Powder metallurgy materials in hot forming

Abstract

The purpose of the present paper is to determine the apparent yield stress of powder metallurgy (PM) materials at high temperatures. A brief introduction concerning the theory of yielding of PM materials is included. The models of loading functions for porous materials are recalled. The experiments have been undertaken by the author to identify the parameters of PM materials in hot forming. Two materials are considered: pure iron and aluminium powders.     

Formation of aluminium nitride during sintering of powder injection moulded aluminium

Abstract

A detailed transmission electron microscopy study of the structure of aluminium nitride formed during sintering of powder injection moulded aluminium is presented. A polycrystalline layer formed on Al particle surfaces exposed to a nitrogen atmosphere. This layer consisted of fine, rod-like crystallites of hexagonal AlN typically aligned normal to the Al surface. A double layer of AlN separated by a thin layer of Al was observed at the interfaces between Al grains. In this report, the structure of the nitride is characterised and its influence on sintering is discussed.     

Effect of pulsed current on reactively synthesised TiB2 consolidated by plasma pressure compaction

Abstract

The effect of pulsed current on TiB₂ formed by reactive consolidation between titanium and boron is reported in this paper. This consolidation was performed using the plasma pressure compaction (P²C) technique. A comparison between the pulsed and control samples reveals that pulsed current reduces grain growth (pulsed samples had an average grain size of 2·79 μm compared to 5·99 μm) while increasing sintering rates (pulsed samples were on average 15·5% more dense). The reduced grain growth and increased densification is due to the removal of adsorbed oxygen from the surface of the powder.     

Liquid phase sintering of ferrous powder by carbon and phosphorus control

Abstract

Powder mixtures composed of liquid forming master alloy powder and coarse iron powder were sintered to near full density by having a high amount (20 wt-%) of liquid phase during sintering. This was made possible by the use of the Fe-P-C system with or without Cu. Without post-sintering treatment, a brittle microstructure was obtained. By means of altered C and P control and decarburisation heat treatment of the as sintered material, the final non-brittle microstructure was achieved. Using the open porosity and liquid phase as a diffusion path, rapid decarburisation is created and the local combination of carbon and phosphorus in the microstructure is avoided. In this way, iron phosphide is not formed on grain and/or particle boundaries. Presence of pores is confirmed to be beneficial for grain growth control.     

Role of reinforcement in sintering of SiC/316L stainless steel composite

Abstract

Austenitic stainless steels with improved corrosion resistance are gaining wide popularity. However, their applications are limited because of their poor tribological properties. The present work was undertaken to improve the overall performance of 316L stainless steel by reinforcing it with SiC. During the processing of the 316L SS composite, the 316L SS matrix was found to interact strongly with the SiC at 1100°C resulting in the formation of low melting Fe–SiC phase. An attempt to process SiC/316L SS composite above this temperature resulted in complete melting of the composite compact.     

Microstructural changes in Mo steels during sintering and effect on electrical conductivity

Abstract

An electrical conductivity measurement method was used for studying the sintering mechanism and microstructural changes of low alloyed PM Mo steels in a temperature range between 600-1300°C. The influences of alloying method (elemental or prealloyed), Mo content (1·5 and 3·5 wt-%), and sintering temperature were investigated. The results show that the effects of, for example, formation of Mo carbide(s), ferrite-austenite phase transformation, as well as liquid phase formation during heating of the steel compacts can be detected by the technique cited. Mo dissolution during sintering of compacts from mixed powders results in a decrease of the conductivity with increasing sintering temperature while compacts from Fe-Mo prealloyed powders exhibit the standard behaviour of higher conductivity after sintering at higher temperature. Moreover, the relationship between Mo dissolution, formation of sintered contacts, and mechanical properties was demonstrated to assess the viability of the conductivity measurement method for studying the sintering behaviour of PM materials and its influence on physical and mechanical properties. An approach was also demonstrated for relating the conductivity to the microstructural parameters, e.g. total porosity and contiguity between solid phase, that would be useful for predicting relative changes in mechanical properties dependent on porosity and pore morphology.     

