Degassing of Al-Si-X powders assisted by flushing with argon or nitrogen

The results reported in this paper showing the effect of different degassing modes on the Al-20Si-3Cu-1 Mg powder are a complement of our previous papers concerning the continuous and non-continuous degassing sequences of the same powder. This research was mainly directed at an improvement in the technique to remove volatile and gaseous contaminants from the porous compact made from the Al-20Si-3Cu-1 Mg powder. This improvement has been possible by modifying the conventional degassing process of the powder as reported previously, namely degassing of the powder assisted by flushing with depurative gases such as argon or nitrogen. It is apparent that flushing with argon or nitrogen increases the efficiency of moisture and hydrogen evolution in comparison with the conventional degassing mode.     

Heating sequence and hydrogen evolution in alloyed aluminium powders

The results reported here, showing the effect of a non-continuous degassing sequence on the Al-20Si-3Cu-1 Mg powder, are a complement of previous work concerning the continuous degassing of the same powder. The degassing experiments were carried out, under high vacuum, in the temperature range 20 to 550 °C in a horizontal furnace heated at a uniform heating rate of 2.5 °C min^–1. The partial pressures of the released gases were monitored and analysed during the heating phase by a computerized Edwards EQ80F residual gas analyser (RGA). RGA measurements indicate that water and hydrogen are the main degassing products. A complete degassing can only be achieved if the sample is heated up to a temperature where the chemical reactions are finished in the applied time. Thermodynamical equations alone are not enough to explain the kinetics of degassing of aluminium powders. The diffusion of aluminium through its surface oxide layer (Al₂O₃), described by the self-diffusion of aluminium, can explain to a large extent the kinetics of degassing aluminium powders.     

Effect of extrusion conditions on mechanical properties of Al-20Si-3Cu-1 Mg alloy prepared from rapidly solidified powder

A study on the optimization of extrusion conditions for a prospective Al-20Si-3Cu-1 Mg alloy prepared from rapidly solidified powder was carried out by evaluating tensile properties at room and elevated temperatures. It was found that extrusion conditions influence the asextruded microstructure and mechanical properties of the alloy to a certain extent. The relationship between the as-extruded mechanical properties and such extrusion variables as temperature, reduction ratio and die shape, can be described by using temperature-compensated strain rate, so that the as-extruded properties can be tailored in a certain range by adjusting this process parameter. In addition, the comparison between the as-extruded and as-T6 tempered tensile properties at elevated temperatures has opened the question as to the necessity of applying the heat treatment to the alloy — a normal practice subsequent to the extrusion. The experimental results suggest that for the material used at temperatures at and above 200 °C, the T6 temper treatment can be eliminated.     

The influence of Saffil δ-alumina fibre reinforcement on gas entrapment and evolution in P/M Al-20Si-3Cu-1Mg- x Al2O3 composite

Compacts of an aluminium/alumina fibre composite, produced by a powder metallurgical (P/M) route, were the subject of a vacuum degassing study. The materials consisted of Al-20 wt % Si-3 wt % Cu-1 wt % Mg pre-alloyed powder mixed with 0%, 5% and 20% (by volume) of Saffil -alumina fibres. The composition of gases liberated during heating at constant rate was monitored with a quadrupole mass spectrometer. Water and hydrogen were the main volatile products released. High water and hydrogen pressures were maintained longest by the composite with the highest volume fraction of fibres. The presence of fibres gave rise to delayed peak hydrogen production when compared to the matrix. Nitrogen adsorption analysis proved that the Saffil fibre RF Milled, possesses a large specific internal surface area and points towards the presence of very fine-sized porosity. Thermogravimetric analyses confirmed a high water content of the fibres of at least 0.86 wt %. Of this, condensed and adsorbed water constitute the largest fraction. Owing to the fineness of the porosity and consequently the low concentration gradients, time as well as temperature (for enhancement of diffusion) are equally important variables to be considered for effective degassing of these composites.     

