Unlike conventional materials removal methods, additive manufacturing (AM) is based on a novel materials incremental manufacturing philosophy. Additive manufacturing implies layer by layer shaping and consolidation of powder feedstock to arbitrary configurations, normally using a computer controlled laser. The current development focus of AM is to produce complex shaped functional metallic components, including metals, alloys and metal matrix composites (MMCs), to meet demanding requirements from aerospace, defence, automotive and biomedical industries. Laser sintering (LS), laser melting (LM) and laser metal deposition (LMD) are presently regarded as the three most versatile AM processes. Laser based AM processes generally have a complex non-equilibrium physical and chemical metallurgical nature, which is material and process dependent. The influence of material characteristics and processing conditions on metallurgical mechanisms and resultant microstructural and mechanical properties of AM proc...

Laser additive manufacturing of metallic components: materials, processes and mechanisms

The core/rim structures of compounds in the Ti(C,N)-xWC-20Ni system have been investigated to determine the effect of WC and nitrogen content on the microstructure of the system. In addition, the relative dissolution rate of WC to Ti(C,N) in the system was studied by analyzing the rim compositions. Variations in the WC content had a much-lesser influence, as an additive, on the general microstructure than that of other carbides that have been used in previous studies. However, the nitrogen content in Ti(C1−XNX) had a significant effect on its microstructure. The composition of the rim structure was determined by the ratio of solutes that were dissolved in the liquid binder, under the given processing conditions. The dissolution rate of WC was ∼2 and 5 times faster than that of Ti(C,N) in the system at temperatures of 1300° and 1510°C, respectively. The results have been interpreted in terms of phase stability and precipitation phenomena.

Formation of Core/Rim Structures in Ti(C,N)‐WC‐Ni Cermets via a Dissolution and Precipitation Process

Laser metal deposition (LMD) additive manufacturing process was applied to produce ultrafine TiC particle reinforced Inconel 625 composite parts. The effects of laser energy input per unit length (LEIPUL) on microstructure development, densification response, and mechanical performance including wear performance and tensile properties were comprehensively studied. A relationship of processing conditions, microstructural characteristics, mechanical performance, and underlying strengthening mechanisms was proposed for a successful LMD of high-performance Inconel based composite parts. It revealed that using an insufficient LEIPUL of 33 kJ/m lowered the densification behavior of LMD-processed parts, due to the appearance of residual large-sized pores in inter-layer areas of the parts. An increase in LEIPUL above 100 kJ/m yielded the near fully dense composite parts after LMD. On increasing LEIPUL, the TiC reinforcing particles became significantly refined and smoothened via the elevated melting of particle surfaces and the dispersion state of ultra-fine reinforcing particles was homogenized due to the efficient action of Marangoni flow within the molten pool. The dendrites of Ni–Cr γ matrix underwent a successive change from an insufficiently developed, disordered microstructure to a refined, ordered microstructure with the increase of LEIPUL. However, the columnar dendrites of the matrix were coarsened apparently at an excessive LEIPUL of 160 kJ/m because of the elevated thermalization of the input laser energy. The formation of the refined columnar dendrites of Ni–Cr γ matrix combined with the homogeneously distributed ultra-fine reinforcing particles contributed to the enhancement of wear performance of LMD-processed composites with a considerably low coefficient of friction (COF) of 0.30 and reduced wear rate of 1.3×10 −4  mm 3 /N m. The optimally prepared TiC/Inconel 625 composite parts demonstrated a ductile fracture mode with a sufficiently high tensile strength of 1077.3 MPa, yield strength of 659.3 MPa, and elongation of 20.7%. The superior tensile properties of LMD-processed parts were attributed to the significant grain refinement effect of the matrix during laser processing and the efficient prohibition of ultrafine reinforcing particles on the mobility of dislocations.

