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Jul 20, 2023

Advances in Bimetallic

Bimetallic deposition capabilities are frequently named as a primary reason for the use of directed-energy deposition (DED) in various industries. Combining multiple materials in one solid part opens

Bimetallic deposition capabilities are frequently named as a primary reason for the use of directed-energy deposition (DED) in various industries. Combining multiple materials in one solid part opens up the possibility to improve performance by intentionally choosing material properties in line with required performance properties in different areas of the workpiece. This benefit is accelerated the more dissimilar the combined metals are and the more the part properties can be modified.

Historically, the use of copper in DED has been limited due to its reflective nature. New process strategies have enabled the early adoption of cladding onto copper components. However, more complex geometries and the high material cost of copper make it desirable to deposit copper as well. Recent advancements in the laser industry make it possible to utilize blue lasers in DED machines that allow for the deposition of more advanced copper alloys as well as pure copper.

Using infrared lasers (IR = 1,040 nm), the absorption coefficient of pure copper is only about 2%. Therefore, copper alloys have been used that increase absorption, such as CuAl10, CuSn8, CU18150, and others. Still, absorption is well below that of more commonly used steels and therefore high-power lasers above 3,000 W find good use with such applications. In addition, preheating of the copper substrate and advanced cladding strategies that make use of more effective deposition angles have had success over the last few years.

Typical applications are focused on cladding Inconel on a copper substrate in various space applications—most commonly rocket nozzles. For example, Figure 1 shows the setup of a copper liner made in CU18150 on a laser-powder-bed-fusion (LPBF) system (LASERTEC 30 DUAL SLM by DMG Mori) clamped in the main spindle of the five-axis, turn-mill LASERTEC 3000 DED hybrid machine. The copper liner in CU18150 was built using a 1,000-W laser on the LPBF system and open cooling channels have been added around the outer diameter. A 3,000-W laser was used on the hybrid-DED machine to deposit Inconel 625 around the circumference and close out the coolant channels accordingly. The reflection of the copper liner is a challenge in establishing a stable process that leads to proper material bonding.

With the right deposition strategy and an advanced toolpath created in a five-axis CAM program such as Siemens NX, successful results can be accomplished as shown in Figure 2. A consistent Inconel layer is applied around the circumference and microstructural analysis shows good bonding between the copper substrate and the Inconel layer.

By using a combination of an LPBF system and a DED system, the size of such components is constrained to typical envelope sizes of a cubic foot, or slightly larger in more recent machine developments. In order to meet the requirements of the space industry, it is desirable to utilize the full envelope of DED-hybrid machines that span diameters of up to 1,250 mm and part lengths of up to 6,000 mm between spindles. Consequently, a DED system that can deposit copper and therefore build the copper liner in the same envelope presents a tremendous opportunity for more advanced space components with very attractive economic benefits.

Recent developments in the laser industry provide diode lasers with higher laser power in alternative wavelength ranges. In particular, green- and blue-light lasers in the visible light spectrum have only recently become economically feasible. Figure 3 shows the advantage of using blue-light lasers in a wavelength spectrum of 450 nm compared to the IR lasers ranging between 900 and 1,070 nm. Blue lasers (450 nm) show further advancement over green lasers (515 nm). Energy absorption is improved for all metals, with the most significant gains made with copper.

A comparison of base elements iron, nickel, and copper shows improved absorption. Copper hardly absorbs any light in the IR spectrum and jumps to the same level of absorption as nickel and iron in the blue-wavelength spectrum. As well, nickel absorbs light at 19% higher efficiency in the blue-laser range. As a result, deposition of pure copper and low-alloyed copper alloys such as CU18150 becomes feasible in the DED process.

The impact on deposition results is remarkable, as shown in the microscopic pictures in Figure 4. Two alloys are compared: one is a low-alloyed copper alloy, aluminum-bronze CuAl1 (1 wt percent Al, >96 wt percent Cu); the other is pure copper. Both alloys are deposited using an IR laser and a blue laser. One can clearly see that the IR laser is already struggling with the aluminum bronze so that pores are created along the parting lines of each deposited layer.

Looking at the pure copper, the IR laser fails to deposit the material properly. Bonding failure is present along the lines of each deposited layer, as well as between weld beads. The weld beads barely adhere, leading to separation of the beads and layers. Meanwhile, the blue laser forms consistent deposition of both the aluminum bronze and the pure copper. The results have been replicated with various copper alloys, as well as on substrates ranging from steel to copper.

The space industry greatly benefits from the possibility to combine materials such as copper and Inconel. Mainly, to improve cooling of propulsion components, such as thrusters and rocket nozzles, copper is used and coated with Inconel to withstand high pressures and provide high strengths at high temperatures.

Previously, rocket nozzles have been produced through complicated process chains, including a combination of casting or forging, machining, electroplating, and some form of cladding. The typical process chain was stretched across various locations leading to many months-long lead times. Additionally, the processes and materials involved are expensive, and can drive cost for a single small-stage rocket nozzle (about Ø600 mm x 800 mm in Z) up to several hundred thousand dollars. Lead times can be as long as nine months for the same component.

By using a hybrid-DED system, the same component can be manufactured on one machine in a single setup. Figure 5 shows a demonstrator part made on DMG Mori’s LASERTEC 125 DED hybrid. In an envelope up to Ø1,250 mm x 790 mm, the copper liner and Inconel shell are deposited layer by layer. Cross-section A shows the view in one layer where the copper channels are fully bonded to the Inconel shell, while Section B looks into the cooling channels vertically along the deposition height, showing a solid connection between each layer of copper and Inconel.

By integrating the deposition and machining processes into one machine, a rocket nozzle can be manufactured within a few days compared to several months in the past. Likewise, significant cost reductions are possible. Taking powder cost and machine time into consideration, the cost for the same nozzle is now ranging in the tens of thousands of dollars compared to hundreds of thousands of dollars previously.

Material quality and process repeatability are crucial to adopting new technologies. Therefore, the LASERTEC 125 DED hybrid is equipped with in-situ melt-pool monitoring, work-distance control (part to DED tool), part-temperature control, and powder-flow monitoring.

Thermal conductivity is a large success factor in die casting and injection molding. More harmonized temperature profiles in the tools and better thermal conductivity yield improved part quality, as well as higher productivity and therefore reduced cost. Conventionally, molds and dies are machined from billet or castings, which have long lead times of several months. Cooling channels are drilled through the die from the outside, limiting the geometry of the coolant channels, so that cooling is different in surface areas of the die that are farther away from the next coolant channel.

Using hybrid-DED machines with blue lasers enables a completely new way of making molds and dies. Figure 6 reveals a die insert that was made from scratch on a LASERTEC 65 DED hybrid with a blue laser. The die core is deposited in copper and the skin of the die insert is deposited in tool steel with a hardness of 55 HRC. Upon depositing the first section (a), coolant channels are machined into the part (b) and the second section is deposited until stopped to continue machining of the coolant channels. This is continued several times until completing the final part (c), which has a solid tool steel skin and a copper core with a coolant channel for rapid thermal conductivity.

Experiments have compared a conventional design of the die insert with a hybrid additively manufactured design. It was found that the thermal behavior of the die improved so that cycle time could be reduced by 76%. Not only does this improve productivity, but it was also found that the castings had a more consistent quality leading to scrap-rate savings of more than $200,000 per year.

The use of blue lasers is in the early phases as they have just become economically available. First applications already show a significant improvement in the use of copper alloys, providing ample use cases that have either been impossible to make in the past or only with significant time and efforts.

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