The effect of extrusion parameters on the fretting wear resistance of Al-based composites produced via powder metallurgy

The work reported in this paper is aimed at establishing the relationship between processing and the wear resistance of the metal matrix composites (MMCs) based on a novel alloy, Al-20Si-5Fe-3Cu-1Mg. The MMCs were processed via a commercially viable powder metallurgy (PM) route, i.e. through mixing the atomized matrix alloy powder with 10 vol% SiC or Al₂O₃ particles, cold isostatic pressing, degassing and hot extrusion. It has been found that the extrusion window of the MMCs is greatly narrowed due to their increased deformation resistance on one hand and incipient melting of their matrix on the other. For a sound MMC extrudate, a reduction ratio over a critical value must be applied. However, a further rise of this ratio leads to deterioration of local interfacial cohesion between the ceramic phase and the matrix dispersed with a high volume fraction of silicon crystals and intermetallic dispersoids, thus degrading the MMCs in tensile properties. Furthermore, fretting wear tests at room and elevated temperatures and with dry and wet contacts show that the MMCs extruded at a higher reduction ratio has a higher mass loss and an increased friction coefficient. The work points to the direction of further research, i.e. on MMCs containing spherical reinforcement instead of commonly used angular particles.     

Superior high-temperature resistance of aluminium nitride particle-reinforced aluminium compared to silicon carbide or alumina particle-reinforced aluminium

Aluminium-matrix composites containing AlN, SiC or Al₂O₃ particles were fabricated by vacuum infiltration of liquid aluminium into a porous particulate preform under an argon pressure of up to 41 MPa. Al/AlN had similar tensile strengths and higher ductility compared to Al/SiC of similar reinforcement volume fractions at room temperature, but exhibited higher tensile strength arid higher ductility at 300–400 °C and at room temperature after heating at 600 °C for 10–20 days. The ductility of Al/AIN increased with increasing temperature from 22–400 °C, while that of Al/SiC did not change with temperature. At 400 °C, Al/AlN exhibited mainly ductile fracture, whereas Al/SiC exhibited brittle fracture due to particle decohesion. Moreover, Al/AlN exhibited greater resistance to compressive deformation at 525 °C than Al/SiC. The superior high-temperature resistance of Al/AlN is attributed to the lack of a reaction between aluminium and AlN, in contrast to the reaction between aluminium and SiC in Al/SiC. By using Al-20Si-5Mg rather than aluminium as the matrix, the reaction between aluminium and SiC was arrested, resulting in no change in the tensile properties after heating at 500 °C for 20 days. However, the use of Al-20Si-5Mg instead of aluminium as the matrix caused the strength and ductility to decrease by 30% and 70%, respectively, due to the brittleness of Al-20Si-5Mg. Therefore, the use of AIN instead of SiC as the reinforcement is a better way to avoid the filler-matrix reaction. Al/Al₂O₃ had lower room-temperature tensile strength and ductility compared to both Al/AlN and Al/SiC of similar reinforcement volume fractions, both before and after heating at 600 °C for 10–20 days. Al/Al₂O₃ exhibited brittle fracture even at room temperature, due to incomplete infiltration resulting from Al₂O₃ particle clustering.     

Effect of Precipitating Phases on Tensile Strength and Microstructural Stability of a Spray-Deposited Al-Si-Fe-Mn-Cu-Mg Alloy

Spray deposition is a novel process which is used to manufacture rapidly solidified bulk and near-net-shape preforms. In this study, AI-20Si-5Fe-3Mn-3Cu-1Mg alloy was synthesized by spray deposition technique. The aging process of the alloy was investigated by differential scanning calorimetry (DSC) analysis and transmission electron microscopy (TEM). The results show that two kinds of phases, i.e. S(Al2CuMg) and σ(Al5Cu6Mg2), precipitate from matrix and improve the tensile strength of the alloy efficiently at both the ambient and elevated temperatures (300℃). In addition, the σ-Al5Cu6Mg2 is a relatively stable phase which improves microstructural stability of the alloy.     

Effect of temperature on the strength and conductivity of a deformation processed Cu-20%Fe composite

The high temperature (22–600 °C) properties were evaluated for a Cu-20%Fe composite deformation processed from a powder metallurgy compact. The ultimate tensile strengths decreased with increasing temperature but were appreciably better than those of similarly processed Cu at temperatures up to 450 °C. At 600 °C, the strength of Cu-20%Fe was only slightly better than that of Cu as a result of the pronounced coarsening of the Fe filaments. However, at temperatures of 300 and 450 °C, the strength of Cu-20%Fe is about seven and six times greater, respectively, than that of Cu, as compared to about a two fold advantage at room temperature. Therefore, Cu-20%Fe composites made by deformation processing of powder metallurgy compacts have mechanical properties much superior to those of similarly processed Cu at room temperature and at temperatures up to 450 °C. The pronounced decrease in electrical conductivity of deformation processed Cu-20%Fe as compared to Cu is attributed to the appreciable dissolution of Fe into the Cu matrix which occurred during the fabrication of the starting compacts where temperatures up to 675 °C were used. While the powder metallurgy compacts used for the starting material for deformation processing in this study did not lead to a high conductivity composite, the powder metallurgy approach should still be a viable one if processing temperatures can be reduced further to prevent the dissolution of Fe into the Cu matrix.     

