorescent die penetrant analysis indicated that the damage was restricted to the near-surface region of both types the tools. Most of this work was carried out by a post-doctoral researcher, Yogesh Singla.

FIA MAGAZINE | MAY 2023 86