K.R. Suresh et al. / Wear 255 (2003) 638–642 6392. Experimental details (case hardened to 80RC , to a depth of 3 mm) was used as counter surface and composites were made into pin having The matrix chosen for this work is ASM 356 Al–Si–Mg 6 mm diameter and 25 mm in height. The pins were madealloy. It is a high strength alloy and renders itself favorably to slide on the steel disc. A sliding track diameter of 98 mmto heat treatment. The composition of the alloy is 7.5% Si, was chosen and the disc was rotated at a speed of 390 rpm,0.345% Mg and balance is aluminum. ‘Beryl’ chemically so that a linear distance of 120 m was covered in 1 min. Theberyllium–aluminum–silicate (Be3 Al2 (SiO3 )6 ) has a density test was carried out for different sliding distances under theof 2700–2800 kg/m3 , having hardness of 7.5–8.5 on Mho’s normal a load of 10 N. The wear was measured in terms ofscale. It has hexagonal crystal structure and retains water of weight loss of the materials measured to 0.0001 g resolu-crystallization up to 800 ◦ C . The chemical composition tion. An average of three trials is presented here. The scatterof beryl particles used here is presented in Table 1. The of three readings were within 5%. Since density of the alloyparticles used here have an average particle size of 20 m. has critical inﬂuence on properties, the same was measured The Al composites having beryl particles varying from 2 for each specimen.to 10% are prepared through gravity casting, their tensileand wear properties are evaluated. The details of which arereported elsewhere . 3. Results and discussion The squeeze casting of Al–beryl casting was carried outby dispersing 2–10% beryl particles. The beryl particles are The densities of gravity cast and squeeze cast matricesheated to 900 ◦ C before adding them so as to eliminate water were measured in order to know the effect of squeeze casting.of crystallization, which is likely to hinder the wetting of It was found to be 2610 and 2790 kg/m3 for gravity cast andAl melt . The squeeze casting was obtained by pouring squeeze cast matrices, respectively, indicating an increasemelt-particle slurry into preheated (200 ◦ C) permanent die of 6.9% for squeeze cast matrix. This improvement in theand punch. It is allowed to solidify under squeeze pressure density may be due to reduction of micro-porosity due toof 80 MPa for a duration of 5 min. High temperature graphite squeezing the casting during solidiﬁcation.powder was used in the die to facilitate removal of cast Fig. 1 shows the tensile test results of gravity cast andblanks from die after cooling. Also, unreinforced alloy was squeeze cast composites for varying percentage of particles.cast in identical conditions for the purpose of comparison. It can be observed from the ﬁgure that the tensile strength of The squeeze cast composites and matrix were machined squeeze cast matrix (0%) is higher than gravity cast matrixto obtain tensile and wear specimens, the tensile test was by 11%. The improvement in strength in squeeze cast con-carried out on samples according to ASTM-E8-95a. All the dition may be attributed to the absence of shrinkage porositycomposites were tested for strength, samples were loaded and ﬁne grain structure .till fracture. Three trials were carried out for the purpose of Application of pressure during solidiﬁcation also resultsrepeatability and the average of them is presented here. in reﬁned grain structure . It can be observed from the Hardness has some inﬂuence on the wear behavior on any ﬁgure that the addition of the beryl particulates enhanced thematerial. Hence, the hardness was measured for the com- tensile strength of the composites. It is true for both gravityposite samples as well as squeeze cast alloy. The hardness cast and squeeze cast conditions up to an addition of 6%.tests were carried out as per ASTM-E-10-93 standard. The The composites attains peak strength on addition of 6 wt.%tests were conducted on three locations on the sample to of beryl particles in both the gravity cast and squeeze castcounter the possibility of indentor resting on hard particle, conditions. However, the squeeze composites, having 6 wt.%which may result in anomalous value. of beryl showed enhancement of 11.3% over the gravity cast Wear tests were carried out under dry condition on a sample of same composition.pin-on-disc apparatus as per ASTM-G-99-95. A steel disc Further dispersion of hard ceramic particles in a soft duc- tile matrix results in improvement in strength . This may be attributed to large residual stress developed during solidi-Table 1 ﬁcation and to the generation of a density of dislocations dueChemical composition of beryl particles to mismatch of thermal expansion between hard ceramic par- ticles and soft Al matrix [29–32]. The hard ceramic particles Composition (%) obstruct the advancing dislocation front, thereby strength-SiO2 65.4 ening the matrix [33–37]. The increase in the strength mayAl2 O3 17.92 also be result of closer packing of reinforcement and smallerBeO 12.25Fe2 O3 0.8 inter-particle spacing in the matrix, and the matrix material’sCaO 1.34 ability to exhibit internal ductility to accommodate local-MgO 0.48 ized internal stresses . As mentioned earlier, squeezeNa2 O 0.55 casting further enhances the strength due to the absence ofK2 O 0.004 micro porosity, better interfacial bond between matrix andMnO 0.05 reinforcement, and grain reinforcement .
