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Aluminum spray forming is a rapid solidification process that results in a fine and homogeneous microstructure with uniform distribution of micron-size silicon particles. SPRAY- Arvind Agarwal* Tim McKechnie* Tungsten cathode Plasma Processes Inc. Huntsville, Alabama I 44 Inert gas Plasma gun n the vacuum-plasma spray (VPS) forming process, powders are simultaneously melted and accelerated onto a rotating mandrel. Hypereutectic Al-Si alloys have been successfully vacuum-plasma spray formed into near net shapes, including thin sheets and thick cylinders. These products have potential for aerospace, automotive, and electrical applications. The spray-formed structure has high strength and refined microstructure with a fine and homogeneous distribution of silicon particles. Strength and stiffness can be tailored by varying the process variables. Hypereutectic aluminum-silicon alloys have been chosen for applications in aerospace, automobile, and electrical equipment because of their low density, high specific stiffness, high-temperature resistance, wear resistance, and low coefficient of thermal expansion. Some of these applications include high-performance automobile engine parts such as connecting rods, rocker arms, cylinder sleeves, piston rings, valve retainers, lightweight optics, and electronic packaging material for aerospace applications. Conventionally, hypereutectic Al-Si alloys (such as A390 containing 16.5 wt% Si) are processed by casting, but this results in growth of large columnar silicon grains, which causes problems with machinability. For example, the automotive industry has experienced difficulties during honing operations because of the large and angular silicon particles. Hypereutectic Al-Si alloys processed by powder metallurgy typically develop deleterious oxides. To overcome the limitations of casting and powder metallurgy processing routes, Osprey Metals Ltd. developed a spray deposition process in which a stream of molten metal/alloy is gas-atomized through a nozzle and deposited on a rotating mandrel. Spray forming is a rapid solidification process that results in a fine and homogeneous microstructure with uniform distribution of micron-size silicon particles. Homogeneous distribution of fine silicon particles translates to higher resistance to scuffing and wear, as well as improved machinability. In one application of the Osprey technology, Daimler-Benz has developed low-friction hypereutectic Al-Si cylinder liners that provide reduced weight and up to 30% lower oil consumption. However, the handling of liquid metal and its atomiza*Member of ASM International Powder feeder Powder feeder Main power supply Arc Copper anode Transfer arc power supply Plasma flame with high velocity molten particles VPS formed Al-Si Rotating mandrel Fig. 1 — Vacuum Plasma Spray (VPS) process illustration. An arc ionizes a gas stream of argon and hydrogen to form a >10,000 K plasma into which powders are fed. Powder material is heated and accelerated in either an inert or reactive environment to form a coating or structure on a rotating mandrel, which is later removed. Cylindrical mandrel shown can be replaced with a more complex shape. tion causes some difficulties during Osprey spray forming. The vacuum-plasma spray (VPS) technology was developed to overcome these difficulties, and has been adopted by Plasma Processes Inc. to fabricate near net shape structures of hypereutectic Al-Si alloys. This article explains the VPS process, describes the microstructural development of the spray formed alloy, and relates the properties to applications. Vacuum plasma spray Conventionally, thermal spray techniques (such as air plasma, vacuum plasma, high velocity oxyfuel) deposit bulk coatings on engineering components. However, free–standing, near net shape structures can also be fabricated by spray forming. Special nozzles have been designed (and patented) to simultaneously melt powders and accelerate the molten particles for deposition on a rotating mandrel. As shown in Fig. 1, the nozzle generates an arc that ionizes a gas stream, forming the plasma with temperatures above 10,000K. Powders are fed into the plasma, where they melt and are accelerated to supersonic speeds. A high electric potential directs the molten particles toward a rotating mandrel, where they deposit and rapidly cool, forming the correct shape by replicating the mandrel shape. Sprayed deposit is removed from the mandrel by mechanical or chemical means. ADVANCED MATERIALS & PROCESSES/MAY 2001 FORMING Aluminum Structures The VPS process can be tailored by varying the precursor materials, particle dimensions, plasma velocity, and mandrel temperature. The density of the deposited materials depends directly on the plasma velocity. The velocity controls the time that the particles are exposed to the heating zone, and the kinetic energy with which they impact the rotating mandrel/substrate. a b The plasma gun and the mandrel are Fig. 2 — Optical micrographs of the vacuum plasma sprayed hypereutectic Al-Si alloy: (a) ascomputer controlled, allowing fabrication of complex shapes. The high tem- sprayed and (b) as-sprayed under cooled conditions. perature of the plasma implies that virtually any material that can be melted is suitable for spraying to controlled thickness and geometries. The process results in a very effective use of precursor materials, because waste is minimal, parts are sprayed to near net shape, and little final machining is required. The process is especially appropriate for materials that are difficult to process b by conventional means or are difficult a to machine. Some of the other advan- Fig. 3 — Optical micrographs of (a) as-sprayed + hot pressed and (b) cast A 390 