N.L. Loh

Cumulative tool wear in machining metal matrix composites Part II: Machinability

Abstract Models for cumulative tool wear are used to study the machinability of aluminium matrix composites reinforced with SiC or Al2O3 particles. Cutting conditions to simulate a finishing operation were employed. Effects of cutting tool materials, heat treatment, and hot isostatic pressing were investigated. Carbide tools can be utilized in a roughing operation, while cubic boron nitride and polycrystalline diamond tools could be used to finish-machine the composites. The latter tools have acceptable tool lives, and cause minimum damage to the sub-surface. The hard aluminium matrix as a result of heat treatment significantly shortens the tool life. Fractured and delaminated particles along or underneath a machined surface could be of concern when composites are used in critical applications.

An overview of hot isostatic pressing

Abstract Hot isostatic pressing (HIP) is a manufacturing process that involves simultaneous application of high temperature and pressure. It was invented in 1955 for diffusion-bonding applications in the nuclear industry and has since found numerous applications in other fields. This paper provides an overview of hot isostatic pressing, looking into the various HIP process steps, temperature and pressure cycles, and the equipment used. Consideration is also given to the application of HIP to the healing of castings, diffusion bonding, powder metallurgy and ceramics. Finally, the latest trends in equipment, research and application are discussed.

Electrochemical polishing of biomedical titanium orifice rings

Abstract The process of electrochemical polishing is applied to finish the surface of the titanium orifice ring used in a St. Vincents prosthetic mechanical heart value. The optimum process parameters for polishing flat specimens were determined from a fractional factorial design and a few single-factor experiments. A good shiny surface finish of 0.08 μm Ra, free from any surface or sub-surface defects, was produced in 45 s of polishing time. Subsequent experimental results with shaped electrodes indicated that the titanium orifice rings could be polished to approximately 0.09 μm Ra, this comparing favourably with the surface finish of 0.08 μm Ra that could be achieved by manual polishing.

Thermal spraying of Ti6Al4V/hydroxyapatite composites coatings: powder processing and post-spray treatment

Abstract Hydroxyapatite (HA) is known for its attractive bioactive properties. Thermal spray techniques (plasma spray and high velocity oxy-fuel) are employed to deposit HA on titanium implants because of their high thermal efficiency and relative economy. However, some of the bioactive properties of HA are lost during thermal spraying. Generally, HA has poor mechanical properties. Titanium is a light metal that has been applied to biomedical engineering because of its non-toxicity and low density. A composite that can elicit the combined bioactive property of HA and the mechanical properties of Tiue5f86Alue5f84V to provide an implant that is both biocompatible and mechanically strong would certainly be desirable. Thermal spray techniques are employed in the present study to process Tiue5f86Alue5f84V/HA composite coatings. The Tiue5f86Alue5f84V/HA coatings can be sprayed onto existing implants to improve post-operation healing. This paper reports the thermal spraying of Tiue5f86Alue5f84V/HA composite coatings using powder feedstock prepared by two powder processing techniques: (i) mechanical alloying and (ii) the ceramic slurry mixing method. The effects of post-spray treatment by hot isostatic pressing (HIP) on the microstructure and other physical properties are investigated also. The surface morphology and microstructure of the as-sprayed coatings and HIPped coatings are examined by scanning electron microscopy. The investigation shows that the as-sprayed coating microstructure is comprised, predominantly, of HA lamella sandwiched between the Tiue5f86Alue5f84V lamellae. The coatings, in particular the HA-rich regions, suffer from high porosity levels. A mercury intrusion porosimeter is used to study the pore-size distribution of the as-sprayed and HIPped samples, the results indicating that the majority of the micropores are drastically reduced. The improvement in the physical properties of the composite was attributed to this reduction. Overall, the results showed that HIP can effectively enhance the mechanical properties of the as-sprayed coatings and improve the porosity levels.

The reduction of dendrite ARM spacing using a novel pressure-assisted investment casting approach

Abstract Dendritic structures are the predominant microstructural constituents of solidified alloys. The primary, and more often the secondary, dendrite arm spacings control the segregation pattern that largely determines the properties of the material. It is advantageous to refine the dendritic arm spacing (DAS) in castings. In this respect, there is a technological advantage to refine the DAS in conventional investment casting. In this paper, the authors describe a newly developed pressure-assisted investment casting process which produces lower DAS values in cast LM25 alloy as compared to those of the conventional investment casting process. The values of DAS obtained in this process were, however, much higher than those of the squeeze-casting process.

