Pulsed Laser Ultrasonic Vibration-Assisted Cutting of SiCp/Al Composites through Finite Element Simulation and Experimental Research

Abstract

Silicon carbide particle-reinforced aluminum matrix composites (SiCp/Al) find diverse applications in engineering. Nevertheless, SiCp/Al exhibit limited machinability due to their special structure. A pulsed laser ultrasonic vibration assisted cutting (PLUVAC) method was proposed to enhance the machining characteristics of SiCp/Al and decrease surface defects. The finite element model was constructed, considering both the thermal effect of the pulsed laser and the location distribution of SiC particles. The model has been developed to analyze the damage forms of SiC particles and the formation mechanisms for the surface morphology. The influence of pulsed laser power on average cutting forces has also been analyzed. Research results indicate that PLUVAC accelerates the transition from the brittleness to the plastic of SiC particles, which helps to reduce surface scratching caused by fragmented SiC particles. Furthermore, the enhancement of surface quality is attributed to the decrease in surface cracks and the beneficial coating effect of the Al matrix. The accuracy of the simulation is verified by experiments, and the feasibility of PLUVAC method to enhance the surface quality of SiCp/Al is confirmed.

1. Introduction

SiCp/Al find extensive applications due to their notable attributes, such as high specific strength, modulus, and excellent wear and corrosion resistance, which make it a typically challenging material to process [1,2,3]. The presence of randomly distributed silicon carbide (SiC) particles in SiCp/Al poses challenges in machining procedures [4]. In fact, surface quality is one of the important indexes that affects the service life of a workpiece [5]. However, traditional techniques often result in SiC particle fracture, fragmentation, and pit, which can adversely affect the overall performance and stability of a workpiece [6,7]. A large number of studies have shown that vibration-assisted machining (VAM) and laser-assisted machining (LAM) methods can improve the machinability of SiCp/Al composites [8,9,10,11].

The ultrasonic vibration-assisted machining (UVAM) has been proven to be effective in improving surface quality [12,13]. For instance, Liu et al. [14] developed a force model for ultrasonic vibration-assisted scratching (UVAS), which took into account the characteristics of SiC particles and determined that the force exerted by UVAS is indeed less than that of conventional machining. Huang et al. [15] demonstrated the effectiveness of an ultrasonic elliptical vibration cutting (UEVC) method for enhancing the critical cutting depth of brittle materials, both in theory and through experimental validations. Meanwhile, Zha et al. [16] showed, through experiments, that the processing forces associated with ultrasonic vibration-assisted cutting (UVAC) are considerably smaller compared to traditional processing methods. However, studies on the surface quality of SiCp/Al are equally as important as the investigation of cutting forces. Zhou et al. [17] successfully predicted the surface morphology of SiCp/Al through finite element analysis and experiments. A study by Shi et al. [18] demonstrated that high-frequency vibrations in UVAM can maintain the integrity of SiC particles. Furthermore, Zheng et al. [19] also pointed out that the degree of surface damage in UVAM is lower compared to traditional processing methods. The experimental results of Bai et al. [20] indicate that UVAM results in fewer surface defects compared to other methods when using the same cutting tool. Additionally, Li et al. [21] illustrated that the improvement in the surface morphology of SiCp/Al through the UVAS method can be attributed to the overlapping trajectories of ultrasonic vibration compensating for holes created during fragmentation and pulled out of SiC particles. Therefore, UVAC can effectively reduce machining forces for SiCp/Al and enhance the surface morphology of a workpiece compared to traditional methods.

