Showing papers on "Powder metallurgy published in 2011"

The investigation of the influence of laser re‐melting on density, surface quality and microstructure of selective laser melting parts

Abstract: Purpose – Selective laser melting (SLM) is a powder metallurgical (PM) additive manufacturing process whereby a three‐dimensional part is built in a layer‐wise manner. During the process, a high intensity laser beam selectively scans a powder bed according to the computer‐aided design data of the part to be produced and the powder metal particles are completely molten. The process is capable of producing near full density (∼98‐99 per cent relative density) and functional metallic parts with a high geometrical freedom. However, insufficient surface quality of produced parts is one of the important limitations of the process. The purpose of this study is to apply laser re‐melting using a continuous wave laser during SLM production of 316L stainless steel and Ti6Al4V parts to overcome this limitation.Design/methodology/approach – After each layer is fully molten, the same slice data are used to re‐expose the layer for laser re‐melting. In this manner, laser re‐melting does not only improve the surface qualit...

Synthesis of nanocomposites and amorphous alloys by mechanical alloying

Abstract: Mechanical alloying (MA) is a powder metallurgy processing technique that involves repeated cold welding, fracturing, and rewelding of powder particles in a high-energy ball mill. Due to the specific advantages offered by this technique, MA was used to synthesize a variety of advanced materials. This article presents two specific examples of synthesis of nanocomposites containing a high volume fraction of the reinforcement phase in Al and TiAl matrices. It was possible to uniformly disperse 50 vol% of nanometric (50 nm) Al2O3 in Al and achieve high strength and modulus of elasticity. Similarly, it was possible to disperse 60 vol% of Ti5Si3 phase in the γ-TiAl intermetallic. Fully consolidated material showed superplastic behavior at 950 °C and a strain rate of 4 × 10−5 s−1. Amorphous phases were produced by MA of blended elemental powder mixtures in several Fe-based compositions. From the systematic investigations carried out, it was possible to deduce the criteria for glass formation and understand the interesting phenomenon of mechanical crystallization. By conducting some controlled experiments, it was also possible to explain the mechanism of amorphization in these mechanically alloyed powder blends. Other examples of synthesis of advanced materials, e.g., photovoltaic materials and energetic materials, have also been briefly referred to. This article concludes with an indication of the topics that need special attention for further exploitation of these materials.

Powder Metallurgical Fabrication and Microstructural Investigations of Aluminum/Steel Functionally Graded Material

Abstract: Aluminum/steel electric transition joints (ETJs) are used in aluminum reduction cell for the purpose of welding aluminum rod and steel bracket components. Solid state welding process used for joining aluminum and steel at the electric transition joints have the drawbacks of cracking and separation at the interface surfaces. Cracking and separation at the electric transition joints are caused by the stress singularities that developed due to the mismatch in thermal and mechanical properties of each material. To overcome the drawback of electric transition joints, aluminum/steel functionally graded may be used as electric transition joints or proposed. Therefore manufacturing and investigation of aluminum/steel functionally graded materials fabricated by powder metallurgy process were carried out through the current work. Different samples with different layers of aluminum/steel functionally graded materials were compacted using steel die and punch at the same compacted pressure and sintered temperature. After investigating the different samples of aluminum/steel functionally graded materials under different fabrication conditions, the suitable fabrication regime was determined with the aid of microscopic observations.

Tungsten–copper composite production by activated sintering and infiltration

Abstract: Activated sintering of tungsten has been used to have infiltrable skeletons. For this purpose the effects of sintering temperature and activator amount were studied and appropriate production parameters were obtained. The results showed that by activated sintering, moderate sintering temperature such as 1400 °C can be used instead of conventional temperatures i.e. > 2000 °C. Activated sintering has the ability to produce fully open and interconnected porosities with desirable density that is ideal for infiltration. This method of production for infiltrated W–Cu composites has not been reported elsewhere.

Characterization of Ti–6Al–4V alloy foams synthesized by space holder technique

Abstract: Ti–6Al–4V foams, biomedical candidate materials, were synthesized by powder metallurgical space holder technique as a result of evaporation of magnesium to achieve desired porosity content. Final products contained porosities in the range ∼43–64% with an average macropore size between 485 and 572 μm and a lamellar type Widmanstatten microstructure composed of α-platelets and β-laths. Unlike the case of bulk Ti–6Al–4V alloy tested under compression loading, compression stress–strain curves of manufactured Ti–6Al–4V foams were similar to those of elastic–plastic foams, which contain a linear elastic region; a plateau stage; and a densification stage. In the plateau region deformation bands perpendicular to the compression axis were developed and cell collapsing took place together with the buckling and fracture of some of the cell walls and edges in a ductile manner. Calculated elastic modulus and yield strength were in the range 1.42–14.7 GPa and 28.2–150 MPa, respectively, and the foam mechanical properties were found to be dependent on micro porous cell wall properties, which in turn depends on neck size between powder particles. Around 330 MPa yield strength value was calculated for porous cell walls by the use of Ti–6Al–4V alloy powder samples sintered in loose and compacted conditions, which were utilized to simulate the cell wall structure of foams. In addition, overall mechanical properties of foams were investigated considering macro porosity fraction, pmacro, and the yield strength of foams exhibited a power law dependence, similar to commonly used minimum solid area models, in the form of A * (1 − pmacro)n, where the proportionality constant “A” was found to be the yield strength of micro porous cell walls.

