N.H. Loh

Spark plasma sintering of hydroxyapatite powders

Dense hydroxyapatite (HA) compacts have been successfully fabricated by a spark plasma sintering (SPS). The sintering behavior of HA powders at different temperatures ranging from 850 degrees C to 1100 degrees C was studied. Results showed that spark plasma sintering resulted in rapid densification to near theoretical density. The HA compact was homogeneously sintered at 950 degrees C in a short sintering duration of 5 min, while maintaining high quality and high relative density (>99.5%). The density, microhardness and Youngs modulus of HA sintered compact initially increased with the sintering temperature, reached a maximum value at around 950-1000 degrees C, then decreased with further increase in the temperature due to the decomposition of HA into beta-tricalcium phosphates. Fracture toughness results showed no significant difference with increasing temperature due to the combined influences of density and grain size. Microstructure analysis showed no noticeable grain growth under different sintering temperatures due to the short time exposure at high temperatures.

Sintering study of 316L stainless steel metal injection molding parts using Taguchi method : final density

Abstract Sintering is a key step in the metal injection molding process, which affects the final density as well as the mechanical properties of the sintered part. To achieve a high final density, the effects of various sintering factors pertaining to the temperature–time profile and sintering atmosphere have to be characterized and optimized as well. This paper reports the use of Taguchi method in characterizing and optimizing the process factors for sintering water-atomized 316L stainless steel (of average size 6 μm). The effects of four sintering factors: sintering temperature, heating rate, sintering time and sintering atmosphere on the final density were studied. The various factors were assigned to an L9 orthogonal array. It was found that all the chosen sintering factors have significant effects on the final density. Optimum fractional final density of 96.14% of wrought material was achieved with a sintering temperature of 1250°C, a heating rate of 20°C min−1 and isothermal heating at 1250°C for 90 min in a vacuum atmosphere. Confirmatory experiments have produced results that lay within the 90% confidence interval.

Production of metal matrix composite part by powder injection molding

Abstract The application of powder injection molding (PIM) to metal matrix composites (MMCs) comprising of 316L stainless steel and TiC powders are presented. The processing steps are discussed and a suitable set of powder loading, mixing procedure, molding condition and debinding schedule was established. Defect-free parts were successfully produced. The effects of sintering parameters on the microhardness and density were studied. The addition of TiC improves the microhardness and density. Increasing the sintering temperature or heating rate also improve the above-mentioned properties.

Mixing and characterization of feedstock for powder injection molding

Abstract Feedstock preparation for Powder Injection Molding (PIM) is a very crucial step since deficiencies in quality of the feedstock cannot be corrected by subsequent processing adjustments. Hence, it is important that the feedstock is homogeneous and free of powder–binder separation or particle segregation. The quality of the feedstock in the mixing process depends on numerous parameters such as mixing time, mixing temperature, sequence of material addition, powder size and shape, formulation of binder, shear rate, and powder loading. The present mixing study was conducted with 316L stainless steel and titanium carbide powders. The main objective was to establish suitable mixing parameters such as powder loading, mixing speed and mixing temperature. The rheological characteristics of the feedstocks, under various mixing conditions, were analyzed using the capillary rheometer. Results show that 54% volumetric powder loading gave satisfactory flow properties when mixed at a temperature of 90°C with a rotor speed of 30 rpm.

Binder system for micropowder injection molding

Abstract To exploit the potential of microsystem technology, micropowder injection molding (μPIM), an economical mass production technology of microparts, is currently being investigated. Present work focused on establishing a suitable binder system for μPIM. Multicomponent binder systems, comprising of different weight percentages of Paraffin Wax (PW), Ethylene Vinyl Acetate (EVA) and High Density Polyethylene (HDPE) were investigated. The findings indicate that 316L stainless steel microparts in dimensions of 100×100×250 μm can be molded, debound and sintered successfully using a 20 wt.% PW+40 wt.% EVA+40 wt.% HDPE binder system.

Characterization of powder injection molding feedstock

Abstract Powder injection molding (PIM) is a cost-effective technique for producing small, complex, precision parts in high volumes. PIM consists of four main processing steps: mixing, injection molding, debinding and sintering. To reduce the design-to-manufacture cycle time for injection molding, simulation software could be used. To have a good understanding of the PIM process and to provide the necessary data for simulation, characterization of the material is essential. This paper presents the characterization of PIM feedstock consisting of 91 wt.% M2 high speed steel (HSS) powder and 9 wt.% PAN250 polymer binder. The mechanical properties (Youngs modulus, Poissons ratio, in-plane shear modulus), thermal properties (coefficient of thermal expansion (CTE), softening point, transition temperature, specific heat, thermal conductivity) and rheological property of the feedstock were established. The CTE of the molded feedstock in three perpendicular directions differed significantly. Except for the specific heat, the mechanical and thermal results showed that the feedstocks properties were generally closer to a polymer rather than a metal. Rheological results exhibited pseudoplastic or shear thinning flow behavior, where its viscosity decreased with increasing shear rate. The feedstock viscosity also decreased with increasing temperature and was found to be suitable for molding.

