Maria Helena Robert

Effect of mechanical alloying on the morphology, microstructure and properties of aluminium matrix composite powders

Abstract A composite powder with a fine homogeneous distribution of the reinforcement phase in the whole particle can be obtained by mechanical alloying. Aluminium PM6061 unreinforced, and matrix composite reinforced with Si 3 N 4 and AlN powder, are milled in a high-energy attritor mill and the powder properties are compared with those of the same composite composition mixed in a horizontal low-energy ball mill. The correlation observed between the apparent densities and the milling time, explained by the morphological and microstructural evolution of the powder particles during the high-energy milling process, is used to determine the steady state of the process. At short milling times, the apparent density decreases as the milling time is extended, due to the deformation dominant at this stage; at longer milling times, it starts to increase with increasing milling time due to the piling up of the flattened particles and fracture of the welded particles. When mechanical alloying reaches the steady state, the apparent density is stabilized. A simple model is proposed to illustrate the mechanical alloying of a ductile–brittle component system. The particle size distribution and the microhardness of the mechanically alloyed particles are determined.

The effects of mechanical alloying on the compressibility of aluminium matrix composite powder

Abstract High-energy milling changes the compressibility of powder material due to the work hardening effect and the changes in the powder morphology. Aluminium 6061 powder alloy reinforced with AlN was mechanically alloyed for different lengths of time and the compressibility of the obtained powder was determined. The results are explained in terms of the plastic deformation capacity of the powders, which is influenced by the hardness and the morphology of the powder. With increased milling time, powder compressibility is reduced. The equiaxed morphology of the as-received unreinforced aluminium powder also induces low compressibility, which is improved by the simple addition of reinforcement particles.

6061 Al reinforced with silicon nitride particles processed by mechanical milling

1. IntroductionPowder metallurgy techniques are known tocontributetothegooddistributionoftherein-forcementparticles,withoutthetypicalsegrega-tionsofcastingcomposites[1].Acriticalstepinthistechniqueistheblendingoftheconstituentpowders,sincedefectssuchasclusteringofthereinforcementparticlescandamagethecompositeproperties.Especiallyinthecaseofthediscontin-uouslyreinforcedMMCs,thedistributionofthereinforcementparticlesdependsontheprocessingrouteinvolved,aswellasonthesizeofthematrixparticlesinrelationtothatofthereinforcementparticles.Adecreaseofthereinforcementparticlesizecanbringaboutanincreaseinbothmechan-icalstrengthandductilityofthecomposite,buttheprobability of particle clustering also increases[2–7].Thedevelopmentofastaticchargeontheparticlesisalsoaprobleminobtainingagoodblend[3].Mechanical milling/mechanical alloying, as amethodofintroducingthereinforcementparticles,assuresbetterdistributionoftheparticlesintheconsolidatedmaterial[8–18].Thisprocessconsistsofrepeatedwelding–fracturing–weldingofamix-tureofpowderparticlesinahigh-energyballmill[19].Thecentraleventisthatthepowderparticlesare trapped between the colliding balls duringmillingandundergodeformationand/orfractureprocesses,dependinguponthemechanicalbehav-iourofthepowdercomponents.Atthepresenttime,therelationshipbetweenthestagesofthemechanicalalloyingwithaductile–brittlesystem,thecharacteristicsofthepowderobtainedineachoneofthesestages,andthemechanicalpropertiesoftheconsolidatedmaterialsisnotwelldeter-mined.Thisworkunderlinestheimportanceofthecorrectdeterminationofthemillingtimetoensurethatthecharacteristicsofthepowderwillbesuchastoenhancethefinalpropertiesofthecompositematerial.2. ExperimentalAluminiumPM6061wasreinforcedwith5%byweightofSi

Structural modifications in rheocast Al–Cu alloys by heat treatment and implications on mechanical properties

Abstract The work shows the effect of heat treatment T6 (solution and controlled precipitation of CuAl2) on the microstructure of rheocast Al-4.5wt.% Cu alloy. Rheocast structures were obtained by partial melting to 40% liquid fraction from dendritic material with initial different grain sizes. Globular structures with grain sizes varying from 15 to 170 μm were produced and characterized for grain size, morphology, and Cu distribution and were mechanically tested, in both as-rheocast and T6 conditions. Results showed that increasing grain size results in a decrease in tensile and yield strengths, the latter according to a Hall–Petch type of relation. Elongation and constriction increase as grain diameter increases to a certain extent; for higher increase in grain diameter, these properties decrease. Concerning the effect of heat treatment in the structure, rheocast material requires more time for complete dissolution of the eutectic phase than cast structures, can keep higher Cu content in solution and prevents grain growth during holding. Greater improvement in mechanical properties is achieved by heat treatment in rheocast material.

Contribution to the study of mechanisms involved in the formation of rheocast structure

Abstract The work presents some results obtained when producing rheocast structures of Al–(4.5wt.%)Cu alloys, by heat treatment of dendritic material (in as-cast and cold deformed conditions) at temperature above solidus . Macro and microstructures were analysed in order to investigate the evolution mechanisms involved in the structure modifications. Results show that coarsening and coalescence phenomena can lead to a significant difference between the sizes of grain and globule, therefore being necessary to distinguish between micro and macroconstituents. Results also show that, for deformed dendritic structure, a phenomenon of recrystallisation plus boundaries wetting leads to structures where one globule of the primary phase corresponds to one grain in the macrostructure.