LAM is a composite processing technique that combines localized high-temperature laser heating with traditional machining, primarily including laser-assisted milling and laser-assisted cutting [22,23]. In the course of processing, laser heating causes a rapid elevation in the material’s temperature, achieving a softening effect or facilitating surface modification [24,25]. Through experimental research, Kim et al. [26] determined that the processing of various SiCp/Al necessitates distinct laser power to attain the goal of reducing cutting forces and enhancing surface quality. Zhao et al. [27] proposed the laser-induced oxidation-assisted micro-milling (LOMM) processing method for 65 vol% SiCp/Al and proved that LOMM provides a great improvement in machining forces and processing quality through experiments. Meanwhile, Zhai et al. [28] concluded, through a large number of experiments, that compared with traditional processing, LAM can reduce the surface roughness of SiCp/Al by 81.73%. Although LAM can improve the machinability of composite materials, the temperature gradient caused by a laser in the workpiece will inevitably produce a thermal damage layer [29]. In order to reduce the thermal damage layer at the same removal depth, it is a feasible solution to reduce the thermal effect depth via a high-energy pulsed laser. When conducting picosecond pulsed-assisted grinding experiments, Azarhoushang et al. [30] discovered that selecting the optimal laser power level can effectively prevent laser-induced damage on the workpiece surface while simultaneously reducing machining forces. Xiao et al. [31] introduced an innovative approach involving ultra-short pulse laser-assisted grinding, which effectively diminishes the grinding force through laser-induced chemical reactions on the surface of Ti₆Al₄V. Therefore, after conducting a feasibility analysis of UVAM and pulsed laser, a novel approach known as the pulsed laser ultrasonic vibration assisted cutting (PLUVAC) method was introduced for the purpose of improving surface quality in SiCp/Al.

In order to improve the machinability of SiCp/Al composites, the PLUVAC method was proposed for the first time. Through the mutual verification method of finite element simulation and experimental research, it was found that the PLUVAC method can accelerate the transition from brittle to plastic SiC particles and reduce the surface damage caused by SiC particle breakage. And the PLUVAC method can reduce the number of cracks and improve the surface quality of the SiCp/Al surface. In addition, the main research contents of this paper are as follows. In this paper, for a comprehensive understanding of the surface formation mechanism of PLUVAC, the finite element model considering the location distribution of SiC particles was built based on the thermal effect of a pulsed laser, the damage forms of SiC particles, and the surface defects in PLUVAC and UVAC were compared. Additionally, corresponding experimental studies were carried out to explore the effect of process parameters on surface roughness (Sa) and surface morphology. An analysis of how pulsed laser power affects the average cutting forces was also conducted. Finally, the accuracy of the simulation model was validated through examinations of the surface defects in SiCp/Al machined using UVAC and PLUVAC.

2. The Principle of PLUVAC

The PLUVAC method combines a pulsed laser with ultrasonic vibration to enhance the processing characteristics of SiCp/Al. The laser’s focal point is positioned at the leading edge of the tool and moves synchronously with the two-dimensional ultrasonic vibration device at a velocity denoted as V. The relative position of the laser spot is denoted as L and the tool tip is 4 mm, as illustrated in Figure 1. Furthermore, the vibration frequency of the two-dimensional ultrasonic vibration device is 20 KHz, the amplitude in the X-direction is 10 μm, and the amplitude in the Y-direction is 4 μm.

Initially, when the pulsed laser irradiates the surface of the workpiece, the material absorbs energy and the surface temperature increases, so as to achieve the purpose of modifying the surface of the workpiece using the pulsed laser. Pulsed laser irradiation not only reduces the hardness of SiC particles but also enhances the plastic flow characteristics of the Al matrix [32,33]. Subsequently, under ultrasonic vibration, a tool is employed to periodically cut the irradiated area. This process is beneficial to transform SiC phase removal from brittleness to plastic [34], reduce the fragmentation of SiC particles, and prevent the broken SiC fragments from entering the back tool face surface and causing secondary damage.

3. PLUVAC Finite Element Simulation of SiCp/Al

To analyze the improved effects of the PLUVAC method, in this section, a comparative analysis of the damages to SiC particles and the surface formation mechanisms in SiCp/Al machined by PLUVAC and UVAC methods is conducted through finite element analysis.

