Laser Additive Manufacturing of High-Performance Materials

Different from conventional materials removal method, additive manufacturing (AM) is based on a novel material incremental manufacturing philosophy. Laser-based AM implies layer-by-layer shaping and consolidation of feedstock, typically powder materials, to arbitrary configurations, using a computer controlled laser as energy resource. The current development focus of AM is to produce complex-shaped functional metallic components, including metals, alloys, and metal matrix composites (MMCs), to meet the demanding requirements from aerospace, defense, automotive, and biomedical industries. In this chapter, the development history of AM technology is briefly introduced and the nomenclature principles for naming different types of AM processes are reviewed. The general processing philosophy of AM is addressed and the typical applications of AM technology are presented.

Laser sintering (LS), laser melting (LM), and laser metal deposition (LMD) are presently regarded as the three most versatile laser-based additive manufacturing (AM) processes. Laser-based AM processes generally have a complex nonequilibrium physical and chemical metallurgical nature, which is material- and process-dependant. The influence of material characteristics and processing conditions on the metallurgical mechanisms and resultant microstructural and mechanical properties of AM-processed components needs to be clarified. This chapter starts with the definition of LS/LM/LMD processes and operative consolidation mechanisms for metallic components. Powder materials used for AM, in the categories of pure metal powder, prealloyed powder, multi-component metals, alloys, metal matrix composites (MMCs) powder, and associated densification mechanisms during AM are addressed. An in-depth review of material and process aspects of AM, including the physical aspects of materials for AM and the microstructural and mechanical properties of AM-processed components, is presented. The purpose of this chapter is to establish a general relationship among material, process, and metallurgical mechanism for laser-based AM of metallic components.

Selective laser melting (SLM) additive manufacturing (AM) process was used to produce nanocrystalline TiC-reinforced Ti matrix bulk-form nanocomposites. The influences of laser energy density on densification activity, microstructural feature, nanohardness, and wear behavior of SLM-processed parts were comprehensibly studied to improve the controllability SLM process of nanomaterials. The TiC reinforcement in SLM-processed nanocomposites typically had a unique nanoscale lamellar structure, which was distinctly different from the initial particulate morphology before SLM. Reasonable physical mechanisms and conditions for the formation of TiC nanostructure reinforcing phase during SLM process were proposed. The microstructural and mechanical properties of SLM-processed TiC/Ti nanocomposite parts were sensitive to the preparation method of the starting nanocomposite powder and the content of TiC nanoparticles. The optimally processed TiC/Ti nanocomposite parts by SLM demonstrated the significantly elevated microhardness and wear performance as relative to the unreinforced Ti parts.

Based on an integrated processing method of “Designed materials,” “Laser-induced in situ reaction,” and “Tailored mechanical performance,” theTiC/Ti₅Si₃ and TiN/Ti₅Si₃ composite parts were produced by selective laser melting (SLM) additive manufacturing (AM) process, starting from the high-energy ball-milled SiC/Ti and Si₃N₄/Ti powder systems. The influence of the applied laser energy density on the densification behavior, microstructural features, microhardness, and wear property of in situ Ti₅Si₃-based composites was studied. The occurrence of balling phenomenon at a low-laser energy density combined with a high-scan speed and the formation of thermal cracks at an excessive laser energy input generally decreased densification rate. The in situ formed TiC and TiN reinforcing phases experienced a successive morphological change as the SLM processing conditions varied. The underlying metallurgical mechanisms accounted for the different growth mechanisms of TiC and TiN reinforcement during SLM process were disclosed. The in situ TiC/Ti₅Si₃ and TiN/Ti₅Si₃ composite parts prepared under the optimal SLM conditions had a near-full density, a significantly elevated microhardness (> 980 HV), a considerably low coefficient of friction (< 0.2), and a reduced wear rate. The enhanced wear resistance was attributed to the formation of adherent strain-hardened tribolayer covered on the worn surface.

Selective Laser Melting (SLM) of a W–Ni–graphite powder mixture was performed to prepare in situ WC-cemented carbide-based hardmetals parts, using two different types of CO₂ laser and fiber laser. The WC phase was developed via a multi-laminated growth mechanism and it experienced block-shaped triangular—elliptical morphological change on decreasing the linear laser energy density of the applied CO₂ laser. As the fiber laser was used, the in situ formed WC crystals generally had a triangular microstructure. An increase in the applied linear laser energy density, which was realized by increasing laser power or decreasing scan speed, resulted in the coarsening of in situ WC crystals in both side length and thickness. The SLM additive manufactured cemented carbide based hardmetals parts possessed a sufficiently high densification level of 96.3 % and a maximum microhardness of 1870.9 HV_0.1. The dominant metallurgical mechanisms responsible for the variations of microstructural and mechanical properties of SLM-processed hardmetals parts were proposed.