Laser additive manufacturing of ultrafine TiC particle reinforced Inconel 625 based composite parts: Tailored microstructures and enhanced performance

Effect of Mo addition on the microstructure and mechanical properties of ultra-fine grade TiC–TiN–WC–Mo2C–Co cermets was studied in this work. Mechanical properties such as transverse rupture strength, fracture toughness and hardness were also measured. Results show that the microstructure exists in black core/grey rim structure and white core/grey rim structure, and the microstructure has an obvious trend to become finer with the increase of molybdenum content. When the added Mo exceeds 10%, ultra-fine TiC-based cermet with an average particle size of less than 0.5 μm is obtained, because of the formation of a Mo-rich rim and the improvement of the wettability between ceramic phase and metallic phase. The transverse rupture strength increases with the increase of Mo content, and the maximum values of the hardness and the fracture toughness were found with 10 wt% and 5 wt% Mo addition, respectively.

Effect of Mo addition on the microstructure and mechanical properties of ultra-fine grade TiC–TiN–WC–Mo2C–Co cermets

HfC, TaC, or WC were individually added to a Ti(C0.7N0.3)–30 wt% Ni system, in order to investigate microstructural changes and the dissolution behaviors of Ti(C0.7N0.3) and the carbides. Of these systems, the Ti(C0.7N0.3)–WC–Ni system proved to be the most favorable for the refinement of microstructure. The fraction for the cross-sectional area of the Ti(C0.7N0.3) cores, which is related to the amount of Ti(C0.7N0.3) dissolved, increases and the thickness of solid solution rims decreases in the order of HfC, TaC, and WC added. This finding indicates that the dissolution rate of Ti(C0.7N0.3) in a Ni melt is the lowest when WC is added. In addition, it was found that the average dissolution rate of Ti(C0.7N0.3) in the HfC-containing system is ∼1.6 and 1.9 times higher than those for Ti(C0.7N0.3) in the TaC- and WC-containing systems at the same sintering conditions. Further, the dissolution rates of TaC and WC were ∼80% of the rate found for HfC during the formation of the outer rim.

Effect of various carbides on the dissolution behavior of Ti(C0.7N0.3) in a Ti(C0.7N0.3)–30Ni system

Ti(CN) based cermets have a core/shell microstructure, which gives rise to outstanding mechanical properties and chemical stability for cutting tool applications. The HfC content and C/N ratio in Ti(C1-x Nx ) were varied in order to investigate the effect of HfC addition on the microstructure of Ti(C1-x Nx )–Ni systems. The presence of HfC in the solid solution, (Ti,Hf)C, was found to cause the surface energy to be more anisotropic in the presence of liquid Ni. A phase separation of (Ti,Hf)C and (Hf,Ti)C was noted when >20 wt-%HfC was added to TiC–HfC–Ni. In addition, the Ti(C0·3 N0·7 ) based system showed the lowest coarsening for all sintering times of the various Ti(C1-x Nx ) systems examined. This study focuses on the role of various Ti(CN) and HfC cermets and how they affect the coarsening of microstructures.

Effect of HfC addition on microstructure of Ti(CN)-Ni cermet system

The effect of the laser engineered net shaping (LENS) process on the microstructure and mechanical properties of (Ti,W)C–Ni solid solution cermets was investigated. Nano-(Ti,W)C–Ni powder was synthesised, spray dried and densified by two different processes: sintering and LENS. Furthermore, all results were compared with conventional micrometre sized TiC–WC–Ni cermets. Solid solution (Ti,W)C–Ni cermets made by sintering resulted in a coreless and ultra fine microstructure, while the LENS processed specimens revealed inhomogeneous microstructure and particle coarsening. The hardness values of the LENS processed specimens range from 15 to 20 GPa, which is higher than for the ultra fine and conventional sintered composites (∼12 GPa). The measured toughness values show greater variability.

S. Kang

Papers

Effect of HfC addition on microstructure of Ti(CN)-Ni cermet system

(Ti,W)C–Ni cermets by laser engineered net shaping