Materials Science Forum Vols. 534-536

Abstract: The increasing demand for automotive industries to reduce the weight of the vehicles has led to a growing usage of Al alloy powder metallurgy (P/M) parts such as camshaft bearing caps, shock absorber pistons and brake calipers [1,2]. In order to control the sintered microstructure and mechanical properties of the aluminium alloy powder metallurgical (P/M) parts, it is essential to establish a fundamental understanding of the microstructural development during the sintering process. Current research at Birmingham University is focussed on the investigation of the sintering behaviour of Al-Cu-Mg-Si powder blends using a combination of Scanning Electron Microscopy, Energy Dispersive Microanaylsis (SEM) and Differential Scanning Calorimetry (DSC). This paper presents a detailed study of the effect of temperature and initial starting materials on the evolution of microstructure during the sintering of Al-Cu-Mg-Si blends for PM.

Abstract: Nanostructured Alumina – 20 vol% 3YSZ composites powder were synthesized by wetmilling technique. The starting materials were a mixture of Alumina micro-powder and 3YSZ nano-powders. Nano-crystalline grains were obtained after 24 h milling time. The nano-structured powder compacts were then processed to full density at different temperatures by high-frequency induction heat sintering (HFIHS). Effects of temperature on the mechanical and microstructure properties have been studied. Al2O3-3YSZ composites with higher mechanical properties and small grain size were successfully developed at relatively low temperatures through this technique. The samples were densified by heating to a sintering temperature in the range of 1100 to 1400 °C, and then fast cooled to 500 °C within short time. A relative density of up to 99% theoretical density of the composites was achieved after sintering at 1370 °C.

Abstract: The effect of the additives, Y2O3 and MgO, on the sintering and properties of Al2O3-TiC composites was investigated. It is known that MgO is used as additive for improving densification and Y2O3 is applied as sintering aid. In this study, the amounts of TiC were varied in the range of 30-47 wt%. The 0.5 wt% MgO and also varied amounts of Y2O3 from 0.3 to 1 wt% were added into the composites. The sintering of Al2O3-TiC composites was performed in a graphite- heating element furnace at different sintering temperature, 1700 and 1900°C, for 2 hr under an argon atmosphere. The results demonstrated that the properties of the composites sintered at 1700°C were much better than those sintered at 1900°C. The comparisons on physical properties, mechanical properties and microstructure of composites with and without additives were reported. Comparing with other samples, Al2O3-30wt%TiC composites with 0.5wt% MgO and 1 wt%Y2O3 exhibited the highest density of approximately 98% of theoretical and flexural strength of 302 MPa.

Abstract: When an alloy such as Ni-W is liquid phase sintered, heavy solid W particles sedimentate to the bottom of the container, provided that their volume fraction is less than a critical value. The sintering process evolves typically in two stages, diffusion-driven macrosegregation sedimentation followed by true sedimentation. During sedimentation, the overall solid volume fraction decreases concurrently with elimination of liquid concentration gradient. However, in the second stage of true sedimentation, the average solid volume fraction in the mushy zone increases with time, and oddly, no concentration gradient is necessary in the liquid zone. In this work, we propose that the true sedimentation results from particle rearrangement for higher packing efficiency.

Abstract: A technology of hardening porous materials of titan powders has been elaborated. The technology is based on passing alternating current with duration of ~10-1…101 s through porous (35…40%) blanks made by method of Sintering by Electric Discharge (SED) by passing a pulse of current with duration of ~10-5…10-3 s. The influence of technological regimes of porous blanks treatment on their structure and properties is investigated. Geometry and dimension of contact necks between powder particles of obtained samples are evaluated. Variations of porosity and strengths as well as microstructure of porous samples materials before and after treatment are investigated. Optimum range of treatment technological regimes is determined within which porosity of 30…35% with maximum strength values.

Abstract: Three 9Cr Fe-base ODS alloys were prepared by MA (mechanical alloying) and HIP (hot isostatic processing) processes. The addition of Ti was confirmed to have a significant effect on the tensile properties at high temperature as well as at room temperature, while the V addition did not clearly show its effect on the microstructure or on the mechanical porperties at both temperatures. Fine and evenly distributed Y and Ti containing complex oxide was detected in the Ti added ODS alloy. They were attributed to the good high temperature strengths. Different cooling rates from the normalizing temperature did not result in a significant change in the grain boundary characters, but rather significant differences were observed in the mechanical properties. This is mainly attributed to the difference of the matrix phase rather than to the grain size distribution or the grain boundary characters.

Abstract: In the study, we attempted to synthesize the cordierite by the reaction of fly-ash and alumina, silicon dioxide, magnesia powders. And also porous cordierite honeycomb filte r for air purification was fabricated from the combination of synthetic cordierite using fly as h and pore forming agent. As the extrusion processing parameters of honeycomb, binder formul ation and extrusion conditions were investigated. Graphite addition was effective for the reduce ment of extrusion pressures. Porous cordierite honeycomb was fabricated with high porosity ( 58%), and good compressive strength (69MPa) with 30wt% graphite at low sintering tempera ture(1150oC).

Abstract: Manganese is an alloying element that improves the strength of ferrite and the hardenability of steels. It could be a valid substitute for expensive and toxic elements (as Mo and Ni) in sintered steels, increasing mechanical properties. The hardenability of four low alloy Mn steels was studied to establish the influence of manganese on the heat treatments. The effect of Mn on steel hardenability is well established. The multiplying hardenability factors in the range 0.05-1% Mn are known, and so the hardenability of the alloy to be investigated can be predicted. The Grossmann approach was adopted, which uses cylinders with different diameters to induce different gradients of cooling rate in the cross section. Quenching experiments were carried out in the vacuum furnace, recording the actual cooling rate (on the external surface and in the central axis). The maximum cooling rate attainable is 10 K/s. Hardness, microhardness and microstructure profiles were determined, and correlated to cooling rate for the different alloying elements and C contents. The correlation of microstructure and microhardness to the actual cooling rate makes the results independent on the process parameters and applicable to each industrial condition, once the actual cooling rate in the parts is known.

Abstract: The effect of tempering temperature and microstructure on dry sliding wear behavior of quenched and tempered PM steels was investigated. For this purpose, atomized iron powder was mixed with 0.3 % graphite and 1-2 % Ni powders. The mixed powders were cold pressed and sintered at 1200°C. The sintered specimens were quenched from 890°C and then tempered at 200°C and 600°C for 1 hr. Wear tests were carried out on the quenched+tempered specimens under dry sliding wear conditions using a pin-on-disk type machine at constant load and speed. The experimental results showed that the wear coefficient effectively increased with increasing tempering temperature. With increasing Ni content, the wear coefficient slightly decreased at all tempering temperatures due to the high amount of Ni-rich austenitic areas.

Abstract: The sinter-bonding behavior of iron based powder mixtures was investigated. To produce the green compacts to be joined the following powders based on Höganäs AB grade NC 100.24 plain iron powder were used: NC 100.24 as delivered, PNC 30, PNC 60 and NC 100.24 + 4%Cu powder mixtures. Dimensional behaviour of all those materials during the sintering cycle was monitored by dilatometry. Simple ring shaped specimens as the outer parts and cylindrical as the inner parts were pressed. The influence of parts’ composition on joining strength was established. Diffusion of alloying elements: copper and phosphorous, across the bonding surface was controlled by metallography, SEM and microanalysis.