February AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | F E B R U A R Y / M A R C H 2 0 1 9 1 9 producing rods, bars, and Z-bars for various applications. Feedstock discs could also be stacked and fused to- gether by FAST to make one large billet. SiC particles were distributed uniform- ly within the Al matrix as shown by el- emental mapping of the composite (Fig. 4). Examination of the SiC-Al matrix interface using a high-resolution trans- mission electron microscope shows a very thin ( ~ 2 nm) amorphous layer around the SiC particles (Fig. 5). Forma- tion of the amorphous phase indicates good metallurgical bonding between the SiC particles and the matrix. Ele- mental mapping of the interface shows that the amorphous region is rich in Mg, Cu, and O due to migration of these el- ements from the matrix under the in- fluence of electric current, and perhaps contributing to formation of the amor- phous phase. Good bonding (i.e., no debonding) exists between the Al ma- trix and SiC particles even though Al has a low melting temperature (660°C) and large coefficient of thermal expan- sion (CTE) while SiC has a high melting temperature and low CTE. MECHANICAL PROPERTIES Hardness was measured on all sintered samples (Al, Al alloy, and Al- SiC composites) processed under the same sintering temperatures and con- ditions. Elemental aluminum has a very low hardness (VHN 31) compared with VHN 131 for Al alloy and even higher hardness for Al alloy with increasing SiC content (20-40 vol%), reaching VHN 207 (a 60% increase over that of Al) at 40% SiC (Fig. 6). This could be achieved only with FAST processing. Tensile properties were measured from the feedstock discs. Tensile test- ing was conducted at a strain rate of 1.376 mm/min, and all tensile sam- ples fractured within the gauge length. Elastic modulus was measured by ul- trasound and by mechanical tensile testing. The elastic modulus of Al alloy with 40 vol% SiC increased by about 30% over the base line Al alloy (Fig. 7). Yield and tensile strengths of the sin- tered products are shown in Figs. 8 and 9, respectively. Yield strength and tensile strength of Al-40% SiC is in- creased over the Al alloy by 80% and 50%, respectively. SEM examination of the matrix of the fractured surface of Al-25% SiC shows a significant volume frac- tion of dimples in the fractured surface around the SiC particles. A sim- ilar fracture mode was observed in the Al-40% SiC composite. The matrix in the fractured region is elongated and de- formed around the SiC particles before fracture. The fracture mode propagates well within the matrix before break- down of the sample. It appears that the matrix is stretched under tension and cracks propagate around SiC particles before rupture. Crack opening size in- creases from inside the matrix toward the outside of the fractured surface. FABRICATION OF COMPONENTS Figure 10(a) shows the capabil- ity of FAST in manufacturing net-shape components from powder to product Fig. 4 — SEM photographs showing uniformdistribution of SiC in Al matrix. Fig. 5 — High-resolution transmission electron microscope photograph of the interface between the SiC and Al matrix reveals a very thin (~1 nm thick) region of amorphous phase. Fig. 6 — Vickers hardness of Al and Al-SiC composites as function of SiC volume fraction produced by FAST (sintered at 550°C and 45 MPa for 10 min).