Materials Science Forum Vols. 638-642

Abstract: The present investigation attempted to optimize the R-D (reduction-diffusion) process for fabricating Sm2Fe17 nanoscale powder from ball-milled powders of samarium oxide and iron oxide using a solid reducing agent of calcium hydrides (CaH2). It was found that the target alloy phase of Sm2Fe17 can be produced by controlling the gas atmosphere in the process of powder preparation to R-D reaction. Powder handling of CaH2 in a protective atmosphere is essential to avoid the formation of Ca(OH)2 which suppresses calcium formation. A switching gas atmosphere of H2 to Ar-H2 during the R-D process at 350oC resulted in a reduction of Fe2O3 and alloying of Sm-Fe, consequently forming nanocrystalline Sm2Fe17.

Abstract: In conventional warm compaction, both powder and die are heated to a certain temperature during compaction. This is a technique for producing P/M compacts with higher green and sintered strength as compared to room temperature pressing. However, there is a certain limit to powder temperature due to flow problems at higher temperatures. Heating the die above this practical limit can further improve properties. In this work, the effect of die temperature on green and sintered properties of Astaloy CrM powder has been investigated. Here, the powder at 135 oC was fed to the die at different temperatures. Density and strength for samples in green and sintered conditions were evaluated for two compaction pressures of 500 and 650MPa and temperatures ranging from ranging from 135 to 165 oC. Comparison of samples compressed at room temperature showed marked improvement in density and strength properties. A 22% increase in density, as well as 40% increase in green strength was observed. Tensile and impact strengths were improved by about 10% and 20% respectively. SEM micrographs showed more rounded pores and hence reduced stress raising sites. The improvement in properties can be mainly attributed to changes in powder morphology and die wall lubrication due to migration of hot lubricant from interparticle space to die walls. The latter will reduce particle spacing and bring about more intimate metal-metal contacts as well as better lubrication on die walls.

Abstract: Nitrogen alloying in steel may greatly increase the strength and corrosion resistance of the material. This paper introduced some research results of high nitrogen stainless steel (HNS) investigation via PM process. Nickel free high nitrogen stainless steels (17Cr12Mn2MoN) and superaustenitic high nitrogen stainless steels (28Cr6Mn2/6Mo10/20NiN) were investigated via gas atomization and HIP processes. Nitrogen alloying behavior during atomization and consolidation processes was investigated. Powders with nitrogen content up to 1% were manufactured by gas atomization process. Nickel free high nitrogen stainless steels with nitrogen up to 0.6% exhibits high strength and ductility at as-HIPed and solution annealed state, and superaustenitic HNS with nitrogen content up to 1% showed very high strength and good ductility at solution annealed state, with b at 1100 MPa, s at 810 MPa and elongation of 43%. PM HNS exhibited excellent corrosion resistance.

Abstract: Ultrafine grained WC cemented carbides were manufactured successfully by reaction sintering with Co-Al intermetallic compounds as binder from mixed powders of a nominal composition of WC-8Co-2Al (wt%). The purpose of adding aluminum to the composition is to promote the mechanical properties of hardmetals by the in situ formation of Co-Al intermetallic phase. To manufacture bulk ultrafine WC hardmetals, ultrafine and even nanocrystalline WC powders were prepared as precursors from the mixed commercial powders by mechanical milling. Experimental results show that the transverse rupture strength and hardness of sintered hardmetal reached the maxima, being 1747.8 MPa and 90.2 HRA respectively, when the milling time expands to 55 h. It is affirmed that mechanical milling weakens the thermostability of WC-8Co-2Al powders, and facilitates the formation of Co-Al intermetallic compounds. However, overlong milling results in decreasing of properties and densities of sintered hardmetals, because the powder particles are severely refined and markedly agglomerated.

