DEVELOPMENT OF SHORT STEEL FIBER REINFORCED ALUMINIUM BASED COMPOSITES BY P/M ROUTE

Abstract

Light alloy MMC systems based on Al and Mg matrices with ceramic reinforcements of particulate, whisker and short fiber morphologies have been well developed and extensively investigated. Metal-metal MMC systems have been considerably less researched on account of intermetallic phase at the interface, which is generally unstable, brittle, suitable for limited structural applications and susceptible to environmental degradation. However, potential structural and non-structural application areas like high temperature application, corrosion resistance, magnetic materials and wear resistant elements have been explored for such class of materials. The aim of the present work is to develop aluminium P/M based composites reinforced with short steel fibers for wear resistant and elevated temperature service. Al-0.5 wt. % Mg matrix powder blend was sintered in N2 atmosphere. The role of Mg as a sintering aid was determined by SEM in backscatter mode to highlight pore morphology and necking phenomenon. Particle to particle bridging and pore filling aided by Mg particles was revealed. A steady growth of the reaction interface with pressing time was observed in Al-0.5Mg-10 wt.% short steel fiber green preforms vacuum hot pressed at 823 K under 50 MPa linearly increasing stress. Line scans along the steel fibers, confirmed the presence of a reaction interface, and formed as a result of inter-diffusion of Fe and Al. The interface growth kinetics followed a parabolic law which was indicative of a diffusion controlled mechanism. The value of parabolic rate constant, determined from slope of the straight line portion of the curve was 1.41 × 10 −12 m2 s−1 at 823 K. The untreated fibers had a cold worked structure composed of ferrite and pearlite. Surface modification and heat treatment of fibers was carried out in a fluidized bed reactor, to alter their surface chemistry and thereby inhibit growth of brittle Fe-Al intermetallics. Nitriding treatments for the fibers resulted the development of case, composed of γ'-Fe4N and ε-Fe2-3N phases with respect to different nitriding durations. Chromizing of the short steel fibers resulted in loss of hardness and absence of any hard carbides due to the low carbon content of steel fibers, 0.38wt.% C and absence of Cr Chromizing treatment on steel fibers was successful in partially inhibiting growth of FexAly type of reaction interface. Aluminizing treatment resulted in formation of a strongly adherent - 2 - layer composed of FeAl3 of about 7μm thickness (890 VHN). FeAl3 is significantly more ductile than Fe2Al5, and hence. Upon sintering under similar conditions, a tongue like growth of this reaction interface towards the Al-0.5Mg matrix is observed with Kirkendall voids at regular intervals. It was observed that density, hardness and porosity increased with increasing content of short steel fibers. Matrix hardness increased to a maximum of 78 VHN for 30.wt.% reinforcement from 34 VHN for monolithic composition Regions lying within agglomerates of fibers were left unfilled during cold compaction; these unfilled spaces are retained as voids post sintering. An appreciable increase in matrix microhardness is observed from 10 to 20 wt.% reinforcement content, whereas the same was not obtained from 20 to 30 wt.%. For 10 wt.%, the volume fraction of fibers (3.7 vol.%) was insufficient to cause matrix hardening; for 30 wt.%, the unfilled voids within agglomerates of the steel fibers, negate the effect of matrix strengthening, as determined, there was a marked increase in porosity volume fraction from 20 to 30 wt.% reinforced composites. In the present investigation, the reaction interface of a maximum 15 μm width was found to be composed of steel fiber/ FeAl3 (θ)/ FeAl (β) and Fe3AlC (κ)/ Fe2Al5 (η)/ Al-0.5 wt.% Mg matrix determined by XRD, EDS and microhardness measurements. Increase in flow stress composites was observed at 250ºC against the monolithic composition. Ball-on-disc dry sliding wear of the composites revealed an order of magnitude increase in room temperature as well as elevated temperature wear resistance with increasing reinforcement content. Sintered compacts in the form of briquettes were close die forged and rolled to remove retained porosity and achieve an overall enhancement in mechanical properties. Increments in hardness and flexural strength were observed. Forging of composites resulted in de-agglomeration and redistribution of short fibers in the Al-0.5Mg matrix. Reinforcement banding as a result of deformation processing- generally observed in hard particulate/whisker reinforced MMC’s was avoided by constrained forging employed in this work. Fiber pull-out and fracture were found to be the primary failure mechanisms. The strength of the composites was found to be lower than that of monolithic composition (141 MPa- 10wt.%; 127 MPa- 20 wt.% and 87 MPa-30 wt.%) with reduction in ductility; which increased to 213 MPa- 10wt.%; 212 MPa- 20 wt.% and 176 MPa for 30 wt.% reinforced composition, which were higher than ROM estimates. - 3 - Reduction of 25:1 by extrusion of sintered billets, conforming to a true strain of 3.21 for Al-0.5Mg, produced elongated ligament like grains, almost nil retained porosity, with gain boundaries oriented along the extrusion direction. Tensile strength was equivalent to that achieved by forging with similar ductility. Preferential alignment of steel fibers, along extrusion direction, was visible for extruded Al-0.5 Mg-25 wt.% steel fiber composites. Fiber breakage resulted due to use of a flat face extrusion die. Fractographs for composite also indicated bunching of fibers, responsible for failure by debonding of fibers from the matrix. Room temperature pin-on-disc wear test of sinter-forged specimens revealed a transition from delamination mode of adhesive wear to less severe abrasive wear mechanism with increasing reinforcement content. Decrease in CTE was observed with increasing reinforcement content. Pre-alloyed AA6061 (Al-0.19Fe-0.68Si-0.30Cu-1.1Mg-0.3Cr) powder was cold consolidated and sintered at 625 ºC; conforming to 10 vol.% liquid fraction, ensuring presence of transient liquid phases of Mg2Si. Sintered densities of 92 for unreinforced AA6061 increased to near 99% upon closed die forging with an ultimate tensile strength of 356 MPa in T6 aged condition. Incorporation of short steel fibers resulted in formation of complex AlFeSi ternary intermetallics for AA6061 based composites, thereby deterioration in ageing response and mechanical properties was observed. An increase in flow stress for hot compression of forged alloy based composites, revealed a lag in flow stress, accompanied by reduction in cavitation induced damage. Comprehensive tribological testing and analysis of wear tracks and debris showed increase in room temperature wear resistance of alloy based composites at higher loads and increased speeds. A decrease in coefficient of thermal expansion as reported for all composites compared to pure compositions.