3.1. Finite Element Model

To facilitate observation of the microstructure and particle dimensions within the 20 vol% SiCp/Al, the workpiece surface is initially subjected to pre-polishing. The average particle size of SiC particles in the SiCp/Al composites processed in this paper is 15 μm, and the polished SiCp/Al surface is photographed using a scanning electron microscope, as illustrated in Figure 2a. Figure 2b describes the constructed simulation model, in particular, the relative position L of the laser spot and the tool tip, which is 4 mm. In order to make the finite element analysis results more accurate, the mesh shape of the model is mainly quadrilateral and the free mesh division technology is used to control the mesh. At the same time, the overall mesh size of the model is refined. The mesh sizes of the SiC particle and the Al matrix are the same, being 1 μm, the total number of grids is 26,544, and the mesh convergence effect is good. By observing the surface of the SiCp/Al workpiece using scanning electron microscopy, it is found that SiC particles are randomly distributed in the Al matrix with an average particle size of 15 μm. Based on this, we create a random distribution model of SiC particles. Detailed cutting simulation parameters are found in

Table 1. Cutting simulation parameters.

Cutting Parameters	Numerical Value	Unit
Cutting speed V	654	mm/s
Cutting depth Ap	10	μm
Longitudinal amplitude	10	μm
Bending amplitude	4	μm
Tool front angle	5	angle
Tool back angle	7	angle

.

3.2. Material Properties

The J-C constitutive model is employed to describe the plastic flow of the Al phase [35,36]. The material constitutive model of the Al matrix is described by the J-C constitutive model. However, in the J-C constitutive model, the fracture or flow of the Al matrix is determined according to the failure criterion of the J-C constitutive model. The specific equation is as follows:

$$D=\sum \frac{∆\epsilon }{{\epsilon }^f}$$

(1)

where ${\epsilon }^f$ is the equivalent strain and $∆\epsilon$ is the strain increment. $D$ is the failure parameter; when this value is 1, the failure of the unit occurs. The equivalent strain can be expressed as follows:

$${\epsilon }^f=\left[D_1+D_{2}exp\left(D_3{\sigma }^{\ast }\right)\right]\left(1+D_{4}ln\frac{\epsilon }{{\dot{\epsilon }}_0}\right)\left(1+D_5\frac{T-T_r}{T_m-T_r}\right)$$

(2)

where ${\sigma }^{\ast }$ is the stress triaxiality, which is the ratio of hydrostatic pressure $P$ to Mises stress $\sigma$, and $D_1-D_5$ is the material failure parameter. The parameter values of $D_1-D_5$ are 0.06, 0.497, −1.551, 0.0286, and 6.8, respectively.

The linear Drucker–Prager constitutive model is employed to simulate the characteristics of the SiC phase [37]. Additionally, in order to represent a more realistic failure behavior of the particles during cutting, a cohesive element model with zero thickness is used to describe the bonding effect between the particles and the matrix. Ultimately, the cohesive unit with zero thickness is incorporated into both SiC particles and Al matrix using the Python programming language. Specific material properties are shown in

Table 2. Material properties.

Parameters	Al Phase	SiC Phase
Density (ton/mm3)	2.7 × 10−9	3.2 × 10−9
Young’s modulus (MPa)	70,000	408,000
Poisson’s ratio	0.3	0.2
Thermal conductivity (W/(m·K))	189	81
Specific heat capacity (J/(kg·K))	2.3 × 10−5	4.9 × 10−6
Yield stress (MPa)	/	1500
Melting point (K)	873.15	2973.15
A (MPa)	270	/
B (MPa)	134	/
n	0.514	/
m	0.703	/
C	0.0082	/
D1	0.06	/
D2	0.497	/
D3	−1.551	/
D4	0.0286	/
D5	6.8	/

.

3.3. Damage Forms Analysis of SiC Particles

During the simulation process, it is crucial to concentrate the pulsed laser energy within a specific region. Figure 3a illustrates the Gaussian distribution of the pulsed laser, and the pulse width of the pulsed laser is set to 100 ns, the pulsed frequency of the pulsed laser is 200 KHz, and the pulsed laser power is 40 W. The actual pulse is equal to a rectangular shape, as shown in Figure 3b. Here, $\left(t_2-t_1\right)$ represents the duration of a single pulse’s energy release, and $\left(t_3-t_1\right)$ denotes the period of a single pulse.