Zr+Er及Zr+Y对Al-Mg-Si-Cu-Mn-Cr合金组织和拉伸力学性能的影响

采用光学显微镜(OM)、电子显微镜(SEM)、能谱分析仪(EDS)、透射电子显微镜(TEM)和拉伸强度试验研究了0.3%Zr+0.2%Er和0.3%Zr+0.2%Y(质量分数)两种元素组合对Al-0.6Mg-0.9Si-0.5Cu-0.5Mn-0.2Cr合金热处理前后微观组织和力学性能的影响。研究结果表明,两种元素组合的加入均能细化铸态晶粒,其中Zr+Y改性合金的晶粒细化更明显,且两种改性合金铸态的强度和塑性均较基体合金提高。在热处理过程中,两种元素组合会影响合金中弥散相的析出,降低合金的弥散强化效果。最终,Zr+Er改性合金的强度较基体合金提高,而Zr+Y改性合金的强度则较基体降低,两种合金的塑性均较基体降低。     

Preparation of Al-20Si-4.5Cu alloy and its composite from elemental powders

Hypereutectic Al-Si-Cu alloys with a low thermal expansion coefficient and good wear resistance are commonly prepared from pre-alloyed powders using atomization. In the present work, an attempt was made to explore the possibility of fabricating the materials from cheaper elemental powders through sintering the compacts of the mixture of a silicon powder and an Al-4.5Cu elemental powder in the liquid state. Another advantage of taking this fabrication route is that it gives an additional flexibility to incorporate Al₂O₃ particles into the alloys to form aluminium matrix composites with a further improved Young's modulus, dimensional stability and wear resistance. Due to the change in the phase constitution brought about by the silicon addition, the sintering scheme for the Al-Cu elemental powder must be modified. The results show that it is well possible to take advantage of the good sinterability of the Al-4.5Cu elemental powder, to maintain the dimensions of the Al-20Si-4.5Cu compacts and to hold their shape during liquid-phase sintering. After consolidation with hot extrusion and heat treatment, the materials show an improved Young's modulus and a lowered thermal expansion coefficient at the sacrifice of strength and ductility. The success in using the elemental powders to produce the hypereutectic Al-Si-Cu alloys and their composites opens up a new flexible and economic way to tailor the properties of the materials.     

Effect of carbon on the mechanical properties of powder-processed Fe-0.45wt% P alloys

The present paper records the results of mechanical tests on iron-phosphorus powder alloys which were made using a hot powder forging technique. In this process mild steel encapsulated powders were hot forged into slabs. Then the slabs were hot rolled and annealed to relieve the residual stresses. These alloys were characterized in terms of microstructure, porosity content/densification, hardness and tensile properties. Densification as high as 98.9% of theoretical density; has been realized. Microstructures of these alloys consist of single-phase ferrite only. Alloys containing 0.45 wt% P; such as Fe-0.45P-2Cu-2Ni-1Si-0.5Mo and Fe-0.45P-2Cu-2Ni-1Si-0.5Mo-0.15C show very high strength. Alloys developed in the present investigation were capable of being hot enough to be worked to very thin sheets and fine wires.     

