Joinability of nickel alloy insert built by laser metal deposition to mild steel via compound casting

• Main Author: Lee, Brian Zhan Rui
• Other Authors: Li Hua
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Materials::Metallic materials::Alloys; Engineering::Materials::Material testing and characterization; Engineering::Materials::Mechanical strength of materials; Engineering::Computer science and engineering::Computing methodologies::Simulation and modeling; Engineering::Manufacturing
• Online Access: https://hdl.handle.net/10356/169549
• Institution: Nanyang Technological University

Internal cladding is almost impossible for parts where their internal geometry is too small or complex to fit cladding equipment. They end up pre-formed completely with high-grade alloys, incurring unfavorable cost even when manufactured with inexpensive traditional methods like casting. Due to this dilemma, a re-engineered process is proposed to sand cast low-grade WCB mild steel over a Nickel superalloy Inconel 718 (IN718) insert pre-built by Laser Metal Deposition (LMD), an additive manufacturing (AM) technique, utilizing waste heat normally dispersed by the sand mold to fuse both materials together. Referred to as Compound Casting (CC), fusion of multi-materials is possible but the process has not yet been explored with AM technologies that enable the internal joining of geometrically complex superalloy inserts. Little is known regarding the joinability of such inserts and their fused characteristics under CC. Thus, research effort is focused on conducting novel first time studies to address these gaps. The first achievement is the original study of CC with an insert comprising of AM- LMD microstructure to discover how its unique traits affect the final IN718-WCB interface. Baseline LMD IN718 microstructure was characterized through a single-track-to-density-cube central composite design. Layer-wise interconnected porosities and cracks with low relative densities (97.1-98.7%) were observed that had little dependency on common AM parameters. Under optimal CC conditions, only IN718 grains at the interface were affected by fusion while the majority retained their original coarse epitaxial form. The abrupt transition from IN718’s original LMD columnar grains to its new CC recrystallized fine chill grains is a prominent dislocation pile-up junction under stress. This characteristic makes the IN718 side of the interface more susceptible to failure despite its superior mechanical properties to WCB. The second achievement is the demonstration of internal joining of a superalloy to common cast metal via the CC method for the first time without compromise of mechanical properties. Two new single dedicated process heat treatment options were developed and successfully meet both steel and superalloy microhardness specifications for sour service. A functionally passing UTS of 491.5 MPa was demonstrated with 100% joint efficiency for the CC internal joining of WCB-IN718. To delineate the origination of defects, unclear due to the multiple manufacturing processes, rigorous comparison of below-par tensile fracture specimens from LMD and CC was conducted. The most terminal LMD build-up defect was rampant oxidation causing severe embrittling while a novel phenomenon was observed that the presence of liquation dendrites, a signature weakness of Nickel-based superalloys, was secondary to the overall poor performance of UTS of multi-material specimens. In actuality, voids and dendrites observed on the fracture surface belong to shrinkage from the CC process and are the main cause of premature failure. The third achievement is the development of a proof-of-concept cycle to identify if an interfacial thermal processing window (TPW) for joining IN718 to WCB is attainable while simultaneously mitigating porosity in the cast metal. Instead of studying just the former, in this cycle, the cast design is continuously modified until both the hypothetical interface temperature and casting integrity is achieved by finite element (FE) simulation. It was found that additional feeding to reduce solidification porosity resulted in excessive interfacial temperatures. Empirically, simulated thermophysical profiles were reproduced with stark similarity and a feasible TPW for joining IN718 to WCB was identified. As actual compatibility of material fusion of the TPW is indeterminable from FE, Molecular Dynamics simulation was explored with promising agreement to TEM evidence.