In PLUVAC, the thermal conductivity, specific heat, absorption coefficient, and convective heat transfer coefficient of SiCp/Al are all functions of temperature. Therefore, the problem of the temperature field in PLUVAC is actually a three-dimensional transient nonlinear thermal conductivity problem. This mainly includes three modes of heat transfer, namely heat conduction, heat convection, and heat radiation. Among them, the basic law equation of heat conduction is as follows:

$$q=-kgradT=-k\frac{\partial T}{\partial n}$$

(3)

where $q$ is the heat flux, $k$ is the thermal conductivity of the material, and $\frac{\partial T}{\partial n}$ is the temperature gradient in the normal direction.

The fundamental equations of heat convection are as follows:

$$q=h_c\left(T_a-T_b\right)$$

(4)

where $h_c$ is the convective heat transfer coefficient, $T_a$ is the solid surface temperature, and $T_b$ is the temperature of the surrounding medium.

The maximum radiant energy flux density equation is as follows:

$$q=\epsilon \sigma T^4$$

(5)

where $\epsilon$ is the emissivity of the object and $\sigma$ is the black-body radiation constant, $5.67\times {10}^{-8}\mathrm{W}/\left({\mathrm{m}}^2·{\mathrm{K}}^4\right)$.

The energy distribution of the pulsed laser satisfies the Gaussian distribution, and the specific equation is expressed as

$$I\left(r,t\right)=I\left(0,t\right)e^{\frac{-2r^2}{r_b^2}}$$

(6)

where $I\left(0,t\right)$ is the power density at the spot center at time $t$, $r$ is the length of any point in the cross-section from the spot center, and $r_b$ is the laser spot radius.

For the pulsed laser, the power density of the center point ($r=0$) in the cross-section during one cycle can be expressed as follows.

$$I\left(0,t\right)=\left\{\begin{array}{cc}\frac{P}{\pi \tau f{r}_b^2}& 0\le t\le \tau \\ 0& \tau <t<T\end{array}\right.$$

(7)

Among them, $r_b$ is a fixed value if and only if the distance between the spot and the light outlet is determined. $P$ is the laser power, $\tau$ is the pulse width, and $f$ is the pulse frequency.

In the simulation setup, the pulse frequency of the pulsed laser is 200 KHz, the pulse width is 100 ns, and the power is 40 W.

As the pulsed laser irradiates the surface, despite variations in the thermal conductivity of the Al phase and SiC phase, both material phases experience different degrees of softening at elevated temperatures. The damage of SiC particles plays an important role in surface quality. Therefore, in the following study, the damage forms of SiC particles during machining with PLUVAC and UVAC methods are comparatively analyzed.

The SiC particle is pressed into the Al matrix. In contrast to the UVAC, when the cutting path intersects the top of the No. 1 SiC particle, a fraction of the No. 1 SiC particle on the cutting path gets pushed into the chip, while the remainder embeds itself into the Al matrix. Furthermore, the remaining SiC particles do not rotate; this leads to a reduction in the pits left by the debonding of the No. 1 SiC particle, as shown in Figure 4a,b. Moreover, the thermal effect of the pulsed laser on the workpiece surface softens the Al phase, enhances the fluidity of the Al phase, and reduces the resistance posed by the SiC particles during their downward movement. However, in the case of UVAC, Figure 4c,d show that the extrusion stress exerted by the tool on the No. 1 SiC particle surpasses the strength limit, leading to the development of significant stress within the No. 1 SiC particle, which eventually leads to fracture and debonding.

SiC particle fracture. Due to the introduction of a pulsed laser, the interface strength between the SiC phase and Al phase decreases, which subsequently leads to a reduction in the sliding motion of the SiC particle in the machining process. Moreover, the hardness of the No. 2 SiC particle undergoes alterations induced by the pulsed laser, resulting in a reduction in its yield strength, accelerating the transition from brittleness to plastic, as shown in Figure 5a. Figure 5b shows that the fractured SiC particle is pressed into the Al matrix. Compared with the UVAC method, the fractured SiC particle is pressed into the Al matrix, which can avoid secondary damage to the SiCp/Al surface caused by the fractured SiC particle entering the back of the tool. Conversely, in the case of UVAC, Figure 5c,d show that the No. 2 SiC particle induces damage by creating pits on the workpiece surface through debonding.

