Objective Wire electrical discharge machining (WEDM) is suitable for the cutting of various metal parts with irregular contours. Fast-speed WEDM has a lower cost and higher efficiency than lower-speed WEDM. However, the surface roughness after fast-speed WEDM is larger and generally up to R_a=3--6 μm. For the polishing of WEDM rough surface, the existing methods mainly include mechanical polishing, chemical polishing, and electropolishing, but they have the disadvantages of low efficiency, high manual strength, high environmental pollution, and difficulty in polishing irregular parts. Laser polishing (LP) is a new surface polishing technology, which emerged during the development of laser processing technology. It has many advantages, such as high efficiency, no pollution, selective polishing, and irregular surface polishing, thereby avoiding the disadvantages of the traditional polishing methods. Presently, research on the pulsed LP (PLP) high surface roughness and its mechanism is inadequate. In this study, the fast-speed WEDM high surface roughness of 316L stainless steel (SS) was polished with a pulsed laser; then, the surface topography before and after polishing was photographed using a microscope with a super depth of field (SDoF) by which the surface profile and roughness were measured. Finally, the evolution law of polished surface morphology and PLP mechanism under different pulse duration values and energy forms was analyzed.

Methods The experimental material was 316L austenitic SS. Before the experiment, a high surface roughness was obtained using a DK77-30 CNC WEDM machine with up to 3.79 μm surface roughness. Then, the surface was polished with a master oscillator power amplifier (MOPA) nanosecond pulsed laser with 100 W maximum average power and 100 μm focused-spot diameter. To prevent the surface oxidation of the material, during the LP process, the workpiece was placed in an atmosphere protection box wherein high-purity Ar was continuously supplied with a 15 L/min gas flow rate. Finally, NIKON stereoscopic and Keyence VHX-5000 microscopes with SDoFs were used to, respectively, photograph and observe the macroscopic/microscopic morphologies of the polished surface. The TR-130A surface roughness tester was used to detect the surface roughness of the sample after polishing.

Results and Discussions As shown in Fig. 4, the microscopic peaks reduced under different pulse duration values. With 10 ns pulse duration, more initial valleys remained [Fig. 3(a)], and the crater-like appearance did not disappear [Fig. 4(a)]. With 50 ns pulse duration, the number of residual valleys decreased, and only a few deep valleys remained [Fig. 3(b)]. With the subsequent increase in pulse duration, the morphology after polishing tended to be consistent [Fig. 3(c--f)], and the contour had no peak and valley but an approximately horizontal and slightly fluctuating line [Fig. 4(g)]. As shown in Fig. 5, different pulsed laser energy forms had different effects on the surface morphology. The peaks reduced under the “low frequency with high energy” energy form, and several valleys remained [Fig. 6(a)]. Under “high frequency with low energy” energy form, valley was filled and surface morphology was smooth [Fig. 6(d)].

As shown in Fig. 7(a), the surface roughness decreased with the increase in pulse duration and was finally saturated (R_a=0.95 μm). However, the fluctuation in a wide range of the surface was difficult to be removed, preventing the roughness from further decrease. As shown in Fig.7(b), surface roughness gradually decreased with the energy form change from “low frequency with high energy” to “high frequency with low energy,” and the melting mechanism of “high frequency with low energy” had a more sustainable energy input and was more suitable for high surface roughness polishing. The gasification mechanism of “low frequency with high energy” was more suitable for low surface roughness polishing.

To further explore the action mechanism of pulsed laser on microscopic peak and valley of high surface roughness, a single pass experiment was conducted with different energy forms, and the change in microscopic peak and valley in the laser action area was observed using a microscope with SDoF. The “low frequency with high energy” form had a better effect on smoothing low peaks and shallow valleys than deep valleys but lacked continuous heat input and melt flow to fill deep valleys (Fig. 8). The “high frequency with low energy” form has a better effect on smoothing the microsurface topography, with continuous heat input and long-time melt flow of the material, resulting in a smooth surface (Fig. 9).

Conclusions In this study, a MOPA nanosecond pulsed laser with a wide range of adjustable pulse duration and repetition frequency was adopted to polish WEDM high roughness surface. The three-dimensional morphology, two-dimensional contour, and surface roughness of the polished surface under different pulse duration values and energy forms were analyzed. The following were observed.

1) Keeping other laser parameters constant, the microscopic surface peaks reduced under different pulse duration values, whereas the valleys showed a trend of gradually being filled as the pulse duration increased, and the surface roughness decreased from the initial R_a=3.79 μm to 0.95 μm.

2) Keeping other laser parameters constant, with the pulse energy form change from “low frequency with high energy” to “high frequency with low energy”, the surface vaporization acting trace gradually became shallower, the peaks reduced and valleys were gradually filled, and the surface roughness decreased from the initial R_a=3.79 μm to 1.37 μm.

3) With different energy forms, when the polishing mode gradually changed from pulsed polishing to quasi-continuous polishing, the surface polishing mechanism evolved from the gasification melting parallel polishing to simple melting polishing, and melting polishing was more suitable for polishing high roughness surface.

4) The main reason PLP could reduce the surface roughness was the smoothing effect of the narrow peaks on the surface. The macroscopic fluctuation of the initial high roughness surface could not be smooth, which limited the further decrease in roughness when the PLP used the high roughness surface.