Heat Treatment Fine-Tunes Wear Resistance of Graphite and Fly Ash Aluminium Composites

Lightweight metals that can shrug off wear are among the most sought-after materials in modern engineering, and a new study has shown that the humble heat treatment step may hold the key to unlocking their full potential. Researchers reporting in PLOS One examined how solution treatment temperature, holding time and cooling method shape the friction and wear behaviour of a hybrid aluminium matrix composite reinforced with graphite and fly ash, two low-cost additives that turn an ordinary casting alloy into a candidate for pistons, brake drums and other sliding components. Using a statistical optimization framework that blends the Taguchi method with the Technique for Order of Preference by Similarity to Ideal Solution, known as TOPSIS, the team identified precise treatment recipes that simultaneously maximize hardness while minimizing both material loss and friction under dry sliding.

The material at the heart of the study is LM26, a widely used aluminium casting alloy prized for its performance at elevated temperatures. By stirring graphite particles and fly ash into the molten alloy through a stir-casting route, the researchers produced a hybrid composite in which soft, self-lubricating graphite coexists with hard, inexpensive fly ash particles. This combination is attractive because graphite can form lubricating films at sliding interfaces while fly ash, a by-product of coal combustion, adds hardness at almost no material cost. The catch is that the final properties of such composites depend heavily on the microstructure of the aluminium matrix, and that microstructure can be deliberately reshaped through heat treatment.

Solution treatment works by heating the alloy to a temperature high enough to dissolve brittle intermetallic phases into the matrix, holding it there to homogenize the structure, and then cooling at a controlled rate to lock in a supersaturated solid solution. The temperature chosen determines how much of the second phase dissolves, the holding time governs how completely that dissolution proceeds, and the cooling method dictates whether dissolved elements remain in solution or precipitate out again. Each of these three levers can alter the hardness of the matrix, the strength of the bond between matrix and reinforcement, and ultimately the way the composite behaves when its surface slides against a counterbody.

To map this three-dimensional parameter space efficiently, the team employed a Taguchi L27 orthogonal array, an experimental design that allows three factors at three levels each to be evaluated with just twenty-seven carefully chosen runs instead of a full factorial sweep. Solution treatment temperatures of 450, 500 and 550 degrees Celsius were paired with holding times of 2, 4 and 6 hours, and each combination was subjected to one of three cooling routes: slow furnace cooling, moderate air cooling or rapid water quenching. After treatment, every specimen was measured for hardness in Brinell numbers, tested for wear rate under dry sliding conditions, and characterized for its coefficient of friction, giving three response variables that often pull in different directions.

The measured values revealed a substantial spread across the treatment conditions. Hardness ranged from 42.3 to 51.2 Brinell hardness numbers, wear rates spanned 0.000667 to 0.000750 cubic millimetres per minute, and coefficients of friction fell between 0.4002 and 0.4501. These are not trivial differences for a component designer: a roughly twenty percent swing in hardness and a corresponding improvement in wear and friction performance can translate directly into longer service life and lower energy losses in sliding machinery. The results showed that as the heat treatment conditions were optimized, hardness climbed from its lowest to highest value while wear rate and friction simultaneously dropped, demonstrating that the three responses can be improved together rather than traded off against one another.

Because no single run was guaranteed to be best on all three counts at once, the researchers turned to TOPSIS, a multi-criteria decision-making technique that ranks each experimental condition by its relative closeness to an ideal solution, defined as the hypothetical condition with the best possible value for every response. The standout performer was Experiment 18, corresponding to a solution treatment at 500 degrees Celsius, a 6-hour hold and water quenching, which achieved the highest relative closeness value of 0.8933. This combination came closest to the ideal balance of maximum hardness with minimum wear and friction among all twenty-seven tested conditions.

Factor-level analysis, which averages the response across all runs containing a given parameter level, pointed to a slightly different optimum within the investigated design space: 550 degrees Celsius, 6 hours and water quenching, designated A3B3C3. The discrepancy between the single best run and the factor-level optimum illustrates a familiar feature of Taguchi analysis, where averaging effects can suggest combinations not directly tested. Both findings, however, converge on the same practical message: longer holding times and rapid water quenching favour the combined tribological response, while the temperature picture rewards careful selection within the studied range.

Analysis of variance added quantitative weight to these observations by partitioning the influence of each parameter on each response. Temperature emerged as the dominant factor for wear rate, coefficient of friction and the overall TOPSIS grade, meaning that the dissolution behaviour of the aluminium matrix during solution treatment exerts the strongest control over how the composite wears and slides. Cooling method, by contrast, had the greatest influence on hardness, consistent with the idea that quenching rate determines how much dissolved solute is retained in solution and how fine the resulting structure becomes. Holding time contributed across all responses but generally played a supporting role to temperature and cooling rate.

Scanning electron microscopy provided the microstructural context for these numbers, allowing the researchers to examine how the graphite and fly ash particles were distributed within the aluminium matrix. A reasonably uniform dispersion of reinforcements is essential for consistent tribological behaviour, since clustered particles can act as stress concentrators and debris sources during sliding, while well-distributed graphite can continuously replenish lubricating films at the wear surface. The microscopy work tied the macroscopic wear and friction measurements back to the underlying structure of the composite, reinforcing the link between heat treatment, microstructure and performance.

The broader significance of the study lies in its demonstration that post-casting heat treatment is not an afterthought for hybrid aluminium matrix composites but a primary design variable. For industries seeking to substitute lightweight composites for heavier ferrous components in engines and braking systems, the ability to dial in wear and friction performance through a documented treatment recipe, validated by a transparent Taguchi-TOPSIS workflow, offers a practical path from laboratory material to engineering component. The findings also highlight the value of fly ash, an industrial waste stream, as a functional reinforcement, aligning improved tribological performance with lower material cost and a measure of environmental benefit.

Subject of Research: Heat treatment optimization of LM26/graphite/fly ash hybrid aluminium matrix composites for tribological performance

Article Title: Effect of Heat treatment parameters on the tribological performance of LM26/graphite/fly ash hybrid aluminium matrix composites using Taguchi-TOPSIS optimization

Article References: Chellamuthu, P., Angappan, P., Duraisamy, P., Agarwal, A., & Dorji, K. (2026). Effect of Heat treatment parameters on the tribological performance of LM26/graphite/fly ash hybrid aluminium matrix composites using Taguchi-TOPSIS optimization. PLOS One, 21(10), e0360156. https://doi.org/10.1371/journal.pone.0360156

Image Credits: AI Generated

DOI: 10.1371/journal.pone.0360156

Keywords: aluminium matrix composites, LM26 alloy, graphite, fly ash, heat treatment, solution treatment, Taguchi method, TOPSIS, tribology, wear rate, coefficient of friction, stir casting