Processing And Characterization Of Fly Ash Particle Reinforced A356 Al Composites

Abstract

Fly ash is a finely divided mineral residue resulting from the combustion of ground or powdered coal in thermal power plants. It primarily comprises SiO (65%), Al O (24%), and Fe O (5%), with a mean particle size of less than 60 microns. Fly ash possesses low density (2.10 g/cm³), is inexpensive, and abundantly available, making it an excellent reinforcement material for aluminium alloys. Additions of fly ash particles reduce the cost and density of aluminium and its alloys, which are otherwise high-energy-consuming materials.

Recognizing the immense potential of Al-fly ash composites, experiments were conducted to fabricate A356 Al-fly ash composites and characterize them for microstructure, ageing behaviour, mechanical properties, damping behaviour, and sliding wear behaviour.

Fabrication Composites were fabricated using the stir casting technique.

Fly ash particles (as-received, ~60 µm; sieved, ~76 µm) were preheated to 800 °C for 2 hours and dispersed into the vortex created by mechanical stirring of A356 Al alloy melt at 770 °C, then cast into 60 mm diameter cast iron moulds.

Composites prepared:

A356 Al-12 vol.% as-received fly ash [C12(AR)]

A356 Al-6 vol.% sieved fly ash [C6(S)]

A356 Al-12 vol.% sieved fly ash [C12(S)]

Unreinforced alloy was also processed under identical conditions for comparison.

Ingots were machined to 55 mm billets and hot extruded.

Microstructure As-cast composites showed relatively uniform distribution of fly ash particles, though clusters and pores were observed.

Hot extrusion improved microstructural integrity, aligning particles and eutectic silicon in the extrusion direction.

Reactions occurred between aluminium and SiO /Fe O , and magnesium with SiO /Al O .

Average densities:

C12(AR): 2.60 g/cm³

C12(S): 2.59 g/cm³

Unreinforced alloy: 2.68 g/cm³

Age Hardening Samples solutionized at 520 °C for 10 hours, then aged at 160 °C.

Fly ash additions moderately accelerated ageing kinetics.

Hardness increase due to ageing was marginal, attributed to insufficient magnesium for Mg Si precipitation.

Mechanical Properties Bulk hardness and matrix microhardness of composites were higher than unreinforced alloy.

In extruded condition, 0.2% proof stress of composites was higher than alloy.

Additions of fly ash increased hardness, elastic modulus, and proof stress.

UTS and proof stress of C6(S) composite were higher than alloy in extruded condition.

Composites with 12 vol.% fly ash showed lower UTS compared to C6(S) and alloy.

Ageing increased proof stress and UTS compared to extruded condition.

Elongation decreased with fly ash addition due to porosity, defects, and reaction products.

C6(S) composites showed higher compressive strength than 12 vol.% composites and alloy.

Fracture Analysis SEM revealed:

Alloy fracture surfaces: dimples (ductile fracture).

Composite fracture surfaces: mixed ductile-brittle fracture.

Dimples from aluminium fracture; brittle fracture in interfacial and eutectic silicon regions.

Cenospheres showed fracturing and good bonding; partially bonded particles showed interfacial gaps and debonding.

Defects and pores contributed to reduced strength and premature failure.

Damage mechanisms: matrix voiding and particle cracking.

Damping Behaviour Studied using Dynamic Mechanical Thermal Analyzer (DMTA) with three-point bending.

A356 Al-fly ash MMCs exhibited improved damping capacity compared to alloy at both ambient and elevated temperatures.

Damping capacity increased with fly ash volume fraction.

C12(AR) composite showed highest damping capacity; C12(S) composite showed highest modulus at room temperature.

Damping capacity increased with frequency up to 20 Hz.

Mechanisms: dislocation damping, interfacial damping (room temperature), grain boundary and interfacial damping (high temperature).

Sliding Wear Studied using pin-on-disc machine against EN32 steel at 1 m/s under loads of 10-80 N.

Wear rate increased with load in all cases.

12 vol.% composites showed superior wear resistance compared to alloy under all conditions.

Peak-aged alloy showed smearing on steel counterface at 50 N due to heat generation and partial melting.

Higher fly ash fraction decreased wear rate due to abrasion of steel disc by dispersed particles.

Friction coefficient increased with fly ash fraction; composites had higher friction than alloy.

Friction coefficient decreased with load in both alloy and composites.

Wear mechanisms:

Alloy: adhesive wear.

Composites: abrasive wear; subsurface delamination at higher loads.