Identification of flow instabilities during hot working of powder metallurgy superalloy IN 100

Abstract

A simple instability condition based on the Ziegler's continuum principles as applied to large plastic flow, is extended for delineating the regions of unstable metal flow during hot deformation of powder metallurgy materials. This criterion has been applied to the existing flow stress data of powder metallurgy superalloy IN 100 and compared with the reported microstructural observations. The optimum hot working conditions for the PM superalloy IN 100 are suggested.     

Utilisation of silicon rubber to characterise tool surface quality during die compaction

Abstract

A new methodology was developed to observe and measure tool wear and tool surface quality during the die compaction process. The newly developed method is a non-destructive test that relies on silicon rubber to transcribe the inner surface profile of the compaction die. After verification of the method, aluminium and iron alloy powders were compacted to quantify tool wear and tool surface quality with two die materials, tungsten carbide and tool steel. The tool surface quality was quantified by recording surface roughness of the die replicas on a surface profilometer.     

Volume changes accompanying the sintering of compacts from mixtures of titanium and iron powders

Conclusions The sintering of compacts from mixtures of titanium and iron powders at temperatures below the eutectic point is accompanied by their shrinkage, the sintering process being mainly determined by the particle size of the starting powders. During the sintering of compacts from mixtures of titanium and iron powders at temperatures exceeding the eutectic point the compacts grow in size, the extent of the growth depending on the particle size of the powders. This phenomenon may be attributed to the effect of heterodiffusion on sintering processes and to the crystallization pressure generated during the formation of intermetallic compounds.Translated from Poroshkovaya Metallurgiya, No. 5(233), pp. 17–21, May, 1982.     

Swelling during sintering of titanium alloys based on titanium hydride powder

Abstract

Compacts were prepared by pressing titanium and titanium hydride powders mixed with nickel powder and sintering under vacuum. Severe swelling was observed only for compacts based on TiH₂ powder. Pressure changes in the vacuum furnace, dilatometry results and mass loss data all indicate that dehydrogenation of TiH₂ powder compacts occurs at lower temperature than any significant sintering. Swelling appears to have been caused by a contaminant in the TiH₂ powder rather than hydrogen. The onset of severe swelling during heating was associated with the formation of liquid phase as the solidus was crossed. However, some swelling appears to take place under solid state sintering conditions. Various results indicate that the mechanism of swelling is high gas pressure within closed pores. Large pores appear to form by breakage of ligaments between small pores followed by opening of the pore. It appears that the use of (uncontaminated) TiH₂ powder in place of Ti powder would allow the benefit of lower green porosity to be retained during sintering to achieve low sintered porosity.     

低温固相反应法制备铈酸镧粉体及其烧结性能研究

以六水合硝酸镧和六水合硝酸铈为原料,一水合柠檬酸为络合剂,通过低温固相反应法制备铈酸镧前驱体,经不同温度煅烧制备铈酸镧粉体。利用红外光谱和综合热分析研究前驱体的结构和热分解过程,并通过X射线衍射分析、扫描电子显微镜观察、透射电子显微镜分析和体积密度测试等手段对不同煅烧温度合成的粉体物相、形貌及烧结性能进行表征。结果表明:当煅烧温度达到600 ℃时,前驱体开始生成铈酸镧晶体,且随着煅烧温度的提高,晶体发育不断完善,晶粒逐渐长大。经800 ℃煅烧可获得单相的铈酸镧粉体,再经1600 ℃烧结,试样的相对密度达到95.5%。     