Debonding and pull-out. As the cutting path intersects the bottom of the No. 3 SiC particle, the damage form of the SiC particle is also different compared with the UVAC and PLUVAC methods. In the case of UVAC, the No. 3 SiC particle is completely pulled out by the cutting force of the tool. Simultaneously, Figure 6c shows that the No. 3 SiC particle ploughs upward into the Al matrix. The collision between the upward-ploughed No. 3 SiC particle and the adjacent No. 4 SiC particle leads to the debonding of the No. 4 SiC particle, as shown in Figure 6d. However, the thermal effect of the pulsed laser destroys the two-phase interface in SiCp/Al, and the No. 3 SiC particle experiences a tensile force, denoted as “F”, along its long axis, which further accelerates the pull-out rate of the No. 3 SiC particle, as shown in Figure 6a,b. Moreover, the rapid pull-out of the No. 3 SiC particle prevents its entry into the tool’s back face, thereby avoiding the secondary processing of the workpiece surface.

By comparing the damage forms of SiC particles through PLUVAC and UVAC, it is observed that the introduction of pulsed laser energy can accelerate the transition from brittleness to the plastic removal of the SiC particle during the UVAC process. Additionally, the rapid pull-out of SiC particles can also prevent the secondary process on the machined surface resulting from debonding SiC particles.

3.4. Surface Defects Analysis

The surface defects in SiCp/Al represent macroscopic manifestations of SiC particle damages. Hence, the surface defects of PLUVAC and UVAC are analyzed separately through FEM simulation.

Figure 7 illustrates the simulation results for UVAC. It is apparent that there are a variety of defects, including the SiC particle fragment, debonding, pull-out, plowing and Al matrix tearing. Firstly, the extrusion stress applied by the tool exceeds the strength limit of the SiC particles, leading to their fragments. Secondly, the extrusion force from the tool destroys the two-phase interface, causing partial debonding of the SiC particles. Simultaneously, as a result of the tool’s ultrasonic vibration, the debonding SiC particles are pulled out, thereby creating pits on the workpiece’s surface. Due to the material properties of the Al matrix, the phenomenon of matrix toughness tearing occurs on the surface. The plastic strain exhibited by the Al matrix will also lead to toughness tearing, thereby producing a large number of surface cracks.

Conversely, a significant amount of plowing occurs on the workpiece surface. This plowing phenomenon can be attributed to two main factors. Firstly, the SiC particles slip continuously in the Al matrix under the tool’s ultrasonic vibration, which results in the plowing effect on the surface. Secondly, as the tool progresses into the next cutting cycle, the extrusion force from the tool’s trailing edge further interacts with the SiC particles on the machined surface, resulting in irregular plowing patterns.

Figure 8 shows that the workpiece surface defects of PLUVAC are significantly less than those of UVAC. The thermal effect of the pulsed laser not only enhances the plasticity of the Al phase but also reduces the hardness of the SiC phase. Furthermore, the defects resulting from SiC particles damages are mitigated by the Al matrix coating, effectively filling in the small pits. This improvement represents one of the pivotal factors contributing to the flattened surface following the pulsed laser action. When the modified SiC particles come into contact with the tool, they tend to plastic fracture, thereby preventing issues such as SiC particle debonding, plowing, and rotation that occur in the UVAC. Furthermore, the fractured SiC particles remain embedded within Al matrix, resulting in a reduction in surface defects on the workpiece. To sum up, the distribution of SiC particles is pivotal in influencing the surface characteristics of SiCp/Al.

