João Batista Fogagnolo

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

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.

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.