固态下SiO2对二元铁酸钙形成过程的影响

 铁酸钙作为高碱度烧结矿黏结相的主要组成,其形成受SiO2的影响,但到目前为止,其影响机理尚不是十分明确。通过XRD和矿相显微镜观察,并结合Rietveld法定量分析,研究了SiO2对二元铁酸钙形成过程的影响。结果表明：Fe2O3与CaO反应,700 ℃时形成Ca2Fe2O5,在800 ℃时出现CaFe2O4,随着温度升高,Ca2Fe2O5逐渐向CaFe2O4转变。SiO2存在时,在铁酸钙形成过程中有Ca2SiO4生成,温度低于900 ℃时,受动力学条件的影响,反应速率较低,Ca2SiO4的量相对较少,另外,SiO2的阻碍作用随其质量分数的增加而增强,进而导致已形成的Ca2Fe2O5与Fe2O3未能继续反应形成CaFe2O4而使其质量分数逐渐增多;高于900 ℃时,随着温度的升高,动力学条件改善,离子扩散能力增强,其中,Fe3+通过CaFe2O4层比Ca2+通过Ca2Fe2O5层更容易,进而促进了CaFe2O4形成反应的进行,Ca2Fe2O5向CaFe2O4转变,但随着SiO2质量分数增加,Ca2SiO4的生成量增多,CaO与Fe2O3的接触面积减小,抑制了CaFe2O4的生成。     

固相烧结低钨含量W-Ni-Fe合金的微观结构与力学性能

采用固相烧结工艺(1 300℃保温1 h)制备低钨含量(质量分数为60%~80%)的W-Ni-Fe合金,测定合金的抗拉强度、抗压强度和伸长率,利用金相显微镜观察合金的显微组织,并通过扫描电镜(SEM)观察合金断口形貌,研究钨含量对固相烧结W-Ni-Fe合金力学性能与微观结构的影响。结果表明:随钨含量降低,合金的孔隙率和平均孔径减小,抗拉强度增大,伸长率显著提高,抗压强度变化不大。W含量为60%~80%的W-Ni-Fe合金,其孔隙率为17.8%~21.4%,抗拉强度为231~262 MPa,抗压强度2 450~2 550 MPa,伸长率为0.3%~2.3%,压拉比为9.45~11.04,都能满足易碎型穿甲弹弹芯材料的性能要求。     

Synthesis and characterisation of Ti6Al4V/xTa alloy processed by solid state sintering

ABSTRACT

This work investigates the effect of Ta particle addition into a Ti6Al4V alloy processed by solid state sintering. The volume fraction of Ta ranged between 0 and 30?vol.-%. The sintering kinetics of powder mixes are evaluated by dilatometry. Sintered materials are characterised by SEM and XRD, and their mechanical properties are obtained from microhardness and compression tests. Sintering behaviour and final microstructure are affected by Ta particles, which slow down the densification, lower the temperature of α-to-β phase transition and stabilise the β phase. Mechanical properties, as microhardness, Young’s modulus and yield stress, depend on the microstructure reached after sintering and on the residual porosity. An equation expressing the Young’s modulus of Ti6Al4V/xTa alloy as function of x and porosity is proposed and validated. The materials with at least 20?vol.-% of Ta exhibited a high strength to modulus ratio, which is suitable for orthopaedic implants.     

Mechanics of sintering materials with bimodal pore distribution. II. Mean-square strain rate for the solid phase of a biporous body and instability of the monomodal pore structure during constrained sintering

Sintering of materials with a bimodal porous structure under conditions of external kinematic limitations that make their macroscopic deformation impossible is considered. It is established that in contrast to materials that contain pores of the same size, with sintering of materials that have a bimodal pore distribution under the conditions indicated above there is deformation within the limits of the matrix material. It is demonstrated that a monomodal pore structure under the same conditions is not stable, in the sense that small disturbance of the uniform pore distribution with respect to size may lead to marked stratification during sintering. Tangential stresses that arise in the solid phase as a result of the existence of two types of pores are evaluated. __________ Translated from Poroshkovaya Metallurgiya, Nos. 1–2(447), pp. 36–44, January–February, 2006.