Xuanpu Dong

Effect of scanning speed during PTA remelting treatment on the microstructure and wear resistance of nodular cast iron

The surface of nodular cast iron (NCI) with a ferrite substrate was rapidly remelted and solidified by plasma transferred arc (PTA) to induce a chilled structure with high hardness and favorable wear resistance. The effect of scanning speed on the microstructure, microhardness distribution, and wear properties of PTA-remelted specimens was systematically investigated. Microstructural characterization indicated that the PTA remelting treatment could dissolve most graphite nodules and that the crystallized primary austenite dendrites were transformed into cementite, martensite, an interdendritic network of ledeburite eutectic, and certain residual austenite during rapid solidification. The dimensions of the remelted zone and its dendrites increase with decreased scanning speed. The microhardness of the remelted zone varied in the range of 650 HV0.2 to 820 HV0.2, which is approximately 2.3–3.1 times higher than the hardness of the substrate. The wear resistance of NCI was also significantly improved after the PTA remelting treatment.

Effects of Process Parameters on Internal Quality of Castings during Novel Casting

In this study, the A356 aluminum alloy castings were obtained using the expendable pattern shell casting process with vacuum and low pressure (EPSC-VL). The effects of process parameters including gas flow rate, vacuum level, and gas pressure on the internal quality of the castings were investigated through measuring the density and section porosity of castings experimentally as well as computing the porosity defects of the castings by simulation. The results showed that the density of castings increased and the porosity defects of castings decreased with the increase of gas flow rate, vacuum level, and gas pressure, respectively. As a result, the internal quality of castings was greatly improved. The simulation results were in accordance with the experimental results and showed that the trend of splash and turbulence of the molten metal during the filling process increased under the excessively high gas flow rate, which may lead to porosity and oxide inclusion defects. In addition, EPSC-VL process had significant advantages in internal quality, microstructure, and mechanical properties compared to expendable pattern shell casting process under gravity casting (EPSC-G) and lost foam casting (LFC). A complicated and thin-walled aluminum alloy part with high quality has been successfully manufactured using this technology.

Microstructural evolution of Mg9AlZnY alloy with vibration in lost foam casting during semi-solid isothermal heat treatment

Abstract The nearly equiaxed grains of Mg9AlZnY alloy were obtained by vibrating solidification in lost foam casting(LFC) and the microstructure of Mg9AlZnY alloy was analyzed. On this basis, the morphology and size of α-Mg grains fabricated by semi-solid isothermal heat treatment(SSIT) at 530 ? and 570 ? holding different time were studied. The results show that the main constituent phases of Mg9AlZnY alloy are α-Mg, β-Mg 17 Al 12 and Al 2 Y, and the Y can greatly refine α-Mg grains. The distribution of α-Mg grains equivalent diameters between 20 and 100 μm is up to 87%, and the average roundness of α-Mg grains reaches 1.37 in the specimen obtained at 570 ? and holding time 60 min. According to the analysis of solidification kinetics and thermodynamic, binary eutectic with low melting point melts firstly on SSIT process. As the liquid fraction increases with the solute diffusibility, both of the shape and size of α-Mg grains change ceaselessly. When the liquid fraction reaches equilibrium, the α-Mg grains are gradually spheroidized under the interfacial tension, and then the α-Mg grains begin to combine and grow. Evolution of α-Mg dendritic grains on SSIT process is obviously different from that of equiaxed grains.

Mold-filling characteristics of AZ91 magnesium alloy in the low-pressure expendable pattern casting process

The magnesium (Mg) alloy low-pressure expendable pattern casting (EPC) process is a newly developed casting technique combining the advantages of both EPC and low-pressure casting. In this article, metal filling and the effect of the flow quantity of inert gas on the filling rate in the low-pressure EPC process are investigated. The results showed that the molten Mg alloy filled the mold cavity with a convex front laminar flow and the metal-filling rate increased significantly with increasing flow quantity when flow quantity was below a critical value. However, once the flow quantity exceeded a critical value, the filling rate increased slightly. The influence of the flow quantity of inert gas on melt-filling rate reveals that the mold fill is controlled by flow quantity for a lower filling rate, and, subsequently, controlled by the evaporation of polystyrene and the evaporation products for higher metal velocity. Meanwhile, the experimental results showed that the melt-filling rate significantly affected the flow profile, and the filling procedure for the Mg alloy in the low-pressure EPC process. A slower melt-filling rate could lead to misrun defects, whereas a higher filling rate results in folds, blisters, and porosity. The optimized filling rate with Mg alloy casting is 140 to 170 mm/s in low-pressure EPC.

Hard-yet-tough high-vanadium high-speed steel composite coating in-situ alloyed on ductile iron by atmospheric plasma arc

A graded high-vanadium alloy composite coating was synthesized from premixed powders (V, Cr, Ti, Mo, Nb) on ductile iron (DI) substrate via atmospheric plasma arc surface alloying process. The resulted cross-section microstructure is divided into three distinct zones: upper alloyed zone (AZ) rich with spherical primary carbides, middle melted zone (MZ) with fine white iron structure and lower heat affected zone (HAZ). Spherical or bulk-like primary carbides with diameter < 1 μmin the AZ are formed via in-situ reactions between alloy powders and graphite in DI. Microstructural characterizations indicate that the carbides are primarily MC-type (M=V, Ti, Nb) carbides combined with mixed hardphases such as M 2C, M7C3, M23C6, and martensite. Disperse distribution of spherical, submicron-sized metal carbides in an austenite/ledeburite matrix render the graded coating hard-yet-tough. The maximum microhardness of the upper alloyed zone is 950 HV0.2, which is five times that of the substrate. Significant plastic deformation with no cracking in the micro-indentations points to a high toughness. The graded high-vanadium alloy composite coating exhibits superior tribological performance in comparison to Mn13 steel and plasma transferred arc remelted DI.