The success of the direct metal deposition (DMD) technique lies in the design of its powder, shielding gas and laser-delivery nozzle (Figure 1a). Powdered metal is channeled into a cone such that its tip intersects with the substrate. A high-powered laser beam is delivered through the axis of the cone to provide the energy to melt and fuse the powdered metal to the substrate. A curtain of shielding gas, which protects the melt pool from atmospheric gases, is created around the metal.[1,2]

During deposition, the laser creates a melt pool on the substrate. The addition of powdered metal to the melt pool increases its volume and creates a bead. By moving the nozzle relative to the substrate, intricate patterns can be created. By stacking these patterns on top of one another, 3-D shapes can be formed. Changes in cooling rates due to variation in part geometry and part-temperature profile can influence the thickness of the layer being deposited.

To counter this effect, companies like DM3D Technology use closed-loop systems. As shown in Figure 1b, three charge-coupled device (CCD) cameras monitor the height of the melt pool in real time and adjust the process parameters to maintain a constant layer thickness.[1,2]

The nozzles are usually mounted on a CNC-based system, which allows very precise control of their motion during the DMD process. When more degrees of freedom are required, the nozzles can also be mounted on robotic arms. Since most alloys can be deposited with just localized shielding provided by the nozzle, the DMD technique can process parts that are much larger than laser powder-bed fusion (LPBF) and binder-jetting AM technologies. For those alloys that are more prone to atmospheric contamination, the deposition process can also be enclosed in an inert-gas chamber.

 

Case Study 1: Low-Cost Build

Heat-checking cracks form due to rapid heating and cooling of a surface.[3] Since forging dies go through large thermal cycles during operation, heat checking is a common problem for the forging industry. This issue can be countered by making the forging dies out of heat-checking-resistant material, which is often expensive and difficult to machine.

A lower-cost approach to counter heat-checking cracks is to make most of the die out of low-cost material and apply heat-checking-resistant material in only the regions that are most prone to heat-checking cracks. Our company was able to demonstrate this methodology by using an undersized low-cost and easy-to-machine S7 blank as substrate (Figure 2a).

A cobalt/chrome alloy called Stellite was then deposited on the surfaces that were expected to encounter large thermal cycles (Figure 2b). This alloy is known for its high-temperature hardness. Figure 2c shows the die after the DMD process, and Figure 2d shows the die’s final machining. The DMD process took approximately one hour.

The die processed using DMD was less expensive than the conventionally manufactured die because of lower material and machining costs. Due to the ease of material machinability and availability, the lead time for manufacturing the die was reduced by 30%. Although conventionally manufactured forging dies had a service life of ~5,000 cycles, the DMD-processed die was functional for 19,000 cycles. This increase in service life was attributed to the fine microstructure that is created during the DMD process.

 

Case Study 2: Reconfiguration

Due to changes in engineering requirements or lessons learned from manufacturing, modification of forging tools is often required. Figure 3a illustrates the model of a hypothetical forging die that is obsolete due to design changes. To reconfigure the die using DMD, it would first need to be machined to a form similar to the blank shown in Figure 2a. It would then go through a DMD process and machined to the final form shown in Figure 3b. The advantages of employing DMD in this case are reduced lead time and retooling costs.

 

Case Study 3: Hardfacing

Figure 4 shows a trimming die used to trim flash after forging. During its operation, the trimming die’s edges experience more wear than the rest of the die. When these edges are damaged, the trimming die is taken out of service.

It is highly desirable to make the trimming edge out of a very wear-resistant material. To hardface the trimming die shown in Figure 4, a chamfer was machined on the trimming edge. As shown in Figure 5a, DMD was used to apply Stellite to the chamfered surface. The DMD surface was then machined to the final form.

Figure 5b shows that the Stellite deposition and its interface with H13 substrate is free of defects like cracks, porosity or delamination. This is indicative of excellent bonding between the substrate and the deposited material. This also suggests that the process parameters used for DMD were adequate. The hardness profile (Figure 5c) shows that the Stellite was able to impart high hardness to the base material.

After the die shown in Figure 4 was hardfaced, it experienced a 260% increase in service life compared to its conventionally manufactured counterparts. The improved performance was attributed to the high wear resistance of Stellite, a good-quality deposit, excellent interfacial bonding between the Stellite and H13 and the fine microstructure inherent to the DMD process.

 

Case Study 4: Repair

Forging dies are often made from high heat-checking-resistant and wear-resistant materials, which are often fairly expensive. As the size of the die increases, so does the manufacturing cost. As an example, it was very expensive to make the approximately 36-inch x 36-inch x 10-inch die shown in Figure 6 out of such a material.

Repairing forging dies to extend their service life can lower lead times and offset initial manufacturing expenses. The die shown in Figure 6 was machined to remove the areas damaged during the forging operation. A high-alloy, high-toughness tool steel called CPM1V was then applied on the machined regions.[4] After the top half of the die was repaired, the same procedure was also applied to the bottom half of the die. The DMD process took approximately five hours and was able to extend the life of the die by 91%.

 

Conclusion

DMD technology has been shown to reduce the fabrication cost for reconfiguring, hardfacing and repair of forging dies. DMD can be an economically viable solution that is able to achieve properties that are comparable or better than wrought material. Furthermore, DMD technology can be used on large parts and in production settings.

References

  1. Dutta, B, and F H Froes, “Additive Manufacturing of Titanium Alloys,” Advanced Materials & Processes, Feb. 2014, pp. 18–23
  2. Dutta, Bhaskar, “Component and Tool Life Extension Using Direct Metal Deposition (DMD),” Additive Manufacturing, Aug. 2013, pp. 15-16
  3. “Heat Checking,” Drilling Lexicon, IADC, 2018, www.iadclexicon.org/heat-checking/
  4. “CPM1V,” Tool Steel and Specialty Alloy Selector, Crucible Industries, www.crucible.com/eselector/prodbyapp/hotform/cpm1vd.html