Papers by Author: Yu Yong Chen

Abstract: TiAl alloy is a high-temperature structural material that is highly sensitive to microstructure. Different hot working or heat treatment processes can refine the grain, release stress, and adjust the phase type and distribution. This study investigates the effect of heat treatment and rolling on the microstructure of powder metallurgy TiAl alloys in detail, resulting in TiAl alloys with high plasticity achieved through hot pack rolling. The heat treatment temperature was set between 1290°C and 1330°C, with a temperature interval of 10°C, a holding time of 30 minutes, and cooling with the furnace. Spheroidization of the grains occurred at 1310°C. Phase analysis of the TiAl alloy was conducted after heat treatment at 1290°C, 1310°C, and 1330°C. The microstructure of the TiAl alloy after heat treatment at 1290°C still consisted of γ phase and B2 phase, with no significant change in the content of γ phase and B2 phase. After heat treatment at 1310°C, numerous α₂ phases abruptly emerged in the microstructure. The TiAl sheet exhibits an elongation of 3.6% at room temperature, which increases to 65% at 700°C. The study investigated the phase transformation process, grain morphology, and microstructure evolution of powder metallurgy TiAl alloy under different heat treatment temperatures and rolling. The relationship between microstructure and temperature of powder metallurgy TiAl alloy was established using electron backscattering diffraction.

Abstract: Ti-47Al-2Cr-2Nb-Y alloy pancake were produced by hot-pack forging. The microstructure of as-forged Ti-47Al-2Cr-2Nb-Y alloy were investigated by optical microscopy and scanning electron microscopy, showing that the forged alloy was composed of fine γ grains and retain cast lamella colonies surrounded by elongated B2 phase. Tensile properties of the material showed that yield strength (YS) and ultimate tensile strength (UTS) were decreased from 500MPa and 612MPa at room temperature to 420 MPa and 462 MPa at 800°C, respectively. With the temperature increasing to 900°C, elongation reached 120%.

Abstract: Ti-43Al-5V-4Nb (at.%) intermetallic compounds with fully lamellar structure were fabricated by forging method without canning using the blended elemental powders. The process route consisted of powder blending, compacting, sintering and final fabrication by hot forging. During sintering, there existed expansion, which is detrimental to forging process. In order to overcome the difficulty, two-step forging was introduced. The total reduction of two-step forging was up to 65%. Nearly fully lamellar structure of TiAl alloy was obtained. Overall, an optimized and potentially lower cost processing route could be identified.

Abstract: Effects of different Mo contents on microstructures and properties of Ti-22.5Al-20Nb-2V(at.%) were analyzed in this paper. Experimental results showed that the grain size was refined and the amounts of equiaxed α2 phases decreased with increase of Mo from 0.6at.% to 1.5at.%. The size of (O+B2) lath was refined when Mo from 0.6at.% to 1.2at.%. However, it seems that Mo easily segregate especially in grain boundary and become seriously with Mo increased. Compression tests showed that the yield strength and ultimate strength of alloys were obviously improved with Mo addition from 0.6at.% to 1.2at.%. Microhardness tests showed the same trend with compression yield and ultimate strength.

Abstract: The fabrication of ceramic moulds with zirconia primary coat for investment casting of gamma titanium aluminide based alloys (hereafter called TiAl alloys) was described in this study. The microstructures and fracture strength of ceramic moulds were characterized. The fracture strength and surface roughness of the ceramic moulds with zirconia primary coat were sufficient for the TiAl investment casting. TiAl blades were used for the technology demonstration in producing TiAl components. TiAl castings produced by the ceramic moulds exhibited an average surface roughness (R_a ) of 6 μm.

Abstract: A novel biocomposite Ti-35wt%Nb-2.5wt%Sn-15wt%HA powders was synthesized by high energy ball mill(HEBM) for various periods of time. The microstructure and characteristics of the milled powder particles were investigated. Results showed that in the composite powders milled for 4h, Ti was still exhibited primary α phase, with the increase of ball milling time up to 8h, Ti transformed into primary β phase and a little α phase, after ball milling for 12h, Ti transformed into β phase fully. the transform temperature was 380.06°C. And TEM and PSD results indicated that nanostructure was obtained after 12h milling..

Abstract: The paper outlines some research work that has been conducted in Harbin Institute of Technology, on the fabrication of TiAl alloys. The review is presented with special emphasis on some different manufacturing routes of TiAl alloys, including investment casting, canned forging and sheet rolling, mechanical milling and rapid solidification. Investment casting has been developed to manufacture near-net shape TiAl blades. Also included are current development of canned forging and rolling for TiAl sheets. Then, TiAl nanocrystalline powders gained through low temperature and two steps mechanical milling were researched. And considerably refined TiAl alloys with Y additions were produced by rapid solidification and the microstructure evolution with Y addition was studied. Details of the processing route and microstructure related to different processing method will be presented.

Abstract: γ-TiAl components due to their unique properties are on the verge of industrial application for increasing operation temperature and improving energy efficiency, especially in aerospace and automobile areas. This paper describes the recent developments of hot precision processing in ingot metallurgy technology including investment casting, canned forging and sheet rolling of TiAl alloys. Alloys development and the melting technology were discussed. Investment casting of TiAl alloys has been successfully developed to manufacture net-shape components. Research aimed at producing TiAl sheets through canned forging and rolling is introduced.

Abstract: In the present study, we tried to make Ti matrix/hydroxyapatite biocomposites using powder metallurgy and chemical treatments. Three composites were produced: Ti-30vol%HA, Ti15Mo-30vol%HA and Ti13Nb13Zr-44vol%HA. After mechanical milling and spark plasma sintering, Ti/HA composite compacts were produced. Microstructure observations and mechanical property tests showed that composites with two components (Ti-HA) have high hardness, moderate compress strength and lower density. The produced Ti/HA bulk composites were subjected to chemical treatment of immersion into phosphoric acid solution and porous structure was obtained. The obtained porous structure was characterized.

Abstract: Effects of heat treatment on the microstructure of as-cast and as-forged Ti-45Al-5Nb-0.3Y alloy are discussed. The as-cast Ti-45Al-5Nb-0.3Y alloy exhibits a microstructure consisting of fine equiaxed grains which average size is almost 100μm. Phase transformation of as-cast Ti-45Al-5Nb-0.3Y alloy greatly depends upon cooling rate. During furnace cooling, the alloy transform to fully lamellar microstructure. During air cooling, massive transformation predominates. During oil cooling, extremely fine fully lamellar microstructure is formed. During water cooling, ordering α2 phases are primary. Thermo-mechanical treatments, through combined action of hot canned forging and heat treatment, were performed on a Ti-45Al-5Nb-0.3Y alloy to investigate their effect on the microstructure of the alloy. The as-forged Ti-45Al-5Nb-0.3Y alloy is comprised of a large number of dynamic recrystallization (DRX) γ grains, curved and broken lamellae, and a small amount of remnant lamellae. And three different microstructures, duplex (DP), nearly lamellar (NL) and fine fully lamellar (FFL), have been obtained through heat treatment at different temperatures (1320-1370°C), respectively.