Hybrid-hybrid machining of SiCp/Al composite

Metal matrix composites (MMCs) are composed of matrix, the parent material, and reinforcements that provide enhanced properties to individual components. Out of various MMCs, SiC particle reinforced aluminium (SiCp/Al) composite has outstanding mechanical properties such as high strength-to-weight ratio, stiffness, excellent wear resistance, ease of production and good corrosion resistance, hence, defined as an aerospace-grade. However, the existence of hard particulate SiC particles leads to machinability issues with conventional machining (CM) techniques. Conventional turning (CT) of SiCp/Al composite is typically characterised by low cutting parameters, thus, low material removal rates during the machining process. A hybrid machining process known as ultrasonically assisted turning (UAT) was introduced in the past to machining metals or alloys, in which, high-power, high-frequency vibration is superimposed on the cutting tool during the conventional machining process. This renowned machining technique results in a decrease in average cutting forces with surface quality improvement for modern alloys. Another hybrid machining process known as laser-assisted turning (LAT) was also introduced to machining metals and alloys, which utilises localised thermal softening before the cutting tool resulting in cutting force reduction at the cutting zone. This technique results in an improved material removal rate (MRR) with less tool damage. In this work, a new machining paradigm, called hybrid-hybrid assisted turning (laser-ultrasonic assisted turning, LUAT) is introduced for the processing of three types (217XG, 225XF and 225XE) of SiCp/AA2124 composites. In this technique, UAT is combined with the laser-assisted technique to gain combined advantages for machining metal matrix composite. To experiment, suitable ultrasonic transducers were designed and analysed first through FE simulation followed by experimental analysis on manufactured prototypes. Then, a selected transducer and laser-optic system were installed on a compact commercial lathe. Before the machining experiment, mechanical and thermal properties of the workpieces were analysed, with studies on the mechanism and characterisation of SiCp/Al composite with ultrasonic vibration during loading and heat-affected zone (HAZ) analysis. Acoustic softening was observed in all SiCp/Al workpieces during the uniaxial compression test with the main vibration applied in the loading direction, thus, the cutting force reduction mechanism was verified. SEM, EDX, micro-hardness, and FE numerical studies were performed in the HAZ to get an insight into the size of HAZ and metallurgical behaviour during laser exposure. It is found that HAZ showed hardened oxide and intermetallic phases that will adversely affect the cutting forces after cooled down. Moreover, it is numerically verified that thermal conduction decreases drastically by particle inclusions, faster conduction towards the particles with an area more parallel to the applied heated surface. Furthermore, reduction of Si element was observed in HAZ and the melt pool, verifying the particle sedimentation, if melting is present. UAT of the studied composites was analysed experimentally and numerically to demonstrate its advantages in terms of cutting forces reduction over various cutting conditions with the carbide tool. However, the evident surface roughness improvement was only visible with composite with the least particle content (217XG). LUAT studies follow with optimisation process through numerous trials that showed approximately 40% cutting force reduction compared to CT using the carbide tool. Surface integrity and roughness improvement were also minimal due to the high tool wear rate in CT, especially in the radial direction. LUAT performed with the PCD tool showed improvement in both cutting forces and surface roughness, showing improvement by an increase of laser power, although cutting forces were higher than that for LAT. It is mainly because the higher impact of ultrasonic vibration neglected the hardened phase but only increased the temperature at the cutting zone. 2D homogenous elastoplastic thermomechanically coupled FE models for the orthogonal turning process were developed for CT and UAT, followed by a 3D homogeneous FE model for the oblique turning process. The temperature profile was observed with moving localised surface heat by power and distance from the laser spot and the tool to predict the temperature of the workpiece nearest to the cutting zone. It is found that due to high thermal conductivity, the heat is quickly dissipated, leaving hardened oxide and intermetallic phases before the cutting tool. The LUAT was predicted in the model at various temperatures of the workpiece in terms of cutting forces. Therefore, it is not always suitable to use developed hybrid-hybrid turning for machining SiCp/Al composite, however, if optimised, it is beneficial in terms of providing thermal softening, expansion of feed grooves, and minimising workpiece damage from particles since particle sedimentation and hardened phases inhibit particle rolling or scratching on the tool-workpiece interface. A preferred setup for the SiCp/AA2124 composite for the used cutting parameter (V_c=30 m/min, f=0.14 mm/rev and a_p=0.15 mm) is to paint the surface in black after surface roughening to enhance absorptivity up to at least 0.3 and use the laser spot size in between 0.3 to 0.7 mm. The preferred energy density for the three workpieces: 217XG (SiC: <0.3 µm, 17 vol%), 225XF (SiC: <0.7 µm, 25 vol%) and 225XE (SiC: < 3 µm, 25 vol%), is 7.5, 6.7 and 8.2 kW/mm2, respectively.