Experimental investigation of yield behaviour of metal powder compacts

Abstract

In this paper the compaction and yield response of two steel and two copper powders are examined. These were chosen to determine how the material response depends on the type of material and the morphology of the powder particles. Experiments were conducted in a computer controlled triaxial cell. Here, concentration is on the response during simulated, frictionless closed die compaction, whereby the radial stress is controlled so as to keep the radius of the sample constant. The compaction process was stopped at regular intervals and a series of probing paths were followed in stress space to construct the yield surface for the compact.

The experimentally determined yield surfaces are compared with yield surfaces predicted by empirical models and micromechanical models of the Fleck type, which assume that the compact consists of monosized spherical particles. During the early stages of compaction the form of the yield surfaces for spherical powders are consistent with Fleck's micromechanical model, but the surfaces become less elongated in the direction of loading at high densities. The yield surfaces for irregular shaped powders are significantly different from the predictions of the Fleck micromechanical model. A modified anisotropic Cam-Clay model is proposed, which is able to predict yield surfaces for the four powders at all densification levels.     

Effect of Al2O3 and B4C particle additions on microstructure of PM copper based brake linings

Abstract

This paper highlights the effect of different ceramic particles on the structure of PM copper based brake linings. The copper based brake linings using a range of ceramic additives (1–6 wt-%) were prepared by powder metallurgy (PM). The optimum conditions for the production of brake linings were determined as compaction under 400 MPa and sintering at 805°C for 20 min in an argon atmosphere. The density of copper based brake linings decreased after sintering with the increase in ceramic powder contents for both Al₂O₃ and B₄C ceramic particles. The microstructural characterisation of produced samples showed that the lower boiling point elements in the as supplied powder vaporise during sintering from the structure and this leads to an increase in the porosity amount of the final component.     

Mechanical properties and wear behaviour of PM aluminium composite reinforced with (Fe3Al) particles

Abstract

Mechanical properties and wear behaviour of an aluminium matrix composite reinforced with Fe₃Al intermetallics have been studied. A 2014 alloy manufactured through mechanical alloying was used as the matrix. Three different Fe₃Al intermetallics have been used as reinforcement, also manufactured through mechanical alloying. The difference between them was the different mechanical alloying times (5 and 20 h were used) and the possibility of carrying out a heat treatment at 1000°C (on the 20 h milled intermetallic) before admixing to the aluminium matrix. The processing of these composite materials included mixing and cold compacting (conventional powder metallurgy) followed by hot extrusion (without caning and degassing). The effect of a T6 heat treatment was also evaluated. The influence of intermetallic additions on the mechanical properties (hardness and tensile strength) and wear behaviour (pin on disk test) was established. All intermetallics showed a good link with the matrix, and high reactivity with it during the heat treatment, as the microstructural study supports.     

Thermophysics characteristics and densification of powder metallurgy composites

Abstract

An analytical densification model describing the final stages of hot pressing and sintering has been developed and found to be consistent with empirical findings. The behaviour of composite powders for the matrices of diamond tools has been studied under hot pressing conditions. Differential scanning calorimetry was used to determine the heat capacity at constant pressure C _p of pure Co, 663Cu, and composite iron- and cobalt based powders (also containing WC, Ni and 663Cu). The relationship between C _p and composite densification has been analysed, and it has been found that optimised rare earth additions to the iron based composite powders can produce C _p characteristics close or equivalent to that of pure Co powders. This modified composite powder has been used to hot press diamond drill and saw bits that show good properties. Employing a densification regime guided by the dynamic model has been found radically to improve stability in service (bend strength, hardness, impact, ductility and porosity).     

激光熔覆陶瓷颗粒增强金属基复合材料研究进展

陶瓷增强金属基复合材料(MMCs)因其优异的耐磨性、韧性、高温蠕变性能和疲劳强度,已被广泛应用于生物医学、航空航天、电子等高端工程行业。激光熔覆是在基体表面,利用激光束使陶瓷及其他特殊粉末与基体表层融化,并自激冷却形成冶金结合涂层的环保新技术,具有沉积效率高、厚度可控、热变形小、冷却快、稀释率小以及冶金结合等优点。本文介绍了激光熔覆陶瓷颗粒的形成方式对MMCs性能的影响,随后讨论了激光熔覆辅助能场及高速激光熔覆技术对MMCs的界面强化效应,并分析了陶瓷颗粒的强化机制和激光辅助能场的作用机制。最后,对目前激光熔覆陶瓷颗粒增强基金属复合材料研究的发展进行了展望。     

