UCLA

The objective of this study is to experimentally provide insights and guidance for rational design of laser additively manufactured high-performance metal matrix nanocomposites (MMNCs) for various applications. Laser additive manufacturing (LAM) has emerged as a popular metal manufacturing platform to accelerate novel material creation and build high performance products with complex geometries that traditional processes have been impossible to fabricate. However, there still exist great challenges in LAM of conventional metals and its alloys such as absence of porosities, poor surface morphologies or hot cracking, deteriorating the resulting material performance. MMNCs consisting two or more different phases give a potential opportunity to obtain enhanced material properties, suggesting a novel route for LAM to tackle the great challenges. Nevertheless, problems arise from agglomeration of nanoparticles and processing difficulties due to the introduction of secondary phase. In this dissertation, a wide variety of MMNCs were laser additively manufactured to experimentally study the nanoparticle effects on powder morphology, laser reflectivity, micro/nanostructure and resulting material performance, providing insightful processing routines for LAM of high-performance MMNCs.

The MMNC powder is one of the major factors for LAM to obtain a desired component. In this study, two fabrication techniques, i.e., nanoparticle self-assembly with assistance of ultrasonic processing or mechanical mixing, were used to produce MMNC powders including aluminum metal matrix nanocomposites (AMNCs), aluminum silicon alloy matrix nanocomposites (AlSi12-TiC), and copper matrix nanocomposites (Cu-WC). MMNC powders with different volume ratio (x) between nanoparticles, i.e., titanium carbide (TiC) or tungsten carbide (WC), and matrix, i.e., Al, AlSi12 or Cu, were prepared, including AMNC with x=0.25 and x=1, AlSi12-TiC with x=0.05; x=0.25, and Cu-WC with x=0.1, x=0.25; x=0.66, respectively. The reflectivity measurements of ultrasonic processed powders show a significant decrease in laser reflectivity at the wavelength of 1070 nm as the nanoparticle fraction increases. Moreover, the analysis of light scattering (LS) and scanning electron microscope (SEM) reveals that a uniform size distribution of ultrasonic processed powders. Nanoparticles were self-assembled at the surface of the matrix powders due to the favorable energy state. Internal microstructures revealed by focused ion beam (FIB) show a uniform distribution and good dispersion of nanoparticles throughout the matrix powders. In addition, to demonstrate the scalability, two different mechanical mixing techniques were developed to produce MMNC powders, namely, wet mechanical mixing and dry mechanical mixing. Whereas the powders produced via wet mechanical mixing show the laser reflectivity of the powders decreases as the nanoparticle fraction increases, while the reflectivity of dry mechanical mixed powder, i.e., Cu-WC (x=0.66), only exhibits a slight reduction due to the less nanoparticle coverage on the matrix copper. The powders (Al system) with a spherical shape and uniform size produced by wet mechanical mixing are similar to those by the ultrasonic processing, demonstrating a good scalability of the technique. For copper matrix system, more efforts are still needed to improve the powder morphology, size distribution, and nanoparticle dispersion and distribution inside the matrix. This study provides a scalable and low cost route for mass production of MMNC powders with high loadings of nanoparticle for LAM.

Experimental studies on LAM of two types of AMNC powders were carried out to investigate the nanoparticle effects on micro/nanostructure and material performance. Assembled powders by both ultrasonic processing and mechanical mixing, were additively manufactured by laser melting using a customized laser additive manufacturing system. AMNCs (with 17 vol.% TiC and 35 vol.% TiC) were successfully laser deposited via laser melting. The material performance shows that the Young’s modulus, yield strength, and hardness of the AMNCs increase as the nanoparticle fraction increases. The AMNC (35 vol.% TiC) delivers a yield strength of up to 1.0 GPa, plasticity over 10 %, and Young’s modulus of approximately 200 GPa. The AMNC (35 vol.% TiC) offers unprecedented performance in terms of specific yield strength, specific Young’s modulus, and elevated temperature stability at 400 �C amongst all aluminum alloys. The exceptional mechanical properties are attributed to high density of well-dispersed nanoparticles, strong interfacial bonding between nanoparticles to aluminum, and ultrafine grain sizes (approximately 331 nm). Additionally, AMNC (15 vol.% TiC) sample was laser deposited via melting of powders produced by the mechanical mixing, offering comparable mechanical properties to that of AMNC (17 vol.% TiC). The study paves a new pathway for laser additive manufacturing of nanoparticles reinforced aluminum for widespread applications.

To achieve comparable mechanical properties of AMNCs, laser additive manufactured AlSi12 matrix nanocomposites, i.e., AlSi12-TiC (x=0.05 and x=0.25), were successfully produced. Micro/nanostructure analysis shows that the grain size of AlSi12-TiC nanocomposites decrease as the fraction of incorporated nanoparticles increases. Additionally, chemical reaction products, i.e., SiC nanoparticles and Al3Ti intermetallic phase, have been identified and observed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The microhardness and Young’s modulus of the laser deposited AlSi12-TiC (x=0.25) were increased to 578 � 42.5 HV and 187.73 � 28 GPa, respectively, showing comparable properties to that of AMNC (35 vol.% TiC), i.e., 330.3 � 30.6 HV and 197 � 27 GPa. The improved results can be attributed to the dispersed nanoparticles and reaction products. This research suggests a new design route to directly deposit high performance aluminum alloys by benefiting from the strengthening effects of the minor phase(s) in alloy while decreasing the amount of incorporated nanoparticles.

The experiments on LAM of Cu matrix nanocomposites were carried out to explore the feasibility on high performance copper materials. While a great number of porosities with ball-liked morphologies appeared after laser melting of the powders on a pure copper substrate, good layer uniformity and densification of the additively manufactured samples were obtained by replacing the pure Cu with nickel or as-cast MMNC substrate, mainly because of less thermal conductivity difference and good wettability between the powders and substrates. The internal microstructures exhibit a uniform nanoparticle distribution but some nanoparticle agglomeration exists in the matrix. The grain structure of laser deposited samples has refined by the laser induced rapid solidification rate and incorporated nanoparticles, showing a smaller grain size than that of as-cast MMNC samples. The study experimentally demonstrates a feasible processing way to directly laser deposit dense Cu matrix nanocomposites.

In summary, extensive experimental studies presented in this dissertation have demonstrated various feasible processing methods of LAM to produce high-performance MMNC.

A wide variety of laser deposited MMNCs produced in this study can provide insights and guidance to LAM on powder fabrication (nanoparticle selection, volume fractions, reflectivity, size and morphology, and scalability) and processing/microstructure/properties relationships. This study also advances the knowledge base for rational design of high-performance MMNCs with desirable properties for various applications.