The nanoscale TiC particle-reinforced AlSi10Mg nanocomposite parts were produced by SLM process. The influence of the SLM processing parameters, especially the “linear laser energy density” (LED), on densification behavior, microstructural evolution, and mechanical properties of SLM-processed nanocomposites were studied. Using an insufficient LED lowered the SLM densification due to the balling effect and the formation of residual pores. The nanostructured TiC reinforcement in SLM-processed parts experienced the significant microstructural variation as the applied LED changed. The sufficiently high densification rate combined with the homogeneous incorporation of nanoscale TiC reinforcement throughout the matrix led to the considerably low coefficient of friction (COF) and resultant wear rate. The obtained microhardness and tensile strength were apparently higher than the unreinforced SLM-processed AlSi10Mg part while maintaining the sufficient ductility. Both the insufficient SLM densification response at a relatively low LED and the disappearance of nanoscale reinforcement at a high LED lowered the hardness and wear performance of SLM-processed TiC/AlSi10Mg nanocomposite parts.

Selective laser melting (SLM) of the SiC/AlSi10Mg composites was performed to prepare the Al-based composites with the multiple reinforcing phases. The influence of the SLM processing parameters on the constitutional phases, microstructural features, and mechanical performance of the SLM-processed Al-based composites was studied. The reinforcing phases in the SLM-processed Al-based composites included the unmelted micron-sized SiC particles, the in situ formed micron-sized Al₄SiC₄ strips, and the in situ produced submicron Al₄SiC₄ particles. As the input “linear laser energy density” (LED) increased, the extent of the in situ reaction between the SiC particles and the Al matrix increased, resulting in a larger degree of formation of Al₄SiC₄ reinforcement. The densification rate of the SLM-processed Al-based composite parts increased as the applied LED increased. A sufficiently high density (~ 96 % theoretical density) was achieved for LED larger than 1000 J/m. Due to the generation of the multiple reinforcing phases, elevated mechanical properties were obtained for the SLM-processed Al-based composites, showing a high microhardness of 214 HV_0.1, a considerably low coefficient of friction (COF) of 0.39, and a reduced wear rate of 1.56 × 10⁻⁵mm³N⁻¹m⁻¹. At an excessive laser energy input, the grain size of the in situ formed Al₄SiC₄ reinforcing phase, both the strip- and particle-structured Al₄SiC₄, increased markedly. The significant grain coarsening and formation of the interfacial microscopic shrinkage porosity lowered the mechanical properties of the SLM-processed Al-based composites.

The interface design, material optimization, and process control during direct metal laser sintering (DMLS) additive manufacturing of (WC–Co)_p/Cu composite parts were performed. By means of the addition of WC reinforcing particles in the form of WC–Co composite powder, the design strategy and formation mechanism of the graded interface during DMLS were investigated to improve the interfacial bonding and material integrity between the WC reinforcement and the Cu matrix. The effects of both laser processing parameters (e.g., laser power, scan speed, and powder layer thickness) and WC–Co reinforcement content on densification behavior, microstructural features (e.g., the particle dispersion homogeneity and the interfacial bonding ability), and mechanical properties (e.g., microhardness and its distribution, tensile strength, and fracture surface morphology) of DMLS-processed (WC–Co)/Cu composite parts were studied comprehensively, in order to propose the effective process control and material optimization methods to improve microstructural and mechanical performance of DMLS-processed metal matrix composites (MMCs). A detailed investigation of the influence of rare earth (RE) La₂O₃ addition on densification and microstructures of DMLS-processed (WC–Co)/Cu composite was performed, thereby proposing a key additive material for the improvement of the laser processing ability of MMCs.

The densification behavior and attendant microstructural characteristics of the direct metal laser sintering (DMLS)-processed nano/micron W–Cu composites under different processing conditions were investigated. The methods for improving the controllability of laser processing were elucidated. A “linear energy density (LED)” parameter, which was defined by the ratio of laser power to scan speed, was used to tailor the powder melting mechanisms. It showed that using a suitable LED between ~ 13 and ~ 19 kJ/m combined with a scan speed less than 0.06 m/s led to a continuous melting of the Cu component, yielding a sound densification rate without any balling phenomenon. A “volumetric energy density (VED)” parameter was defined to facilitate the integrated process control by considering the combined effect of various processing parameters. It was found that setting the VED within ~ 0.6 and ~ 0.8 kJ/mm³ favored a better yield of high-density DMLS parts. The influence of Cu-liquid content on densification and microstructures of DMLS-processed nano/micron W–Cu was also studied. It showed that at a suitable Cu elemental content of 60 wt.%, a series of regularly shaped W-rim/Cu-core structures were formed after DMLS processing. The metallurgical mechanisms for the formation of such a novel structure were proposed.