Role of alloying elements in microstructure evolution and alloying elements behaviour during sintering of a near-β titanium alloy

Abstract: A near-β Ti–5Al–5Mo–5V–2Cr–1Fe alloy was produced using the blended elemental powder metallurgy approach. The microstructural development and distributions of the alloying elements were investigated during transformation of the heterogeneous powder compacts into the final homogenous alloy product. The influence of different alloying elements and their combination on microstructural evolution and chemical homogenisation is discussed. X-ray mapping and line scans found the order of the rates at which the alloying elements diffuse into the Ti matrix. It was concluded that the addition of 1% Fe as alloying element with the fastest diffusivity in titanium has a positive influence on the characteristics of sintered alloy.

Effect of Particle Size on the Microstructures and Mechanical Properties of SiC-Reinforced Pure Aluminum Composites

Abstract: This article examined the effects of particle size and extrusion on the microstructures and mechanical properties of SiC particle-reinforced pure aluminum composites produced by powder metallurgy method. It has been shown that both particle size and extrusion have important effects on the microstructures and mechanical properties of the composites. The SiC particles distribute more uniformly when the ratio of the matrix powder size and SiC particle size approaches unity, and the smaller-sized SiC particles tend to cluster easily. The voids are found to coexist with the clustered and large-sized SiC particles, and they significantly decrease the density and mechanical properties of the composites. Extrusion can redistribute the SiC particles in the matrix and decrease the number of pores, thus make the SiC particles distribute more uniformly in the matrix, and enhance the interfacial bonding strength. The decrease in the SiC particle size improves the tensile strength and yield strength, but decreases the ductility of the composites.

Comparative processing-structure-property studies of Al-Cu matrix composites reinforced with TiC particulates

Abstract: Effects of TiC content and size on the microstructure, density, hardness and wear resistance of Al–4 wt.% Cu matrix composites were investigated. Al–4 wt.% Cu matrix composites were fabricated using two main processing routes of powder metallurgy (PM) and casting. In the PM process, Al–4 wt.% Cu matrix alloy reinforced with 10 wt.% TiC particulates of two different average sizes (13 μm and 93 μm) were mechanically alloyed and sintered. In the second process, Al–4 wt.% Cu– x TiC ( x = 5, 10, 15 and 20 vol.%) composites were fabricated using a flux-assisted casting. Both the wear resistance and the hardness of cast composites improved with TiC content. The sintered composite containing smaller TiC particles (0.6–3.5 μm) exhibited higher hardness than that reinforced with coarser TiC (0.8–5.6 μm) particles. However, the detachment of carbide particles from wear surfaces of the composite reinforced with TiC particles (0.6–3.5 μm) results in the deterioration of its wear resistance.

Effect of SiC content on the processing, compaction behavior, and properties of Al6061/SiC/Gr hybrid composites

Abstract: Aluminum matrix composites reinforced with SiC and graphite (Gr) particles are a unique class of advanced engineered materials that have been developed to use in tribological applications. The conventional techniques for producing these composites have some drawbacks. In this study, a new method, namely In situ Powder Metallurgy (IPM), is applied for the preparation of Al6061/SiC/Gr hybrid composites. In this method, the stir casting and the powder metallurgy synthesizing processes are combined into an integrated net shape forming process. 0–40 vol.% of SiC particles with an average size of 19 μm, along with 9 vol.% of uncoated Gr particles, were introduced to the molten 6061 aluminum alloy. Then, the slurries were stirred in a specified time–temperature regime resulting in mixtures of the SiC, Gr, and aluminum powder particles. The powder mixtures were cold pressed in six different pressures (between 250 and 750 MPa) and sintered. Finally, the produced composites were heat treated and their hardness and wear properties were investigated. Homogenous distribution of the SiC and Gr particles within the powder mixtures and the hybrid composites is clear from the SEM images. The results also show that the SiC particles decrease the compressibility of the hybrid powders and improve the hardness of composites. The best wear resistance is achieved in the hybrid composite containing 20 vol.% SiC particles.

