In friction drilling, frictional heat generated by a conical tool allows it to penetrate a workpiece. It subsequently results in the formation of a bushing, that extends the hole length due to plastic deformation of the material. During initial contact the rotating friction drill comes in contact with the surface of the work piece. Localized heat generated between the workpiece and the tool softens the material. Because of plastic deformation, the tool penetrates further, resulting like bushing-like extended structure. The material begins to flow upward around the tool and downward beneath the surface. Because of frictional heating, distinct zones are developed which are characterized by material softening and microstructural changes. The mechanism of bushing formation during friction drilling of snail shell-reinforced aluminium matrix composites could be comprehended by observing at the microstructural changes across different regions of bush (Fig. 2). Zonal regions, designated A through E, reveal distinctive features that resulted as a consequence of coupled mechanical and thermal effects occurred during the friction drilling process.
Bushing formation showing different zones.
Microstructural analysis of bushing zones
SEM micrographs, segmentation boundary images and particle analysis at different zones (a) Head Petal Region – Zone A, (b) Thorax Region -Zone B, (c) Upper Critical Region –Zone C, (d) Lower Critical Region –Zone D and (e) Tail Petal Region –Zone E.
Friction drilling produces an increase in heat due to high rotational speeds of the tool that causes snail shell-reinforced aluminium matrix composite to soften. As evidenced by the formation of bushing and plastic deformation, different areas have remarkably different microstructure and surface properties (Fig. 3). As it can be seen from the SEM micrographs and segmentation boundary images, there are microstructural alternation from one zone to another along five different zones.
Zone A: head petal region
The surface was examined using scanning electron microscopy, which revealed a mixture of long and irregular particles. This suggests that there was a large amount of plastic deformation at the location where the tool was inserted. A small number of cracks may be observed on the surface of the bushing as a result of the significant amounts of material displacement. An examination of the border segmentation reveals that the particle does not appear to be dispersed uniformly. Poor material displacement is considered as a consequence of the minimum initial impact of the tool, which leaves material to be extruded without complete compaction. This ultimately results in poor material displacement. During the process of inserting the instrument, the Head Petal Region is subjected to a significant amount of shear pressure. This causes the surface to become rough and causes the particles to break apart as a result of the rapid movement of the material under the influence of high temperatures and mechanical deformation. In Zone A, elongated and coarse grains are found because of severe plastic deformation. The average grain size is about 20 μm.
Zone B: thorax region
When compared to Zone A, the micrograph reveals that the surface homogeneity has been enhanced. It has been seen that the deformation patterns have become smoother, and there are fewer cracks, which is an indication of enhanced material flow. The segmented image emphasises particles that are smaller and more evenly scattered throughout the image. It can be observed that the particles are gradually aligning themselves, which is indicative of regulated plastic deformation. During the Thorax Region, the material that has been softened experiences a flow that is smoother and more uniform as the heat generation becomes more stable. This leads to a refined particle structure and a reduction in the number of cracks3. In Zone B, partially refined grains with an average diameter of about 12 μm could be seen. Grains are aligned due to favorable heat flow conditions.
Zone C: upper critical region
This region displays relatively smooth surface features with minimal defects. Uniform material flow and elongated particle structures are observed, which correlate with the thermal softening. The boundary segmentation shows highly aligned particles with consistent size. Minimal particle fragmentation occurs due to optimized thermal conditions. The Upper Critical Region benefits from optimal heat-softening effects, enabling smooth plastic deformation. The particle structure indicates significant material flow with minimal defects, contributing to a better surface finish. In Zone C, a fine with an average diameter of about 5 μm could be seen. Equiaxed grains are formed due to dynamic recrystallization and steady-state heat conditions.
Zone D: lower critical region
In this area, it is observed more surface discontinuities, and an increase in the bushing height and a decrease in bushing thickness. Some cracking does start to initiate, despite the fact, the particles are experiencing elongation. Segmented image shows that the particles are of different sizes compared to the particles in Zone C. Thermal softening occurs, which leads to a partial material flow Transition implies the initiation of heat loss in the lower critical field ranges. However, the thermal effects diminish and only partial material solidification and localized defects persist. In Zone D, elongated grains with an average diameter of about 14 μm could be seen. Coarser are formed due to partial recrystallization and some heat dissipation.
Zone E: tail petal region
In this area, the surface is rough and there is surface tearing. Large particles and some fragmented particles are visible in the segmented image. It displays that particles are dispersed in a random way and there is incomplete plastic flow. There is disruption to the flow of material due to rapid cooling and immediate solidification as the tool exits out. In Zone E, because of immediate cooling, much coarser grains with an average diameter of about 24 μm could be seen. Grains are irregular in shape because of disrupted heat flow conditions and quick dissipation.
Table 3 provides a detailed analysis of the microhardness distribution across different zones of a bushing formed during friction drilling of a snail shell reinforced aluminium matrix composite.