Extrusion of mechanically milled composite powders

Mechanical alloying has come to the fore in fields such as the production of intermetallic compounds, supersaturated solid solutions, amorphous materials and metal matrix composites. With this process, composite powders are obtained with characteristics that are impossible to achieve employing conventional powder metallurgy techniques. In this work, aluminium powder AA6061 is mixed with silicon and aluminium nitrides in a conventional mixer and in a high-energy ball mill to obtain composite powders that are subsequently uniaxially cold pressed and hot extruded. The necessary pressure to extrude the composite cold pressed powders varies with the powder condition, being lower for the composite powder conventionally mixed, higher for the composite powder after a short time of milling, and intermediate after a longer time of milling, due to the morphological and structural changes typical of the mechanical alloying process.

Thixo-forming of Al–Cu alloys

Abstract Semi-solid processing (SSP) is now an established technology, there being many companies who are increasing the use of thixo-forming techniques. The principal reason for this interesting position is that SSP enables the combination of the high mechanical properties resulting from plastic deformation with the economic advantages of near-net-shaping. This work analyses the formability of Al–4.5% Cu alloys in the semi-solid state by forging and by extrusion (thixo-forging and thixo-extrusion). Forging and direct extrusion tests were carried out in order to make feasible the production of simple-shaped geometry. Forming forces were monitored during the processes in order to enable the analysis of the deformation behavior. The products obtained showed good quality in terms of dimensional accuracy and surface finish, comparable to those obtained by conventional forming, for both processes. For the particular geometry studied, the maximum force required for thixo-forging was 35% of that required to forge from the solid state. For thixo-extrusion, the maximum force required was 20% of that required for conventional extrusion. These values can be reduced by increasing the liquid fraction in the feed stock (which, however, must be kept low to prevent material decohesion during deformation) or decreasing the degree of deformation. Macrostructures of the products showed highly homogeneous material, with fine equi-axial grains, free from residual stresses. Some separation of liquid to the outer regions could be observed, which, however, can be avoided by increasing the deformation rate. The microstructures showed higher Cu content in solution than in conventional forged and extruded products, which can lead to more convenient conditions for further homogenization treatments.

New Design of Aluminium Based Composites through Combined Method of Powder Metallurgy and Thixoforming

The present study deals with a new design of aluminium alloy based composites reinforced with SiC particles and Si/Al2O3 powders through combined methods of powder metallurgy and thixoforming. Moreover, recycled machining chips are used as raw material, specifically AA7075 chips generated in the aeronautical industry. The proposed method is based on forming at high temperatures a compacted mixture of metal chips and reinforcing particles, with the metal in thixotropic semi-solid condition. Composites containing different SiC weight fractions (10, 20 and 30%) were produced and had their microstructure analyzed. Mechanical properties were evaluated by means of micro-indentation tests. General results show the feasibility of producing composites by the proposed route. Products with good mechanical properties could be obtained. The process, even still not completely optimized as some improvement still must be achieved, can bring a new possibility for the production of a noble material from recycled wastes, particularly important in the high energy spending Al industries.

The effects of mechanical alloying on the extrusion process of AA 6061 alloy reinforced with Si3N4

The synthesis of materials by high energy ball milling of powders was first developed for the production of complex oxide dispersion-strengthened nickel alloys for structural, high temperature applications but has been attracting attention in the field of fabrication processes like the production of intermetallic compounds, supersaturated solid solutions, amorphous materials and metal matrix composites. However, due to the high level of deformation imposed, the aluminum mechanically alloyed undergo extensive grain growth during the extrusion process, resulting in serious damage in the extruded materials. This work investigates the effects of mechanical alloying on the extrusion of AA6061 aluminum alloy and the same alloy reinforced with silicon nitride. In both cases, the energy of deformed particles produced extruded bars with coarse grains in the core, while in the periphery the higher rate of deformation in the extrusion process has prevented this coarsening, resulting in a material with heterogeneous microstructure and with poor mechanical properties. This grain growth can be prevented by a higher percentage of reinforcement in the composite materials or by annealing before extrusion.

Production of Aluminum/SiC/NiAl2O4 MMCs by Thixoforming of Recycled Chips

The work explores the production of Al/SiC and Al/SiC/NiAl2O4 composites by thixoforming compacted mixtures of AA7075 machining chips and reinforcing particles. It is analyzed the influence of processing parameters in the various processing steps, such as mixing, compacting, heating and thixoforming, in the final quality of products. Results showed the general viability of producing composites by the proposed technique; the utilization of recycling material is particularly important in the high demanding energy processing sector, as the Al industry. NiAl2O4 particles and, in less extent, also SiC particles, can penetrate into the liquid phase present in the thixotropic microstructure within the chip, promoting their disaggregation; reinforcement distribution in the composite depends on appropriate choose of processing parameters.