Al alloy. tages of spray forming by VPS include: • Compositional flexibility: Materials include metal Hypereutectic Al-Si alloys with processing histories matrix composites (MMC), ceramic matrix comPrimary silicon Microhardness, posites (CMC), and functionally gradient materials Processing history particle size, µm Density, % Knoop (FGM), • Minimal oxidation: The droplets are entrained As-sprayed ≤5 99.8 151 ±16 in the ionized gas, hence are not exposed to oxgyen. As-sprayed ~2 99.2 136 ±16 • High density: The small size of the silicon par(cooled) As-sprayed + 20 100 101 ±12 ticles results in high density. Hot Pressed • Refined microstructure: Rapid solidification enCast A 390 Al 50 100 102 ±8 ables fine microstructure. • Potentially lower cost of fabrication: Very little silicon particles (Fig. 2b), with the result that the waste means efficient use of materials. density of the as–sprayed structures is higher than Properties of spray formed structures 99%. Hot isostatic pressing of the as-sprayed alloy Rings and flat tensile specimens have been ma- results in coarsening of silicon particles up to 40 chined out of spray formed cylinders, indicating ex- mm (Fig. 3a). Eutectic silicon agglomerates into cellent machinability. The key innovation is the ability coarse particles and undergoes grain growth during to keep the silicon particles small and well distrib- the HIP operation. The microstructure of cast hypereutectic A-390 uted. The microstructure of the as-sprayed hypereutectic Al-Si alloy (Al-23 wt% Si) constitutes a homo- (Fig. 3b) provides a comparison with the vacuum geneous distribution of fine eutectic silicon and small plasma sprayed alloy and hot pressed alloy. primary silicon particles (~5 μm) in the aluminum Vacuum plasma sprayed hypereutectic Al-Si alloy matrix (Fig. 2a). This is possible with VPS processing contains finer and rounded silicon particles (<5 μm), because of the high temperatures and rapid cooling, which is a significant improvement over the large which provide a high nucleation rate for particles, and columnar particles (50 to 100 mm) in the A390 alloy, which are detrimental for machining and yet do not allow time for particle growth. A proprietary cooling technique further enhances wear resistance. The table lists the microstructural cooling rates and consequently reduces the size of features of these alloys. Continued ADVANCED MATERIALS & PROCESSES/MAY 2001 45 0.02 0.015 As sprayed A 390 0.01 0.005 5 10 Time, min. 15 20 Fig. 4 — Wear resistance of A390 and spray formed Al-Si. The tensile strength of as-sprayed hypereutectic Al-Si alloy was as high as 345 MPa (50 ksi), which is comparable to the wrought A 390 aluminum alloy. High tensile strength of the as-sprayed alloy is caused by fine silicon particles that produce classical second-phase “composite” strengthening, and impede dislocation movement. Hot pressing of assprayed alloy results in a reduction of tensile strength because the material fractures at the coarsened silicon particles. Dry sliding wear resistance of the as-sprayed alloy evaluated by a block-ondisk tribometer suggests a comparable or lower wear rate than wrought A 390 aluminum (Fig. 4). Structural applications Apart from automotive and electrical applications, hypereutectic Al-Si alloys have potential ap- Fig. 5 — Spray formed ring of hypereutectic Al-Si with a varying degree of stiffness. Diameter of the ring is 125 mm, width is 5 mm and thickness is 0.5 mm. plications for structural parts in the aerospace industry as a replacement for beryllium-based alloys. Traditionally, beryllium has been selected for truss structures, optical mirror shells, satellite attach fittings, and solar panel support structures because of its low density, high stiffness, low thermal expansion, and high thermal conductivity. Despite its usefulness, beryllium is not an ideal material because it is expensive and too brittle to work. However, hypereutectic Al-Si possesses properties similar to those of beryllium at lower cost. For example, a hypereutectic Al-Si based X-ray mirror can be fabricated by a replication technique. The “replicated optics” technique is a process for producing multiple thin mirror shells from a single shaped and polished mandrel. Each shell is built up on the surface of a correctly figured (negative) mandrel, usually made of glass or nickel-coated aluminum. The mandrel is super-polished to a ~5 nm high surface finish that has the negative figure of the required optic. The spray-formed material is then removed from the mandrel, usually by taking advantage of the large difference in thermal expansion between the two components. No additional polishing or shaping of the shells is needed. The technique has considerable merit in that many shells can be fabricated for a single shaping and polishing of the mandrel, and the resulting shells can be very thin. A 25 mm diameter spray formed X-ray mirror of hypereutectic Al-Si successfully replicated the optical finish on the inner surface. Optical-finish mandrels have been fabricated by NASA Marshall Space Flight Center and Goddard Space Flight Center. This technique can be adopted to fabricate mirror shells of larger sizes, such as 62 mm and 125 mm diameter. Stiffness of the spray formed hypereutectic Al-Si could be controlled by varying the process parameters (Fig. 5). ■ For more information: Arvind Agarwal and Tim McKechnie, Plasma Processes Inc., 4914 D Moores Mill Road, Huntsville, AL 35811; tel: 256/851 7653; fax: 256/8594134; e-mail: [email protected] ; Web site: www. plasmapros.com. How useful did you find the information presented in this article? Very useful, Circle 293 Of general interest, Circle 294 Not useful, Circle 295 46 Circle 24 or visit www.adinfo.cc ADVANCED MATERIALS & PROCESSES/MAY 2001