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 7 Fig. 1 — Schematic diagram of field assisted sintering technique (FAST). to sinter the product with nearly 100% of theoretical density. It is also possible to retain a subgrained microstructure in the sintered product, which offers su- perior physical and mechanical prop- erties, attributes not cost effectively achievable using conventional compac- tion and sintering methods. In addition, FAST offers significant processing ener- gy savings of 30-40%. Penn State is the only academic institution in the U.S. that has three FAST units (25, 250, and 325 ton capaci- ties), capable of producing plates up to 13.5 inches in diameter and 4 to 9 inch- es high (Fig. 2). PRODUCTION OF Al-SiC ALLOY MATRIX COMPOSITES This article discusses the fabrica- tion of Al-SiC composites using FAST. Materials include: • Al (base line): 2.8 g/cm 3 density • 6061 Al alloy (Al-5.8Fe-3.8Cr-3.3Ti): 2.8 g/cm 3 density (ECKA powder supplied by SCM Metal Products) • 6061 Al alloy + 20% SiC (0.7 µm particle size): 2.8 g/cm 3 density (AMC620XF) • 6061 Al alloy + 40% SiC (0.3 µm particle size): 2.9 g/cm 3 density (AMC620XF) A great demand exists for weight re- duction of components in sectors such as transportation, commu- nication, satellites, infrastructure, and energy. The biggest payoff is in the ener- gy sector, which includes aerospace and automotive applications that are able to achieve weight savings—as these reduc- tions translate to fuel efficiency, higher payload capacity, and less wear and tear. Among the most common materials in these sectors are aluminum alloys. How- ever, conventionally developed Al alloys using precipitation hardening along with extended solid solubility and mechani- cal alloying have reached their optimum performance. Powder metallurgy (PM) meth- ods have produced a new generation of high performance aluminum alloy ma- terials with significant improvements in strength and wear properties. Powders are consolidated into feedstock materi- als via hot pressing (HP) and hot isostat- ic pressing (HIP) to produce fully dense products before hot extrusion and roll- ing. Current aluminum alloy powders are frequently restricted to use in low- stress applications such as automotive cam caps and power tools. Further, the mechanical properties of existing alu- minum alloy powders do not meet the requirements for a wide range of more demanding applications. Development of metal matrix composites (MMCs) such as reinforced aluminum and titanium matrix com- posites offers a further improvement in properties, but they are limited to use in high performance, high profile govern- ment programs and a few commercial applications associated with high man- ufacturing costs [1, 2] . MMCs also include materials such as directionally solidified eutectic al- loys, oxide dispersion strengthened al- loys, and lamellar structures in alloys. There has been limited research on alu- minum-base metal matrix composites. This article addresses new opportuni- ties for the development and applica- tion of aluminummatrix composites, or AMCs. It is well documented in the liter- ature that AMCs offer a superior com- bination of properties to those of rival monolithic Al alloys. However, AMCs have not yet captured their full application potential. The primary AMC in- dustrial scale manufactur- ing processes comprise liq- uid state processing includ- ing stir casting, infiltration, spray casting, and in situ (reactive) processing, as well as solid state process- ing (PM processing) where Al powder is blended with particulates or short fibers. PM processing offers high flexibility in terms of load- ing powder with a range of size, shape, and vol- ume fractions of second- ary phases (including fibers and particulates). However, the main shortcoming is the segregation of secondary materials such as fiber or particulate, which leads to unpredict- able performance. This problem must be addressed to realize the full applica- tion potential. This article discusses an innova- tive approach to cost effectively man- ufacture AMC net-shape components and feedstocks using the field assisted sintering technology (FAST) process. FIELD ASSISTED SINTERING TECHNOLOGY Compared with hot pressing and hot isostatic pressing, FAST is a relative- ly newmanufacturing technology capa- ble of compacting and sintering powder materials (metals, ceramics, and com- posites) in a very short time to near the- oretical density. Premixed powder is placed into a mold and compacted at a rapid heating rate (up to 500°C/min) by concurrently applying high density cur- rent (10,000 A), high pressure (75 MPa), and temperature up to 2400°C, which deforms and fuses particles together by concurrent volumetric heating (Joule heating) and external radiant heating (Fig. 1). This lowers the sintering tempera- ture by 150-200°C and activation energy by 30-40%. The processing cycle takes about two to 10minutes at temperature