Fracture features of a silicon-dispersed aluminium alloy extruded from rapidly solidified powder

The tensile fractography of an AI-20Si-3Cu-1 Mg alloy consolidated from rapidly solidified powder by extrusion has been investigated using optical and electron microscopy, and related to the processing conditions as well as the tensile behaviour of the alloy at room and elevated temperatures. The alloy studied shows distinct fracture features owing to the presence of dispersed silicon crystal particles with a bimodal distribution in size and of prior powder particle boundaries in the extrudate. It has been found that at room temperature cracks initiate by cracking the primary silicon crystal particles. Crack propagation occurs along the interfaces between the eutectic silicon crystal particles and the matrix and also between the prior powder particles, where microvoids are formed by the interfacial decohesion. At 300 °C, the fracture of the alloy involves microvoid nucleation, growth and coalescence at the interfaces between the silicon crystal particles and the aluminium matrix and between the prior powder particles. It has also been observed that the fractographic features of the alloy correspond well to the processing conditions including extrusion temperatures and subsequent heat treatment. The importance of minimizing the coarsening of the silicon crystals in processing in order to use the full strength potential of the alloy investigated is emphasized.     

Artificial ageing of Al-Si-Cu-Mg casting alloys

The T6 heat treatment is commonly used for gravity cast Al-Si-Cu-Mg alloys. The influence of the alloying elements Cu and Mg and the artificial ageing temperature on the age hardening response were investigated. Artificial ageing was conducted at 170 °C and 210 °C for various times for three alloys, Al-7Si-0.3Mg, Al-8Si-3Cu and Al-8Si-3Cu-0.5Mg, cast with three different solidification rates (secondary dendrite arm spacing of about 10, 25 and 50 μm). The coarseness of the microstructure has a small influence on the yield strength, as long as the solution treatment is adjusted to obtain complete dissolution and homogenisation. The peak yield strength of the Al-Si-Mg alloy is not as sensitive to the ageing temperature as the Al-Si-Cu and Al-Si-Cu-Mg alloys are. The ageing response of the Al-Si-Cu alloy is low and very slow. When 0.5 wt% Mg is added the ageing response increases drastically and a peak yield strength of 380 MPa is obtained after 20 h of ageing at 170 °C for the finest microstructure, but the elongation to fracture is decreased to 3%. The elongation to fracture decreases with ageing time in the underaged condition as the yield strength increases for all three alloys. A recovery in elongation to fracture of the Al-Si-Cu-Mg alloy on overageing is obtained for the finest microstructure, while the elongation remains low for the coarser microstructures. The quality index, Q = YS + K?, can be used to compare the quality of different Al-Si-Mg alloys. This is not true for Al-Si-Cu-Mg alloys, as K depends on the alloy composition. Overageing of the Al-Si-Mg alloy results in a decrease in quality compared to the underaged condition.     

Microstructures and Mechanical Properties of TiCp／A1-4.5Cu-0.8Mg Composites by Direct Reaction Synthesis

Direct reaction synthesis (DRS), based on the principle of self-propagating high-temperature synthesis (SHS), is a new method for preparing particulate metal matrix composites. TiCp/Al-4.5Cu-0.8Mg composites were fabricated by DRS. Particulate composites were fabricated with Ti carbide (TiC) particles, generally less than 1.0μm. The reacted, thermal extruded samples exhibit a homogeneous distribution of fine TiC particles in Al-4.5Cu-0.8Mg matrix.Mechanical property evaluation of the composites has revealed a very high tensile strength relative to the matrixalloy. Fractographic analysis indicates ductile failure.     

Effect of consolidation temperature on mechanical properties of rapidly solidified Al–7Mg–1 Zr alloy

Abstract

The mechanical properties achieved via the extrusion of non-degassed billets prepared from an inert gas atomised powder of nominal composition Al–7Mg–lZr are reported. The alloy was extruded over the temperature range 350–550°C and the tensile mechanical properties and plane strain fracture toughness were evaluated. It was found that the yield strength remained fairly constant over the entire temperature range, with only a small decrease in strength observed at the highest extrusion temperature. The strength could be related to microstructure using standard models for solid solution, dispersoid, and substructural strengthening mechanisms, and the last was found to make the greatest contribution. The sensitivity of strength to subgrain size was found to be nearly three times higher than that for pure Al. The optimum combination of strength and fracture toughness was obtained for extrusion at 500°C (yield strength 400 MN m^?2; T–L K_Iv 21 MN m^?3; elongation 20%). The poor values of K_lv obtained at other temperatures were attributed to coarse dispersoids (highest extrusion temperature), undeformed powder particles (lowest extrusion temperature), and inhomogeneous dispersoid distributions (intermediate temperatures). It is concluded that extrusion process control plays an important role in determining the mechanical properties of consolidated rapidly solidified powders. Considering the excellent ductility and toughness obtained, vacuum degassing before extrusion may not be essential in the processing of inert gas atomised powders of a non heat treatable composition.

