Kon-Bae Lee

In the present study, the composite sheets were produced by powder rolling the ball-milled Al/SiC mixture. The effect of milling speed (350–550 rev min^–1) and milling duration (8 and 12 h) on the Al particle morphology and subsequent mechanical properties were examined for composites with different volume fractions of SiC. From the microstructural investigation, SiC particles are evenly distributed and well dispersed in the Al matrix; those are prepared under higher milling speed (550 rev min^–1) and higher milling time (12 h). Furthermore, the flattened-shaped particle morphology was obtained under lower milling conditions, whereas the reduced-granule shape was obtained under higher milling conditions (550 rev min^–1, 12 h). Consequently, the Al/SiC composite sheets made from a granule-shaped Al/SiC mixture show higher density, microhardness, and tensile strength due to significant consolidation and SiC uniformity. In addition, the tensile strength of 430.26 MPa has been achieved for the Al/2vol.-%SiC sheet compared to pure Al (393.49 MPa) under similar processing conditions.

This study compares the tensile properties of commercial aluminum matrix composites (AMCs) with those of AMCs produced via a nitrogen-induced self-forming process. This process is a newly developed AMCs manufacturing process that takes advantage of the price competitiveness and productivity of large-scale products produced via the liquid process. Additionally, this process has the freedom of choice of the reinforcement phase and the homogeneous dispersibility of the powder process. Compared to commercial monolithic 6061 alloys, 6061 aluminum alloy matrix composites exhibit increased Young’s modulus, yield strength, and ultimate tensile strength by 59%, 66%, and 81%, respectively. This study also compares the tensile properties of AMCs with different matrix compositions, including 2009 and 7050 aluminum alloys. The study shows that AMCs produced using the nitride-induced self-forming aluminum composite (NISFAC) process exhibit comparable or superior tensile properties to those obtained using existing commercial powder metallurgy (P/M) processes.

This paper investigates the effect of the size and volume fraction of SiC, along with that of the processing temperature, upon the nitridation behavior of aluminum powder during the nitridation-induced self-formed aluminum composite (NISFAC) process. In this new composite manufacturing process, aluminum powder and ceramic reinforcement mixtures are heated in nitrogen gas, thus allowing the exothermic nitridation reaction to partially melt the aluminum powder in order to assist the composite densification and improve the wetting between the aluminum and the ceramic. The formation of a sufficient amount of molten aluminum is key to producing sound, pore-free aluminum matrix composites (AMCs); hence, the degree of nitridation is a key factor. It was demonstrated that the degree of nitridation increases with decreasing SiC particle size and increasing SiC volume fraction, thus suggesting that the SiC surface may act as an effective pathway for nitrogen gas diffusion. Furthermore, it was found that effective nitridation occurs only at an optimal processing temperature. When the degree of nitridation is insufficient, molten Al is unable to fill the voids in the powder bed, leading to the formation of low-quality composites with high porosities. However, excessive nitridation is found to rapidly consume the nitrogen gas, leading to a rapid drop in the pressure in the crucible and exposing the remaining aluminum powder in the upper part of the powder bed. The nitridation behavior is not affected by these variables acting independently; therefore, a systematic study is needed in order to examine the concerted effect of these variables so as to determine the optimal conditions to produce AMCs with desirable properties for target applications.

The nitridation-induced self-formed aluminum matrix composite (NISFAC) process is based on the nitridation reaction, which can be significantly influenced by the characteristics of the starting materials (e.g., the chemical composition of the aluminum powder and the type, size, and volume fraction of the ceramic reinforcement) and the processing variables (e.g., process temperature and time, and flow rate of nitrogen gas). Since these variables do not independently affect the nitridation behavior, a systematic study is necessary to examine the combined effect of these variables upon nitridation. In this second part of our two-part report, we examine the effect of nitrogen flow rates and processing temperatures upon the degree of nitridation which, in turn, determines the amount of exothermic reaction and the amount of molten Al in the nitridation-induced self-formed aluminum matrix composite (NISFAC) process. When either the nitrogen flow rate or the set temperature was too low, high-quality composites were not obtained because the level of nitridation was insufficient to fill the powder voids with molten Al. Hence, since the filling of the voids in the powder bed by molten Al is essential to the NISFAC process, the conditions should be optimized by manipulating the nitrogen flow rate and processing temperature.