Interstitial elements in titanium powder metallurgy: sources and control

Abstract: The effect of interstitials on the mechanical properties of cast and wrought titanium alloys has been extensively reported but less information is available on the effect of contamination during PM processing. The sources of interstitial contamination when processing titanium powders by compaction, isostatic pressing, powder injection moulding (PIM) and innovative foaming processes are reviewed, focusing specifically on oxygen. The initial powder characteristics (surface area, size), process parameters (time, temperature) and environment (atmosphere, binder, support) may all have significant impact on the final interstitial content. It is, therefore, important to identify and control the sources of contamination by interstitials. A case study on PIM is provided to illustrate the relative contribution of the different sources.

Optimisation of stainless steel cold spray coatings using mixed particle size distributions

Abstract: Porosity in type 316 L stainless steel cold spray coatings is studied from two different aspects. The first is to examine, for a given level of porosity, the effect of coating thickness on corrosion behaviour. Beyond a critical coating thickness the substrate is no longer attacked in polarisation tests, which has implications for coatings that contain some porosity (including some thermal spray coatings). The second aspect examined is to approximate the stainless steel particles as non-deforming, and apply the powder metallurgy practice of mixing particle size distributions to improve coating density. The results show that coatings sprayed using mixed particle size distributions can have similar properties to those sprayed using fine particles alone, but without the processing difficulties of fine particles such as inconsistent powder feeding or nozzle fouling.

Development of a Sn–Ag–Cu solder reinforced with Ni-coated carbon nanotubes

Abstract: In this study, Ni-coated carbon nanotubes (Ni-CNTs) were incorporated into the 95.8Sn-3.5Ag-0.7Cu solder alloy using the powder metallurgy route. Up to 0.3 wt% of Ni-CNTs were successfully incorporated. The effects of Ni-CNTs on the physical, thermal and mechanical properties of Sn–Ag–Cu solder alloy were investigated. With the addition of increasing weight percentages of Ni-CNTs, the composite solders showed a corresponding decrease in density values and improved wetting properties. The thermomechanical property results showed an improvement in thermal stability for the composite solders. Mechanical characterization revealed an improvement in ultimate tensile strength (up to 12%) and 0.2% yield strength (up to 8%) with the addition of 0.05 wt% Ni-CNTs in the solder.

Development of Aluminium Based Hybrid Metal Matrix Composites for Heavy Duty Applications

Abstract: The present study deals with the investigation of dry sliding wear behavior of aluminium alloy based composites, reinforced with silicon carbide particles and solid lubricants such as graphite/antimony tri sulphide (Sb2S3). The first one of the composites (binary) consists of Al. with 20% Silicon Carbide particles (SiCp) only. The other composite has SiCp and solid lubricants: Graphite + Sb2S3 (hybrid composite) at solid state. Both composites are fabricated through P/M route using “Hot powder perform forging technology”. The density and hardness are measured by usual methods. The pin-on-disc dry wear tests to measure the tribological properties are conducted for one hour at different parameters namely load: 30, 50 and 80N and speed: 5, 7 and 9m/s. The tested samples are examined using scanning electron microscope (SEM) for the characterization of microstructure and tribolayer on worn surface of composites. The results reveal that wear rate of hybrid composite is lower than that of binary composite. The wear rate decreased with the increasing load and increased with increasing speed. The results of the proposed composites are compared with iron based metal matrix composites (FM01N, FM02) at corresponding values of test parameters. These iron based metal matrix composites are also fabricated by P/M route using ‘Hot powder perform forging technology’. The comparative study reveals that the proposed composites have lower friction coefficient, less temperature rise and low noise level; however they have little higher wear rate. It is concluded that the hybrid composite has acceptable level of tribological characteristics with blacky and smooth worn surface.

Surface Integrity of C-40 Steel Processed with WC-Cu Powder Metallurgy Green Compact Tools in EDM

Abstract: Electric discharge machining (EDM) is an electrothermal process where recast layer on the machined surface and heat-affected (HAZ) zone just below the machined surface are common phenomena. Thus, the assessment of surface integrity in EDM is a very important task. In this study, an attempt was made to modify the surface integrity of C-40 steel in EDM. WC-Cu powder metallurgy (P/M) green compact tools were used so that the tool material could be transferred to the work surface and thereby the surface characteristics could be altered. WC was used as tool material because of its ability to produce a hard layer over the work surface. A detailed experimental study was performed. The effect of various process parameters on the surface characteristics were presented with the support of analyses such as material transfer rate (MTR), tool wear rate (TWR), surface roughness, optical microscopy, scanning electron microscopy (SEM), energy dispersive x-ray (EDX), x-ray diffraction (XRD), microhardness testing, etc. It...