640 K.R. Suresh et al. / Wear 255 (2003) 638–642 Fig. 1. Tensile strengths of gravity cast and squeeze cast Al composites.3.1. Hardness gravity casting. However, the peak hardness observed for addition of 10 wt.% of particles in both the cases. The hard- Fig. 2 shows that the hardness of squeeze cast matrix is ness in squeeze casting conditions may be due to combinedhigher than that of gravity matrix by about 4.5% approxi- effect of denser matrix and hard ceramic particle .mately. The application of pressure during solidiﬁcation insqueeze casting minimizes porosity and makes the metal 3.2. Wearmore dense, making the matrix to resist surfacial plastic de-formation, rendering higher hardness to the matrix. The dis- Dry sliding wear of both gravity cast and squeeze castpersion of beryl particles enhances the hardness, as particles composites of different weight percentage particles is indi-are harder than Al alloy, the materials render their inherent cated in Table 2. It can be noted from the table that theproperty of hardness to the soft matrix . The peak hard- composites show lower weight loss indicating the beneﬁcialness of 64 BHN was found to be for an addition of 10 wt.% effect of addition of beryl particles. It may be attributed toof beryl particles in gravity cast condition. hardness of the material a dominating factor affecting the The squeeze cast composites having 6% beryl (the com- wear resistance [42–44]. The decrease in wear weight lossposition which has shown peak strength in both the cases) may also be attributed to higher load bearing capacity ofwas found to be 19.8% more harder than the corresponding hard reinforcing material . Fig. 2. Hardness of gravity cast and squeeze cast Al–beryl composites.
K.R. Suresh et al. / Wear 255 (2003) 638–642 641Table 2Wear weight loss (mg) of Al–beryl composites for different sliding dis-tances under gravity cast (GC) and squeeze cast (SqC) conditionsBeryl (%) Sliding distance 600 (m) 1200 (m) 1800 (m) 2400 (m) GC SqC GC SqC GC SqC GC SqC 0 5.2 4.5 11.4 7.9 16.2 8.2 20.8 11.2 2 4.9 4.0 7.8 5.3 12.6 7.6 20.3 10.0 4 4.6 3.6 7.4 4.8 10.9 5.8 18.6 9.4 6 4.4 3.0 6.0 4.1 9.8 4.9 14.1 8.5 8 4.3 2.3 5.6 3.3 9.0 4.7 12.3 7.310 4.1 1.9 5.2 2.3 8.5 4.2 11.2 6.4 Fig. 4. Wear track of 6% beryl–Al composite in gravity cast condition. The processing of composites also inﬂuence the wear rate.It can be seen from Table 2, that squeeze cast compositesshow lesser wear weight loss when to gravity cast compos-ites for any sliding distance. From the component designer’s point of view, it is essen-tial that the material should retain its strength character. Inthe present study, it was observed that the composite with6% beryl particle shows peak strength, both in gravity castand squeeze cast condition, hence the wear studies of thiscomposite is presented here. Fig. 3 depicts the wear weightloss of these composites under different sliding distances.The squeeze cast composite showed decreases of 31% com-pared to gravity cast composite over a sliding distance of600 m. When the sliding distance was increased to 2400 mthe squeeze cast composite showed decrease of 40% whichis almost 1.2 times lesser wear weight loss of gravity castcomposites for same distance. The squeeze cast composite shows lower wear weight loss Fig. 5. Wear track of 6% beryl–Al composite in squeeze cast condition.as can be seen from Fig. 3. The photomicrographs of thewear track of the composite in both the processing condi-tions are presented in Figs. 4 and 5. More debris can be seen than that of gravity cast sample), better interfacial bond be-in the tracks of gravity cast composite Fig. 4, while in Fig. 5 tween the particle and the matrix than in the gravity sam-shows uniform wear track with reasonably lower debris. The ples, reducing the possibility of particle pull out which maylower wear weight loss of squeeze cast sample may be due result in higher wear. Besides, denser squeeze cast samplesto the fact that the material is more denser (density is higher resist surfacial deformation during hardness testing indicat- Fig. 3. Wear weight loss of 6% beryl–Al composites for different sliding distances.
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