Surface characteristics of bushing
The evaluation of 3D topography along with the surface properties is crucial for understanding the quality of drilled holes. In friction drilling of aluminum, a bushing formed because of the frictional heat generated by a rotating, conical tool pressed against the aluminum workpiece. As the tool penetrates, it softens and displaces the material without cutting. This leads to the formation of several distinct zones within the bushing. This involves a high-resolution analysis of the material surface in both 2D and 3D formats, along with a roughness profile. Bushing wall height variations are seen in a 2D color-coded surface map. Heights are color-coded, with peaks in reds and oranges and valleys in blues and greens. This 2D map shows regions exposed to heat that are identified as “HAZ” (Heat-Affected Zones) where property changes are experienced by high temperature. Light green, dark green, light blue, dark blue, light red, dark red, and red spectrum variations, corresponding to differences in spectral colour intensities, suggest an uneven material flow and some rough surface, which is likely due to the interaction between the tool and the reinforced particles. The 3D Surface Topography Map is a more comprehensive, three-dimensional view of the surface topography. The height differences use the same colour gradient scale, with peaks in red and valleys in blue. It assists with identifying particular high peaks on the bushing wall surface. These peaks are probably due to the abrasive characteristics of the particles composing the snail shell, which interact with the tool when drilling. HAZ is indicated in this image, where heat has softened the aluminum matrix surrounding the reinforcement particles, leading to roughness and inhomogeneity in material flows. Those peaks and valleys in this zone are formed as a result of the coupled thermal and mechanical phenomenon that contributes to the roughness. The surface roughness profile consists of positive values representing peaks and negative values indicating valleys. Periodic peaks and valleys could be attributed to the non-uniform distribution of snail shell particles, microstructural changes within the HAZ and non-uniform heat distribution and cooling rates. Surface roughness is crucial in friction-drilled bushings, since it affects the mechanical properties, such as wear resistance and the ability to form strong connections with fasteners.
Surface Topography at different zones − 2D & 3D surface roughness map and surface roughness profile (a) Head Petal Region -A, (b) Thorax Region -B, (c) Upper Critical Region – C, (d) Lower Critical Region – D and (e) Tail Petal Region.
The 2D and 3D surface roughness maps (Fig. 4a) for Zone A of the Head Petal Region show the red for peaks and yellow-green for valleys. These variations indicate surface irregularities due to limited material flow and extrusion during tool entry. HAZ appears with darker areas closer to the peaks. The red peaks correspond to the regions of severe plastic deformation, while the green-yellow valleys show the zones with poor material flow. Material has been strongly pushed upwards in the region with red peaks, and valley-like yellow and green regions indicate incomplete material flow. Surface roughness in the range of 15–20 μm is observed in the surface roughness profile.
Surface roughness is lesser in the Thorax Region (Zone B) in comparison with Zone A, and smoother textures are uniformly spread out as yellow-green regions (Fig. 4b). The 3D peak red regions are relatively lower, indicating better material flow and reduced surface roughness. Here HAZ are the transition zones where the material is subject to less heat and deformation. Increased plasticity due to stabilized heat generation reduces the peaks and valleys, and the balance between the mechanical and thermal effects promote a more homogeneous structure. The improved uniformity shows a reduced number of peaks and valleys in the surface roughness profile, with roughness in the range of 8–12 μm.
The finest surface texture is observed in Zone C in the Upper Critical Region (Fig. 4c), where the light green and blue areas are predominant on the maps. Even in their deepest valleys or tallest red peaks, they show less surface irregularities and even material flow and minimum irregularities in the surface. Heating Affected zones are homogenously dispersed, promoting the desired thermal and mechanical conditions. The blue-green regions show low particle fragmentation and perfect microstructure alignments. The Upper Critical Region experiences a steady heat input that results in ideal circumstances for softening and controlled flow of material. The maps show uniform material flow (green and blue) with minimal deformation or tearing. The surface is the smoothest with the roughness value in the range of 3–5 μm.
Surface roughness slightly increases, with little red spots signifying localized peaks and yellow and green areas predominating in the local critical region (Fig. 4d). As thermal softening decreases, the surface changes from the homogeneity of Zone C to a more uneven structure. The roughness maps show a decrease in the intensity of colour transitions in the heat-affected zones. In the roughness maps, the green areas represent smoother regions, whereas the yellow areas represent areas of incomplete and partial material flow. Coarser grains and slightly rougher surface textures are the result of dynamic recrystallization being limited by reduced thermal softening. The change from green to yellow areas demonstrates how heat dissipation affects material flow. The roughness profile indicates increased roughness in comparison to Zone C with surface roughness in the range of 6–10 μm.