MST/1721     

连接SiCp/6063Al复合材料的Al-Si-Cu-Ce-Ti箔状钎料制备及其性能研究

采用快凝甩带技术制备了6组不同Ti含量的(Al-10Si-20Cu-0.05Ce)-xTi急冷箔状钎料,并对SiCp/6063Al复合材料进行真空钎焊,然后对钎料及接头的显微组织和性能进行分析。结果表明,急冷箔较常规铸态钎料的组织细小、均匀;固、液相线降低,熔化区间变窄;随着Ti含量的增加,急冷箔中片状Al-Si-Ti金属间化合物相增多,导致钎料脆性增加;6组钎料在复合材料上润湿性较差,但在6063Al合金上润湿性良好。在580℃钎焊温度、保温30min条件下,采用1%Ti含量急冷箔状钎料成功连接了SiCp/6063Al复合材料,钎焊接头组织致密、完整,急冷箔状钎料与6063Al合金基体连接界面可进行充分的冶金结合,且接头剪切强度达到104.9 MPa;钎焊前采用夹具增加接头压力可显著提高接头的连接质量。     

Microstructures and Mechanical Properties of TiC_P/Al-4.5Cu-0.8Mg Composites by Direct Reaction Synthesis

Direct reaction synthesis (DRS), based on the principle of self-propagating high-temperature synthesis (SHS), is anew method for preparing particulate metal matrix composites. TiCp/Al-4.5Cu-0.8Mg composites were fabricatedby DRS. Particulate composites were fabricated with Ti carbide (TiC) particles, generally less than 1.0 μm. Thereacted, thermal extruded samples exhibit a homogeneous distribution of fine TiC particles in Al-4.5Cu-0.8Mg matrix.Mechanical property evaluation of the composites has revealed a very high tensile strength relative to the matrixalloy.Fractographic analysis indicates ductile failure.     

铸态Mg-4Al-4Si-0.75Sb合金的显微组织与力学性能

采用重力铸造法制备Mg-4Al-4Si-0.75Sb(AS44-0.75Sb)(质量分数/%,下同)镁合金,研究铸态合金的显微组织和室温力学性能。结果表明:铸态AS44-0.75Sb合金主要由α-Mg基体、β-Mg17Al12相、Mg2Si相和Mg3Sb2相组成;加入0.75Sb后形成高熔点的Mg3Sb2相,显著改善了Mg2Si相的形貌,使粗大的骨骼状Mg2Si转变为相对细小的汉字状Mg2Si。铸态合金的硬度HV为65.9,屈服强度为136.4MPa,抗拉强度为172.3MPa,伸长率为3.3%;拉伸断裂形式为准解理脆性断裂。     

Microstructural features and final mechanical properties of the iron-modified Al-20Si-3Cu-1 Mg alloy product processed from atomized powder

The potential piston alloy Al-20Si-5Fe-3Cu-1Mg has been experimentally extruded from rapidly solidified powder, and subsequently heat treated. The effects of adding iron to the alloy on the microstructural evolution during the solution and ageing treatment subsequent to extrusion have been examined. The study shows that iron-bearing intermetallic particles modify the recrystallization behaviour of the present alloy during solution treatment at 470 °C in a complex way, through blocking the migration of recrystallized grain boundaries from particle-depleted areas, and pinning subgrain boundaries in particle-rich areas, thus leading to a partially recrystallized duplex structure in the final product. The observed two-fold role of the intermetallic particles is a consequence of their inhomogeneity in distribution, which in turn results from the processing history of the powdered alloy. It is also observed that, in the presence of the intermetallic particles, the excessive coarsening of the silicon particles dispersed in the -Al matrix (as occurs to the base alloy during the heat treatment) is lessened. The retained subgrain boundaries provide heterogeneous nucleation sites for precipitation occurring during ageing. Most of the precipitates are characterized by being associated with iron, and the precipitating behaviour of copper and magnesium in the present alloy with the iron addition is accordingly altered. The resultant tensile properties of the alloy at room and elevated temperatures have been assessed, with reference to those of the base AI-Si-Cu-Mg alloy. The results indicate that the present alloy with the iron addition has a fairly high hot strength up to a temperature of 300 °C, which offers an important improvement ensuring its reliable application in automotive engines.