Papers by Author: Yan Long

Abstract: Ultrafine grain WC hardmetals were manufactured successfully from mixed powders of nominal composition of WC-6Co-1.5Al(wt%). To manufacture bulk ultrafine grain WC alloys, nanocrystalline WC powders as precursor were prepared for sintering from the mixed commercially available powders by high energy milling of 15 h. Then the electrical current sintering process of the milled powders was further studied. Experimental results showed that the density, hardness and transverse rupture strength of the sintered alloys reached 14.224 g/cm3, HRA 94 and 1660 MPa, respectively, by a appropriate coupling of pulse- and subsequent constant-current sintering and with the total sintering time of 6 min. It seemed difficult to obtain desirable sintered hardmetal by a single type of electric current sintering. However, the application of intensive pulse electric current was favorable to sintering, although it was difficult to achieve good WC alloys only by the pulse electric current sintering. A higher density WC alloy could be prepared by enlarging the electric current when only constant electric current was applied to sinter the milled powders. Unfortunately, the WC grains substantially coarsened concurrently so that the mechanical properties of the as sintered alloy decreased obviously.

Abstract: Constant current plus pulse current electrical sintering was introduced into the sintering of a ball-milled nanocrystalline Fe-2Cu-2Ni-1Mo-0.8C mixed powder. Effect of pulse current charging time on the properties of sintered products was studied. Constant electric current sintering was also used for comparison. A nanostructured iron-based material was obtained within 4 minutes of sintering. It has a density of 7.7 g/cm3 (relative density of 98.7 %) with an average grain size (iron-matrix) of 58 nm and carbide particle size of less than 100 nm, a hardness of HRC 64 and transverse rupture strength of 2004 MPa. When only pulsed electric current sintering was used with a sintering time of 5 min, a fine grained iron-base material with density of 7.74 g/cm3 (relative density of 99.2%), a hardness of HRC 65 and transverse rupture strength of 2045 MPa was obtained.

Abstract: A new process of pulsed electric current sintering was developed. It combines compaction with activated sintering effectively and can manufacture bulky nano-crystalline materials very quickly. Pulsed electric current sintering of high-energy ball-milled nano-crystalline iron-based powders is investigated in this work. A nanostructured steel is obtained with high relative density and hardness by this process. The average grain size of iron matrix is 58nm and the carbide particulate size is less than 100nm. The densification temperature of ball-milled powders is approximately 200°C lower than that of blended powders. When the sintering temperature increases, the density of as-sintered specimen increases but the hardness of as-sintered specimen first increases and then decreases. Microstructure analysis results show that the decrease of hardness is caused by the dramatic grain growth of iron matrix.

Abstract: Fe-2Cu-2Ni-1Mo-0.8C (wt. pct) alloys were successfully fabricated from elemental mixed powders by high energy milling and pulse electric current sintering, and the effects of milling time and sintering parameters on the densification degree, microstructure, mechanical properties and fracture characteristic of sintered material were investigated. Results showed that with increasing milling time, pulse electric current peak and sintering time, the density and transverse rupture strength of sintered alloy were improved. However, higher pulse electric current peak and/or longer sintering time were unfavorable to the density and strength of sintered alloy, because of generating overhigh sintering temperature. The density and transverse rupture strength of sintered alloy could reach 7.74
103 kg/m3 and 2231 MPa, respectively, and the corresponding fracture morphology was characterized as intergranular fracture.

Abstract: A novel technique was developed for direct joining between ethylene propylene rubber and high ductile spheroidal-graphite cast iron using functional polymeric nanofilm without any adhesive. The functional polymeric nanofilm was created on the surface of cast iron by means of polymer plating with a synthesized organic compound of 6-diallylamino-1,3,5-triazine-2,4-dithiol monosodium salt. High peel strength joints of the rubber to polymer-plated cast iron were achieved by adding an accelerating agent of perhexane 3M under crosslinking condition of 418 K for 7 min. As the thickness of polymeric nanofilm was around 8.5 nm, peel strength of the joints was high to 8.9 kN/m and its broken-out section was rubber cohesive failure with 100% rubber coverage. Rubber/cast iron joints possessed good heat-resistant property. The film thickness and perhexane content had large effects on water resistance of joints. It is considered that high joining property results from chemical bond at interface between polymer-plated cast iron and rubber chain.

Abstract: Iron powders were successfully consolidated by a Pulse Electric Current Sintering (PECS) process in a short heating duration of 6 min. Microstructure and chemical composition of the sintered samples were studied by an optical microscope, scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX). Experimental results showed that both the densification of powders and inter-diffusion between elements were accelerated by increasing of the pulse peak. No noticeable grain growth was observed in the sintered samples. When the peak of pulse current increased, the density and transverse rupture strength of the sintered samples were improved.