烧结温度对Al_3Ni金属间化合物增强铝基复合材料力学性能的影响

通过粉末冶金原位合成法制备Al3Ni金属间化合物增强铝基复合材料。采用X射线衍射,扫描电镜,硬度测试和压缩强度测试,研究烧结温度对复合材料微观结构和力学性能的影响。结果表明:在铝基体中成功获得了均匀分布的金属间化合物Al3Ni增强相;随烧结温度从570℃上升到590℃,复合材料的密度从2.435 g/cm-3上升到2.990 g/cm-3,维氏硬度从~24升高到~37;经590℃烧结制备的复合材料表现出了高的压缩强度(255 MPa)和伸长率(~40%)。     

纳米SiC增强纯Al基复合材料的微观组织和力学性能

与采用微米尺度SiC颗粒为增强相制备的Al基复合材料相比,以纳米SiC颗粒为增强相制备的Al基复合材料具有更加优异的力学性能,可极大提高SiC增强Al基复合材料的服役可靠性及应用范围。采用传统粉末冶金方法制备纳米SiC颗粒增强纯Al基复合材料,研究烧结温度和增强相体积分数对复合材料微观结构和力学性能的影响。研究表明,烧结温度和增强相体积分数均对复合材料的微观结构和力学性能有重要影响。随烧结温度升高,复合材料中的残留微孔减少,密度和强度均得到显著提高。含体积分数为3%纳米SiC颗粒的复合材料在610℃具有最高的强度,进一步提高纳米SiC颗粒的含量并不能提高材料的力学性能,这主要是由于当纳米SiC颗粒的体积分数超过3%时将出现明显的团聚,从而降低强化效应。     

碳纳米管和氧化铝颗粒协同增强铜基复合材料的制备与性能研究

本文首先利用碱式高锰酸钾对纯化后的CNTs进行改性处理,然后用分子水平法制得前驱体CNTs/Cu复合粉末,最后用内氧化方法,结合放电等离子烧结获得CNTs、Al_2O_3/Cu复合材料。结果表明:CNTs、Al_2O_3/Cu复合材料的维氏硬度(136)和抗拉强度(226 MPa)均优于两个增强相单独作用的铜基材料;材料的断后伸长率超过纯铜(40.1%),达到43.6%,表现出非常好的塑性;CNTs和Al_2O_3两个增强相对铜基材料导电率起到了协同增强作用,达到了1+12的效果。     

Al_2O_3含量对碳纳米管增强Cu基复合材料性能的影响

利用机械合金化法(MA)、磁力搅拌法(MS)、放电等离子烧结工艺(SPS)制备材料样品,研究了Al2O3含量对碳纳米管(CNTs)增强Cu基复合材料性能的影响。结果表明,加入Al2O3与碳纳米管增强相后的Cu基复合材料与纯Cu相比,磨损率降低了70.9%~85.7%,维氏硬度提高了11.6%~24.5%。当添加1.0%CNTs和1.6%Al2O3(质量分数)时所制备的复合材料的综合性能最优:相对密度为97.5%,维氏硬度为75.2 HV,热导率为272.45 W/(m·K),电导率为4.39×107Ω-1·m-1。     

Investigation of corrosion and wear properties of Fe based metal matrix composites consolidated by sintering and hot isostatic pressing

Abstract

Many industrial applications, e.g. processing of polymers, suffer from high costs caused by corrosion and wear. Particularly the combination of both increases the requirements for the materials used. Corrosion resistant cold work steels were developed to withstand the combined attack. Resistance is achieved by a sufficient content of chromium in the metal matrix and by carbides dispersed in a martensitic matrix. A further gain in wear resistance is possible by adding hard phases to the steel to produce a particulate reinforced metal matrix composite (MMC). The common consolidation process for such MMCs is hot isostatic pressing, but they can also be processed by solid state or liquid phase sintering. This work focuses on detailed investigations of the properties in dependence on the processing route. The results show that the resulting corrosion and wear resistance depend not only on the processing method, but also on the incorporated hard phases in combination with the manufacturing method. In addition, the unreinforced metal matrices were compared to the MMC.