4. Experimental Setup

The PLUVAC experimental platform consists mainly of the pulsed laser system, the ultrasonic vibration cutting system, and the force measurement system. The pulsed laser system includes a nanosecond pulsed laser head (P100Q laser, Wuhan Raycus Fiber Laser Technology Company, Wuhan, China) and a fixture. The pulsed laser system is fixed on the moving guide rail of the CNC lathe to ensure that the pulsed laser beam is irradiated in the position in front of the tool tip. The pulsed laser head is installed on the fixture that can move in the X-Z plane, rotate around the Y axis, and adjust the pitch angle. During the experiment, the pulse frequency of the pulsed laser is 200 KHz, the pulse width is 100 ns, and the pulsed laser power is 40 W. The ultrasonic device is fixed on the moving guide rail of the CNC lathe, so that the ultrasonic device and the pulsed laser system can move synchronously during the cutting process. Finally, the force measurement system includes a dynamometer (Kistler 9109AA, Kistler Group, Winterthur, Switzerland), an electric charge amplifier, and a data collector, wherein the Kistler dynamometer is fixed under the ultrasonic device, and the forces during the machining process are measured in real time, as shown in Figure 9. The Sa of the machined SiCp/Al is measured several times using a white-light interferometer (ZYGO NewView 8000, AMETEK, Inc., Berwyn, PA, USA), and the surface morphology of SiCp/Al is examined using scanning electron microscopy (SEM, FEI company, Hillsboro, OR, USA).

5. Results and Discussion

In this section, firstly, the effects of different process parameters on the PLUVAC method and UVAC method are comparatively studied, and the variation in Sa values is analyzed. Then, the effect of pulsed laser power (P) on the average cutting forces is analyzed. Finally, the accuracy of the simulation is verified by analyzing the surface morphology obtained using the PLUVAC and UVAC methods.

The single-factor experiment method is used to explore the effects of workpiece speed (n), feed rate (f), cutting depth (Ap), and pulsed laser power (P) on surface roughness (Sa). The specific experimental design is shown in

Table 3. Single-factor experiment.

Processing Parameters	Workpiece Speed (n)/(rpm)	Feed Rate (f)/(μm/r)	Cutting Depth (Ap)/(μm)	Pulsed Laser Power (P)/(W)	Pulsed Frequency /KHz
Section 5.1	300, 500, 800, 1000	10	8	40	200
Section 5.2	500	8, 10, 12, 15	8	40	200
Section 5.3	500	10	6, 8, 10, 12	40	200
Section 5.4	500	10	8	10, 20, 30, 40, 50, 60	200

.

5.1. Effect of the Workpiece Speed (n) on the Sa

The implementation of pulsed laser technology has a substantial impact on the Sa of SiCp/Al in comparison to UVAC. As is evident from Figure 10, it is apparent that the Sa values for both processing methods initially decrease and then increase with an increase in workpiece speed (n). Notably, the lowest Sa value is attained at a workpiece speed (n) of 500 rpm. At a lower workpiece speed, the pulsed laser proves effective in sufficiently softening the SiCp/Al, thereby reducing the yield strength and enhancing a processing outcome. However, with an increase in workpiece speed (n), a significant elevation in Sa is observed. There are two possible reasons for this phenomenon. Firstly, as the workpiece speed (n) approaches the critical speed for ultrasonic vibration, the separation periods between the tool and workpiece diminish, which probably results in an increase in actual contact time and changes in the shear angle, ultimately resulting in heightened Sa. Secondly, the higher workpiece speed (n) reduces the duration of heating applied by the pulsed laser to the workpiece, leading to a decreased degree of softening in the workpiece and a less desirable processing outcome.

5.2. Effect of the Feed Rate (f) on the Sa

It is seen from Figure 11 that the Sa after PLUVAC is notably lower than that following UVAC. In both machining methods, the Sa values exhibit an increasing trend with an increase in the feed rate (f). This can be explained by the following factors. Firstly, the increase in the feed rate (f) reduces the heating time of the pulsed laser on the surface of SiCp/Al, which leads to the poor modification effect of SiCp/Al composites and an increase in Sa. Secondly, the residual area height gradually increases with the increase in the feed rate (f), which worsens the surface of SiCp/Al and increases the Sa value. Finally, from the analysis of the force of SiC particles in the SiCp/Al material, the increase in the feed rate (f) will directly increase the impact force of the tool on SiC particles, increase the breakage probability of SiC particles, and increase the Sa value of SiCp/Al.

