Aluminum Matrix Composites Manufactured using Nitridation-Induced Self-Forming Process

Abstract

Conventional manufacturing processes for aluminum matrix composites (AMCs) involve complex procedures that require unique equipment and skills at each stage. This increases the process costs and limits the scope of potential applications. In this study, a simple and facile route for AMC manufacturing is developed, a mixture of Al powder and the ceramic reinforcement is simply heated under nitrogen atmosphere to produce the composite. During heating under nitrogen atmosphere, the surface modification of both Al and the reinforcement is induced by nitridation. When the oxide layer covering Al powder surface is transformed to nitrides, temperature in the local region increases rapidly, resulting in a partial melt of Al powder. The molten Al infiltrates into the empty space among Al powder and reinforcement, thereby enabling consolidation of powders without external forces. It is possible to fabricate AMCs with various types, sizes, volume fractions, and morphologies of the reinforcement. Furthermore, the manufacturing temperature can be lowered below the melting point of Al (or the solidus temperature for alloys) because of the exothermic nature of the nitridation, which prevents formation of un-wanted reactants. The relative simplicity of this process will not only provide sufficient price competitiveness for the final products but also contribute to the expansion of the application scope of AMCs.

Effect of Material and Process Variables on Characteristics of Nitridation-Induced Self-Formed Aluminum Matrix Composites—Part 2: Effect of Nitrogen Flow Rates and Processing Temperatures

“…When heated in argon, the maximum temperature inside the bed was 631 • C, which is lower than the programmed furnace target temperature of 640 • C but higher than the solidus of the Al 6061 alloy (582 • C), hence melting still occurred. However, unlike the powder heated in nitrogen, little shrinkage was observed after solidification ( Figure 1d) and molten Al alloy leaked out of the bed surface ( Figure 1e) due to the poor wettability between the molten Al and SiC [2,3] as in the previous studies. Hence, high-quality composites were not produced in this case.…”

“…We recently developed the nitridation-induced self-formed aluminum composite (NISFAC) process as a facile and innovative route to manufacturing Al matrix composites (AMCs) [1][2][3]. Whereas, previously, the limited wettability between the reinforcement and the Al matrix had to be overcome by the use of high-energy mechanical stirring (e.g., stir casting) [4][5][6][7][8][9][10], high-pressure infiltration of molten Al into a preform [11][12][13][14][15][16], or high-pressure consolidation of the powder mixture (powder metallurgy) [16][17][18][19][20][21], a key feature of the NISFAC process is its flexibility with respect to the selection of reinforcement regardless of the level of wettability [1][2][3]. Although the nitridation of aluminum has been investigated by a variety of in-situ techniques such as directed metal oxidation [22], pressureless metal infiltration [23], and reactive gas injection [24], etc., there have been limited studies on the nitridation-induced forming of aluminum matrix composites.…”

“…Although the nitridation-based NISFAC process is much simpler and easier than conventional processes, the degree of nitridation and, hence, the amount of molten Al generated during the process, is influenced by all relevant variables, including the composition of the Al matrix, the type, size, and volume fraction of reinforcement, and the fabrication temperature and time [1][2][3]. Therefore, it is essential to optimize the process conditions for the selected Al matrix and reinforcement (or filler) system.…”

Effect of Material and Process Variables on Characteristics of Nitridation-Induced Self-Formed Aluminum Matrix Composites—Part 1: Effect of Reinforcement Volume Fraction, Size, and Processing Temperatures

“…As a result, the degree of nitridation also decreased from 49.8% to 31.3% (Table 1). In previous work, we reported that the degree of nitridation and resultant powder-bed temperature change varied significantly according to the volume fraction of the reinforcement [16,[18][19][20][21]. Before heating, the aluminum powder in the powder bed was covered with an amorphous Al2O3 layer as shown in Figure 1a.…”

“…In a previous study, we developed a new process for AMCs (namely, the nitridation-induced self-formed aluminum composite or NISFAC process) [16,17], which overcomes the issue of poor wettability between the aluminum and the ceramic reinforcement. In this process, composites can be produced for any type, size, and volume fraction of reinforcement simply by heating a mixture of aluminum powder and ceramic reinforcement under a nitrogen atmosphere in a sealed furnace to prevent the ingress of air.…”