In the roughness maps of the tail petal region, high surface roughness brought on by disturbed material flow is noticeable with red peaks and yellow troughs (Fig. 4e). Significant surface irregularities caused by rapid cooling and material tearing are highlighted by the lack of consistent green or blue regions. Red spots correspond to areas of significant material extrusion and tearing; on the other hand, yellow valleys show partially disrupted material displacement. Rapid cooling and uneven deformation result from the sudden withdrawal of the tool, leading to disruption of the flow of material. Subsequently, it results in high surface roughness and coarse grains that are marked by the red peaks and yellow troughs with the surface roughness in the range of 15–25 μm.
The surface roughness values across different bushing zones in different values is presented in Table 4.
Effect on bushing length
Effect of process variables on Bushing Length.
Higher spindle speeds generate more heat that softens the aluminium matrix more effectively. This is increased friction causes the spindle to generate more heat. This, in turn, makes it possible to have a controlled flow of material along with the reinforcements45. As a consequence of this, the material that has been softened is more easily displaced and extruded, which ultimately results in longer bushing length that is more noticeable and longer. When feed rates are lower, there is more time for the tool to come in contact with the material. This eventually raises the amount of heat produced resulting in uniform and longer bushing length. Higher feed rates lead to inadequate heating and incomplete material flow. The tool moves quickly through the material, reducing the extent of extrusion and shortening the bushing length. Workpieces that are thicker more quantity of material is extruded and displaced, which ultimately results in a longer bushing length. When workpieces are thinner, there is less material available for extrusion, which results in shorter bushing length. Larger drill diameters help to achieve longer bushings by displacing more material. Smaller diameters, shortens bushing length. However, offers greater precision in bushing quality and wall thickness as revealed in Fig. 5.
Effect on bushing thickness
Effect of process variables on Bushing Thickness.
Increased spindle speeds produce greater heat because of more friction. It results in the aluminium matrix becoming more workable. Because of that, the material is able to flow around the tool with more ease, resulting in a thinner bushing. This is because the material is displaced outward rather than accumulated around the hole. Lower spindle speeds produce less heat, the aluminium continues to maintain its relatively high level of stiffness. Because of the decreased plastic deformation, the material comes together to form a longer bushing. The longer the tool-material interaction at lower feed rates, results in more amount of heat generation and leads to more plastic deformation as shown in Fig. 6. Because of this prolonged heating, the material tends to flow outward rather than upwards. This ultimately results in a thinner bushing. Higher feed rates limit the amount of heat generation, which in turn limits the flow of aluminium material. There is less plastic deformation, resulting in accumulation of material around the edge of the hole. This tends to increase in the thickness of the bushing. Working with thicker workpieces allows for a greater displacement of material during the friction drilling process. Because of the availability of more material around the hole, it results in typically thicker bushings. This occurs when sufficient heat is not produced and there is an incomplete partial material flow. On the other hand, thinner workpieces have less material for displacement around the circumference of the hole, resulting in reduced bushing thickness. When larger drill diameters penetrate, they displace more material, and this displacement can lead to the formation of a thicker bushing. Other parameters, such as feed rate or spindle speed, do not provide enough heat, and this thickening effect is more significant for larger diameters. Smaller drill diameters displace less material than larger drill diameters. Because of the availability of more material around the hole, it results in typically thicker bushings. This occurs when sufficient heat is not produced and there is an incomplete partial material flow. On the other hand, thinner workpieces have less material for displacement around the circumference of the hole, resulting in reduced bushing thickness. When larger drill diameters penetrate, they displace more material, and this displacement can lead to the formation of a thicker bushing. Other parameters, such as feed rate or spindle speed, do not provide enough heat and this thickening effect is more significant for larger diameters. Smaller drill diameters displace less material than larger drill diameters. In most cases, this leads to a thinner bushing, when combined with higher spindle speeds and lower feed rates.
Effect on roundness
When the spindle speed is raised, there is more heat generation due to the friction between the tool and the workpiece. This heat makes softens the material and it undergoes plastic deformation. Figure 7 reveals that higher spindle speed cause uneven plastic flow, eventually resulting in decrease of the roundness values. When the spindle speed is lower, there is insufficient heat generation because of poor material flow and there is deterioration in roundness.
Effect of process variables on Roundness.
Lower feed rates however yield longer time for heating and softening of the material. This leads to a better roundness achieved from the material deformation. Higher feed rates lead to intermittent tool-material contacts which result in uneven deformation of the material and thus may result in reduced roundness of the product. At this very low feed rates the large amount of heat developed can be harmful on hole geometry and roundness. Drilling of thicker workpieces takes more time and more heat generation. When heat generation is not sufficient, it deteriorates the roundness of material due to uneven deformation. Larger drills necessitate increased energy to produce adequate heat over an increased contact area. Suboptimal spindle speed and feed rate may lead to uneven deformation, adversely affecting the roundness. Reducing the drill diameter generates heat rapidly due to increased friction, frequently resulting in enhanced roundness. Conversely, excessive heat may induce local deformation, drastically affecting roundness.