5.3. Effect of the Cutting Depth (Ap) on the Sa

Figure 12 illustrates a significant increase in Sa value with the cutting depth (Ap), regardless of the machining method. This effect is similar to the impact of the feed rate (f) on Sa. The primary reason for this phenomenon is the reduction in cutting stability as the cutting depth increases, resulting in an increased number of machining surface defects. Additionally, the workpiece contains a substantial quantity of high-hardness SiC particles, and increasing the cutting depth (Ap) enhances the likelihood of interaction between the tool and these hard SiC particles, leading to the degradation of surface quality.

5.4. Effect of the Pulsed Laser Power (P) on the Sa

The Sa initially decreases and then increases as the pulsed laser power (P) is varied from 10 W to 60 W, and, at 40 W, the Sa is minimized, as shown in Figure 13. This observed trend can be attributed to the intensified pulsed laser power (P), which enhances the conversion of light energy into heat energy. Therefore, the workpiece reaches a temperature at which it undergoes complete softening, thereby improving the machinability of the material. However, excessive pulsed laser power (P) can lead to an increase in the irradiated spot area, which will result in severe workpiece ablation, and, consequently, a significant deterioration in surface quality [38].

In order to verify the feasibility of the PLUVAC method for processing SiCp/Al composites, the changes in surface roughness Sa with different processing parameters (the workpiece speed n, the feed rate f, the cutting depth Ap, and the pulsed laser power P) are compared between PLUVAC and UVAC methods. The results show that the workpiece speed n has the greatest impact on Sa, followed by the feed rate f and the pulsed laser power P, and, finally, the cutting depth Ap. In addition, at $n=500$ rpm, $f=8$ μm/r, $Ap=8$ μm, and $P=40$ W, the UVAC method obtains the lowest Sa value of 0.404 μm. At $n=500$ rpm, $f=10$ μm/r, $Ap=6$ μm, and $P=40$ W, the PLUVAC method achieves the lowest Sa value of 0.27 μm. In summary, the PLUVAC method can improve SiCp/Al composite machinability.

5.5. Effect of the Pulsed Laser Power (P) on the Average Cutting Forces

In order to more clearly study the effect of the pulsed laser power P in the PLUVAC method on the average cutting force in the machining process, the processing parameters at the lowest Sa value obtained using the PLUVAC method are used to conduct experiments. That is, $n=500$ rpm, $f=10$ μm/r, $Ap=6$ μm, and $P=40$ W.

Figure 14 shows that at lower pulsed laser power levels, the temperature of the pulsed laser remains relatively low, and the material experiences limited softening effects. This results in the modified layer thickness falling short of the cutting depth, making the changes in average cutting forces less apparent. As pulsed laser power (P) is increased, a noticeable reduction in average cutting forces becomes evident. However, when the pulse laser power (P) exceeds 40 W, the average cutting forces gradually increase, which can be attributed to the melting of the Al matrix in the processing area and the formation of chip nodules on the tool surface, which increases the degree of tool wear, and then increases the average cutting force.

5.6. Comparative Analysis of Surface Defects between PLUVAC and UVAC Methods

5.6.1. Complete Fragment and Partial Fracture of SiC Particles in UVAC

From Figure 15a, it is evident that SiC particles undergo fragmentation into particles of varying sizes and are dispersed across the workpiece surface. On the one hand, the extrusion force exerted by the tool exceeds the hardness of the SiC particle, resulting in the fragmentations of the SiC particles due to the external force. On the other hand, the fragmentation of SiC particles occurs as a result of interactions with neighboring particles that have been pulled out. Furthermore, when only a minor fraction of SiC particles is positioned above the cutting path, brittle fractures in SiC particles occur along their longer axis direction in response to the vibration impact from the tool, as shown in Figure 15b.

5.6.2. Debonding and Pull-Out of SiC Particles in UVAC

Figure 16 provides a diagram illustrating the defects linked to the debonding of SiC particles. Figure 16a,b illustrate the debonding of SiC particles within the Al matrix. When the tool comes into contact with only a small portion of the SiC particles, the extrusion force of the tool will destroy the interface phase, leading to the debonding of SiC particles within the Al matrix. Furthermore, Figure 16c,d show that numerous irregular pits and holes appear on the surface. When the volume of SiC particles exposed at the workpiece’s surface exceeds that within the Al matrix, SiC particle debonding occurs.