Abstract: Air-atomised pure aluminium powder with additions of 10 at.% of AgO, PtO2 or PdO was mechanically alloyed (MAed) by using a vibrational ball mill, and MAed powders were consolidated into bulk materials by a spark plasma sintering (SPS) process. Mechano-chemical reactions among pure Al, precious metal oxide and stearic acid, added as a process control agent, during the mechanical alloying (MA) process and subsequent heat treatments were investigated by X-ray diffraction. The mechanical properties of MAed powders obtained under various heat treatment conditions and those of the SPS materials were evaluated by hardness tests. Mechano-chemical reactions occurred in Al/precious metal oxide composite powders during 36 ks of the MA process to form AlAg2, Pt and Al3Pd2 for the Al-AgO, Al-PtO2 and Al-PdO systems, respectively. Further solid-state reactions in MAed powders have been observed after heating at 373 K to 873 K for 7.2 ks. The hardness of MAed powders initially increased significantly after heating at 373 K and then generally decreased with increasing heating temperatures. The full density was obtained for the SPS materials under the conditions of an applied pressure of 49 MPa at 873 K for 3.6 ks. All the SPS materials exhibited hardness values of over 200 HV in the as-fabricated state.

Abstract: The goal of the work is fabrication coatings with the pressureless forming method or laser treatment retaining the relatively high ductility of the coated tool's core. The paper presents selection of the binder portion and type, and also of the metallic and carbides powders (WC) being the constituents of the polymer-powder slurry which was applied onto the prepared surfaces of the test pieces from the conventional HS6-5-2 high speed steel. This materials was compared with the same conventional HS6-5-2 high speed steel heat-treatable steel after laser treatment conditions and alloying additions contained in WC. Investigation indicate the influence of the alloying carbides on the structure and properties of the surface layer of investigated steel depending on manufacturing conditions and power implemented laser (HPDL). In the effect of laser alloying with powders of carbides occurs size reduction of microstructure as well as dispersion hardening through fused in but partially dissolved carbides and consolidation through enrichment of surface layer in alloying additions coming from dissolving carbides. The resistivity to thermal fatique of laser remelted steel is higher than steel after heat treatment. It shows the possibility of applying the worked out technology to manufacturing or regeneration of chosen hot working tools.

Abstract: Impregnating resin into the open pores of a sintered iron compact is well known to improve the machinability of the compact. However, the causes of this phenomenon require further investigation. The purpose of this study is to clarify the main cause of the improvement in machinability on resin impregnation. In this study, sintered iron was machined and the influences of resin impregnation on its thermal properties, coefficient of friction, and flow stress (deformation resistance) were investigated. The results indicate that the great improvement in machinability produced by resin impregnation is mainly due to a reduction in the plastic deformation (fracture strain) for chip generation, lowering the degree of work hardening and consequently reducing the cutting force required.

Abstract: Aluminium RAl-1 and its alloy Al17Si5Fe3Cu1.1Mg0.6Zr composite materials were manufactured from powder mixtures by cold pressing, hot closed-die forging at 480oC and heat treatment. Powders ranging in composition in 20 wt.% steps of the alloy were mixed in a Turbula mixer for 1 h. The preforms with alloy concentrations of 80 and 100 % were hot consolidated at 480oC and closed-die forged at the same temperature. The effect of chemical composition on microstructure and mechanical properties in bending and compression was examined. Bend strength ranged from 400 to 540 MPa, compression strength from 415 to 744 MPa and hardness from 32 to 203 HB. Simulated distribution of component materials for a cross-section of the forging and shapes of the materials were analysed using LARSTRAN/Shape finite element program and are qualitatively comparable with the results obtained by forging.

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: High speed steels (HSS) have been widely used worldwide. This paper introduced some research results of high speed steels via Powder Metallurgy process. High speed steels AHPT15 (W12Cr4V5Co5 ) were investigated via gas atomization, hot isostatic pressing, hot working and heat treatment processes. Microstructure of the steels, precipitation of carbides and the influence of nonmetallic inclusions on mechanical properties of the steels were investigated. PM HSS showed fine and uniform microstructure and exhibited excellent toughness and wear resistance, with bend strength up to 3500 MPa and hardness up to 68.0 HRC.