Papers by Author: Zhi Yu Xiao

Abstract: In order to develop high density powder metallurgy forming technology, a new concept combining high velocity compaction and warm compaction called warm high velocity compaction (WHVC) was presented. A new warm high velocity compaction forming equipment which adopts gravitational potential energy instead of hydraulic cylinder as hammer driver was designed. By means of the newly developed equipment, a preliminary study on warm high velocity compaction was performed. 316L stainless powder compacts with green density of 7.47 g/cm3 were obtained; the density is much higher than those prepared by conventional high velocity compaction. These results demonstrate that the newly designed equipment can basically meet the demand of warm high velocity compaction and the new forming method is superior to the conventional high velocity compaction. In addition, Densification mechanism of WHVC was also discussed.

Abstract: High velocity compaction (HVC) is one of the latest technologies of powder metallurgy (PM), while the prices threshold of Hydropulsor’s HVC presser limited massive application of this technique in vast minor PM enterprises in Mainland China. In the light of this, the paper delivers a new design of HVC apparatus which adopts mechanical springs groups instead of hydraulic cylinder as hammer driver. The virtual prototype and simulation of new HVC apparatus is brought about. The result of simulation shows that new design can basically meet the demand of HVC.

Abstract: Bending fatigue behavior of a sinter-hardened high density (7.4 g/cm3) Fe-2Cu-2Ni-1Mo-1C material fabricated by die-wall lubricated warm compaction of partially-diffuse alloyed powder was studied by bending ultrasonic fatigue testing. Results showed that fatigue strength decreases continuously with the increasing number of cycles. The fatigue failure yet occurs in the regime of exceeding 107 cycles and exhibits no traditional horizontal plateau between 106 and 107 cycles. Fatigue strength was 194 MPa, 239 MPa and 293 MPa at 108, 107 and 106 cycles respectively. Scanning electron microscopy revealed that cracks initiated from large pores on the surface and from pore clusters near the sub-surface. The fatigue cracks initiated both at single and multiple sites. Crack propagation was mainly in a trans-crystalline rupture mode. Fatigue striation and cleavage plane were observed in the crack propagation region and dimples were observed in the fracture zones.

Abstract: Powder metallurgy process is a net-shape manufacturing technique that can eliminate or reduce machining time. It is economical and environmental friendly since no scrap is being produced and no high energy consuming process such as melting is involved. Unfortunately, conventional powder metallurgy is not capable of producing complex parts. However, a recently developed binder-treated warm compaction technique can overcome this problem by increasing the flowability of the mixed powder. In this study, by using a commercially available water-atomized iron powder, a cross-shaped part was prepared by warm compaction of a binder-treated iron-base powder at approximately 80 °C and then sintered at 1120 °C. Microstructure, mechanical property and shape consistency of the prepared part were examined. Results showed that parts with high density and high green strength can be obtained without significant shrinkage or expansion. The present paper demonstrated that the binder-treated warm compaction process can expand the capability of powder metallurgy techniques to produce complex parts.

Abstract: Warm flow compaction is a new, simple and economy process that can produce complex powder metallurgy parts. A special apparatus for measuring the lateral flow capability of the powder was designed and manufactured. It can be used to analyze the feasibility of forming complex cross-shaped parts. The preparation of the powder blend and effects of pressing speed, temperature and axial pressure on the lateral flow capability of a tungsten-base powder mix were investigated. Results showed that the lateral-pressure increased with increased pressing speed and increased applied axial-pressure. The lateral-pressure decreased as lateral-distance increased. Biggest lateral-pressure was obtained when the powder blend was pressed at around 96 °C, at which the binder start to melt. In this study cross-shaped powder metallurgy parts were successfully prepared by warm flow compaction.

Abstract: Warm compacting and sintering behaviors of pre-mixed Fe-2Ni-2Cu-1Mo-1C powders were studied. Results showed that green density increased with compacting temperature and then fell slightly; the maximum green density was obtained at about 120°C. Green compact density and spring back effect of the pre-mixed powder increased gradually as the compacting pressure increased. Sintered density first increased and then fell as the temperatures went up under different sintering temperatures for 50 minutes, but the trends of sintering dilatation were first reduced and then increased. Sintered density first reduced and then increased with the prolonged sintering time at 1300°C, but the trends of dimension change after sintering were the very reverse. Tensile strength, elongation and hardness of the warm compacted Fe-2Ni-2Cu-1Mo-1C materials generally increased as sintering temperature and sintering time went up.

Abstract: Warm compaction is a low cost process to make high density and high performance iron base powder metallurgy parts. Based on results obtained from the dynamic compacting curve, ejection force curve, X-ray diffraction, micro-hardness of iron powder, friction condition and lubricant properties, densification mechanism of warm compaction can be drawn. In the initial stage, the rearrangement of powder particles is the main factor. It contributes more in the densification of warm compaction than that in cold compaction. However, in the later stage, the plastic deformation of powder particles is the primary factor. The increase in plasticity at high temperature can harmonize the secondary rearrangement of powder particles. During the compaction, the polymer lubricant has great contribution to the densification of the powder, since it improves the lubricating condition and effectively decreases the friction in the forming process and thus enhances the compact density. The dynamic compacting curve of warm compaction can be divided into three phases. The first is the particle rearrangement dominant phase; the percentage of particle rearrangement in warm compaction is higher than that in cold compaction by 15-31%. The second is the elastic deformation and plastic deformation dominant phase. The third is the plastic deformation dominant phase. The study of the powder densification mechanism can direct engineers in designing and producing warm compaction powders for high density parts.

Abstract: Conventional powder metallurgy processing can produce copper green compacts with density less than 8.3 g/cm3 (a relative density of 93%). Performances of these conventionally compacted materials are substantially lower than their full density counterparts. Warm compaction, which is a simple and economical forming process to prepare high density powder metallurgy parts or materials, was employed to develop a Ti3SiC2 particulate reinforced copper matrix composite with high density, high electrical conductivity and high strength. In order to clarify the warm compaction behaviors of copper powder and to optimize the warm compaction parameters, effects of lubricant concentration and compaction pressure on the green density of the copper compacts were studied. Copper compact with a green density of 8.57 g/cm3 can be obtained by compacting Cu powder with a pressure of 700 MPa at 145°C. After sintered at 1000°C under cracked ammonia atmosphere for 60 minutes, density of the sintered compact reached 8.83 g/cm3 (a relative density of 98.6%). Based on these fabrication parameters a Ti3SiC2 particulate reinforced copper matrix composite was prepared. Its density, electrical conductivity, ultimate tensile strength, elongation percentage and tribological behaviors were studied.

Abstract: A 15 wt.% NbC particulate reinforced iron-based composite was prepared by using warm compaction PM technique. It possesses a high relative density of 98%, a tensile strength of 515 MPa, a hardness of HRC 58 and a remarkable tribological behavior. Warm compaction was used because it can provide compacts with high green density and also increase the formability of the mixed powder. Furthermore, it can provide green strength that is strong enough to handle compacts before sintering. Block on ring tribotester was employed to study the compact’s friction and wear behavior using GCr15 steel as counterpart. A load of 980 N was used. The friction coefficient was 0.085 when the number 20 engine oil as lubricant was used. The results showed that the sintered composite has excellent wear resistivity. This material was then applied in the valve-guide cup of a combustion engine and subject to 500 hour bench test. The cup showed good performance for this test and did not shown any severe wear on the working surface after the test was completed. Therefore, it is a suitable material for parts that are exposed to severe wear condition.