iTSSe TSS ADVANCED MATERIALS & PROCESSES | APRIL 2024 35 iTSSe TSS GTV K2 HVOF JOINING TECHNOLOGIES HVOF coatings can support joining processes like brazing, soldering, and welding in several ways as summarized in Table 3[3]. Coatings produced by the liquid fuel HVOF GTV K2 torch are typically formed by spraying particles that impact substrate surfaces in mostly solid state. This particle impact results in metal-metal contact areas resulting in high bond strength and high cohesive strength of coatings. Using a transition filler material, the joining of metal and ceramic components and dissimilar materials can be achieved. For example, GTV K2 sprayed AISI316L coatings on aluminum alloys can be joined with case-hardening steel 16MnCr5 by laser welding (Fig. 4). Interface damage between coating and aluminum substrate due to thermal impact during welding as well as crack formation is securely avoided. Only very few fine micron sized pores are formed at the interface between weld metal and coating. Even for unfavorable butt joints, the GTV K2 sprayed AISI316L transition joints achieve higher joint strength than conventional transition joints produced by roll cladding. HVOF copper coatings provide a surface for workpieces to endure the thermomechanical loads imposed by the soldering process. A copper coating on an extruded aluminum component permits easy soldering to copper plates to form an efficient aluminum/copper composite heat sink. HVOF is also utilized to deposit brazing filler material on complex shaped geometries with long term stability using relatively inexpensive mixtures of elemental powders (Fig. 5). GTV K2 pseudo alloy AgCuTi active brazing filler coating permits the joining of molded Si3N4 ceramic turbochargers to case-hardening steel 16MnCr5 shafts with joint strength exceeding the strength of the ceramic. The automation of these processes enables mass production and cost saving as discussed in the following section. ADVANCED THERMAL SPRAY PROCESS AUTOMATION AND DIGITIZATION Automated production lines are being used increasingly more often in thermal spray and laser cladding processes to improve productivity and reliability of coating properties. This is especially the case for coating of safety relevant workpieces like turbine components or automotive brake rotors (Fig. 6) as there is need to control the production process securely and to maintain records of all relevant parameters during the coating of individual workpieces. Coating process variables including machine setpoints are complemented with sensor readings to provide real time analysis of the HVOF process. Key HVOF process variables, including combustion chamber pressure, powder feed hopper pressure, and net cooling power yield a snapshot of the process status and whether the process is being conducted within permissible limits. TABLE 3 — HVOF JOINING APPLICATIONS Application Substrate Coating Material 2 Comments Transition joints Aluminum AISI316L Case-hardened steel Unweldable materials Heat sinks Aluminum Copper Copper Extruded clad structure Complex shapes Case-hardened steel AgCuTi Si3N4 Ceramic-metal bonding Fig. 4 — Laser weld of a 16MnCr5 steel workpiece and an aluminum alloy workpiece with K2 sprayed AISI316L transition joint coating. Fig. 5 — K2 spraying of an active filler coating on a workpiece with hyperbolic tip. 6 FEATURE
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