The production of aluminium alloy composites using a cold isostatic press and extrusion approach

Abstract Whilst experimental results indicate that metal-matrix composites possess higher specific mechanical strengths, one of the main difficulties in implementing these materials on a large scale has been the costly and often complicated processes used. This paper presents the preliminary results obtained in developing a powder metallurgy process involving an approach based on the use of a cold isostatic press together with subsequent extrusion. Three systems have been compared: an unreinforced Al alloy system; an Al alloy-Ni short-fibres composite; and an Al alloy-Al 2 O 3 p particulates composite. Mechanical mixing of the matrix powder and fibre reinforcement was followed by a high-pressure isostatic pressure compaction. Pressurization was done in the range of 10–400 MPa. The green compacts were extruded subsequently at 5–10 mm/min, and reduction ratios of 5 and 10, at both 400 and 500°C. Whilst the initial high-pressure compaction step was sufficient to reduce the porosity to as low as 5% when a pre-degassing step was used, both composite systems could not achieve porosity of less than 10% even after sintering at 600°C. However, by using the subsequent extrusion step, reduction to less than 5% porosity was achieved at 500°C, for both of the composite systems.

Cold-hot isostatic pressing of Mar M200 superalloy powders

Abstract Powder metallurgy based on cold isostatic pressing, vacuum sintering and hot isostatic pressing (HIP) reveals the possibilities of mass producing critical parts in near net shape. The multi-stage process consists of: (i) cold isostatic compaction to a green perform; (ii) supersolidus vacuum sintering of compact to more than 90% density; and (iii) final consolidation via hot isostatic pressing to densify the preform. Three-point flexural tests were carried out on the sintered and HIPped specimens. Microstructure and porosity evaluations were conducted using scanning electron microscopy and image analysis.

Hot isostatic pressing of cast SiCp-reinforced aluminium-based composites

Abstract Two as-cast SiC particulate reinforced composites were treated by hot isostatic pressing (HIP) under different pressures and temperatures. The microstructures and tensile properties of the composites were characterized in the as-cast and HIPped conditions to study the effects of the HIP treatment. It was found that ductility of the as-cast composites was increased greatly by the HIP treatment but the yield stress was reduced drastically. The reduction of internal defects was identified to be the major factor for the improvement of the ductility. After the T6 treatment, the HIPped specimens were better than the as-cast specimens in strength as well as in ductility. The effects of the HIP temperature and pressure were also studied, and it was found that in the chosen temperature range of 450–550°C and pressure range of 100–150 MPa, increasing the temperature tended to improve the tensile properties, whilst increasing the pressure had little effect on the strength and ductility.

Measurements on fibre distribution in a fibre-reinforced Al metal-matrix composite (MMC) (matrix intercept-length measurements)

Abstract The fibre distribution in an MMC is an important factor which determines the nucleation sites [1] during solidification, this in turn, affects the properties of MMC materials. In order to control effectively the fibre distribution in an MMC, it is necessary to know the degree of fibre clustering and to understand the factors affecting the uneven distribution of fibres within the alloy matrices. The present work describes details of fibre distributions within a metal-matrix composite using matrix intercept-length measurements on three different samples: squeeze-cast samples (the original sample used for re-melting and resolidification experiments): slowly-cooled samples (quenched after solidification at a cooling rate of 0.24 K/s): and rapidly-cooled samples (quenched directly from the liquid state). In addition, a computer-generated random circular distribution is considered. The results of measurement of the samples are presented and discussed.

Casting and HIPping of Al-based metal matrix composites (MMCs)

Abstract The commercially available Al (trade mark A-356 Al) alloy, with approximately 7 wt% Si, 0.4 wt%Mg and other small amount of elements, is the most common type of Al foundry alloy due to its excellent castability. The needle-like Si phase embedded in the Al matrix is the most typical structural feature of this alloy system and the morphology of the Si phase has been shown to be important in determining the mechanical properties of this alloy. Much work has been done to modify the Si phase morphology by heat treatment or addition of grain refiners such as Sodium and Strontium. The present work describes the microstructural changes by the addition of alumina particles into molten Al, focusing on the effect of the alumina particle on the location, amount and morphology of the Si or eutectic phase in the matrix. Since the alumina particles are not wettable by molten Al, small amount of reactive agent such as Li was added to the Al. Reasonable alumina particle distribution within the matrix Al was achieved by low temperature mixing and gravity casting technique under open atmosphere. The cast MMC bars were processed by Hot Isostatic Pressing (HIPping) in order to densify the MMCs. The cast and HIPped MMCs were examined and mechanically tested. Results show that the 0.2% Yield Strength was improved over the unreinforced Al alloy.