Microstructures and mechanical properties of powder injection molded Ti-6Al-4V/HA powder.

Taguchi method with an L9 orthogonal array was employed to investigate the sintered properties of Ti-6Al-4V/HA tensile bars produced by powder injection molding. The effects of sintering factors at the 90% significance level: sintering temperature (1050 degrees C, 1100 degrees C and 1150 degrees C), heating rate (5 degrees C/min, 7.5 degrees C/min and 10 degrees C/min), holding time (30, 45 and 60 min) and cooling rate (5 degrees C/min, 20 degrees C/min and 40 degrees C/min) were investigated. Results showed that sintering temperature, heating rate and cooling rate have significant effects on sintered properties, whereas the influence of holding time was insignificant. It was found that a sintering temperature of 1100 degrees C, a heating rate of 7.5 degrees C/min and a cooling rate of 5 degrees C/min increased the relative density, Vickers microhardness, flexural strength and flexural modulus. However, a further increment of sintering temperature to 1150 degrees C did not show any discernable improvement in the relative density and Vickers microhardness, but there was a slight increase of 0.6% and 0.9% in the flexural strength and flexural modulus, respectively. Mechanically strong Ti-6Al-4V/HA parts with an open porosity of around 50% were developed.

Sintering of injection molded M2 high-speed steel

Abstract The sintering of injection molded M2 high speed steel (HSS) was studied. The sintering parameters investigated were: sintering atmosphere, heating rate, sintering temperature and sintering time. Mechanical properties and SEM microstructures were used to determine an optimum sintering schedule. It was found that sintering in vacuum is better than in nitrogen as densification is faster in the former. In vacuum sintering, effective sintering took place in the range 1190–1210°C, whereas in nitrogen atmosphere, effective sintering occurred form 1250°C to 1270°C. A heating rate of 10°C/min is preferred since a higher heating rate of 30°C/min resulted in distortion. Near full density was obtained within 10 min after reaching an optimum sintering temperature of 1210°C. However, the maximum ultimate tensile strength of 752.6 MPa was obtained after 2 h of sintering in a vacuum.

Ball burnishing of 316L stainless steel

The process of ball burnishing AISI 316L stainless steel was studied. Taguchi techniques for the statistical design of experiments were employed to establish the optimum parameters for achieving good surface finish (response) on flat specimens. An L16 (45) orthogonal array was chosen to study the effects of five burnishing parameters (control factors). The roughness of the pre-burnished surface was included as a noise factor. It was found that the type of ball material, the depth of penetration, the burnishing speed and the type of lubricant significantly affect, at a 99% level of confidence, the surface finish of the burnished specimens. Feed was significant at a 95% level of confidence. The minimum roughness could be achieved using ZrO2 as the ball material, kerosene as lubricant, a depth of penetration of 20 μm, a feed of 110 μm and a burnishing speed of 300 mm/min. A confirmation test yielded an average surface finish of 1.017 μm (Rtm), which fell within a 90% confidence interval constructed about the predicted optimum. Using this set of parameters, a 3D curved surface was similarly burnished. The curved surface was generated using the solid modeler CATIA running on an IBM 4381 P13 mainframe. Pre-burnishing milling and ball burnishing were performed in a single setting on a LeBlond Makino FNC40 vertical machining centre. Surface roughness values varying between 0.6 and 0.9 μm (Rtm) were obtained for the curved surface.

An investigation into the ball burnishing of an AISI 1045 free-form surface

This paper reports on the work undertaken in the ball burnishing of a free-form surface made from AISI 1045 medium-carbon steel. A geometric model of a free-form surface was generated using the catia solid modeler running on an IBM 5080 work-station. The NC codes generated were then down-loaded onto a 4-axis vertical machining centre for rough-milling, ball-end milling and finally ball burnishing. Preliminary experiments with 3-D curved surfaces indicated that grease is a better lubricant than machine oil in the ball-burnishing process. Using a full factorial experiment, it was found that the quality of surface finish attainable in the ball-burnishing process is affected significantly by the depth of penetration, the burnishing speed and the feed-rate. Four two-factor and four three-factor interaction effects were found to be significant at the 99% confidence level. The results of the ball-burnishing experiments on 3-D curved surfaces were analysed and a set of machining parameters selected therefrom were applied to the ball burnishing of the free-form surface. The best surface finish of the free-from surface obtained was estimated to be between 0.29 and 0.55 μm Rtm.