5.6.3. Surface Cracks of Workpiece in UVAC

Figure 17 reveals a significant presence of surface cracks. These cracks primarily result from the plastic deformation of the Al matrix, while the extrusion of SiC particles also plays a role in their formation. In addition, the fragmented SiC particles become embedded in the workpiece surface for secondary processing, under the action of the back cutter face, which leads to the development of surface cracks.

5.6.4. Surface Defects Analysis of PLUVAC

The PLUVAC method significantly decreases the surface defects of SiCp/Al. Figure 18a illustrates that the compression of SiC particles reduces the fragmentation of SiC particles and then avoids the secondary processing of the surface caused by fragmented SiC particles. Following pulsed laser irradiation, the transition from brittleness to the plastic removal of SiC particles is promoted, and the fragmentation and fracture of SiC particles are reduced, as shown in Figure 18b. Moreover, a notable reduction in cracks on the surface is evident. This reduction can be attributed to the enhanced plastic flow capacity of the Al matrix and is facilitated by heat softening effectively compensating for surface pits and cracks following the UVAC method, as shown in Figure 18c. Notably, Figure 18d illustrates that the PLUVAC method results in a substantial coating on the workpiece’s surface when the tool presses the Al matrix onto the processed surface, concealing defects such as holes, cracks, and pits [39].

Through an observation of the surface morphology of SiCp/Al, machined using PLUVAC and UVAC methods, it can be found that the PLUVAC method reduces the fragmentation of SiC particles compared with the UVAC method, as shown in Figure 15 and Figure 18b, which is similar to the simulation results, as shown in Figure 7 and Figure 8. The simulation results show that in PLUVAC, SiC particles basically have no breakage behavior. In addition, the PLUVAC method reduces the number of surface cracks of SiCp/Al, which is observed in Figure 17 and Figure 18c. At the same time, it can also be found from the simulation results that the number of surface cracks processed using the PLUVAC method is significantly reduced. Finally, simulations and experiments show that the coating behavior of the Al matrix is found on the SiCp/Al surface processed using the PLUVAC method, as shown in Figure 8 and Figure 18c,d.

6. Conclusions

In this paper, the PLUVAC method is proposed to enhance the machinability of SiCp/Al. The surface defects of SiCp/Al processed using the PLUVAC method have been analyzed by combining finite element simulation and experimental study. The main conclusions are as follows:

(1) The feasibility of the PLUVAC method in enhancing the machinability of SiCp/Al has been demonstrated via finite element analysis and experimental investigations. Simulation and experimental results reveal that surface defects in the UVAC method are primarily attributed to SiC particle fragmentation, fracture, debonding, pull-out, ploughing, and surface cracks. In contrast, the PLUVAC method accelerates the transition from brittleness to plastic of SiC particles. Additionally, a reduction in the quantity of cracks on the surface and the Al matrix coating effect further enhance surface quality.

(2) The influences of different processing parameters on the Sa of SiCp/Al using PLUVAC and UVAC methods have been investigated through single-factor experiments. The experimental results indicate an initial decline followed by an increase in Sa as the workpiece speed (n) and pulsed laser power (P) are raised, whereas the Sa escalates with a higher feed rate (f) and cutting depth (Ap). In addition, the lowest Sa of SiCp/Al using the PLUVAC method was 0.27 μm, and the process parameters are as follows: workpiece speed (n) is 500 rpm, feed rate (f) is 10 μm/r, cutting depth (Ap) is 8 μm, pulsed laser power (P) is 40 W, and pulse frequency is 200 KHz.

(3) The influence of pulsed laser powers on average cutting forces in PLUVAC has been investigated through experiments. The experimental findings indicate that as the pulsed laser power rises, the average cutting forces exhibit an initial reduction followed by an increment. When the pulsed laser power is too low or too high, the modification of the surface by the pulsed laser shows counteraction, resulting in increased average cutting forces. The pulsed laser power (P) needed to obtain the lowest average cutting force is 40 W.