Optimization of Process Parameters for the Laser Polishing of Hardened Tool Steel

Abstract

In mold making, the mold surface roughness directly affects the surface roughness of the produced part. To achieve surface roughness below 0.8 μm, the cost of surface finish is high and time-consuming. One alternative to the different grinding and polishing steps is laser polishing (LP). This study investigates and models the LP of tool steel (X38CrMoV5-1-DIN 1.2343), typical for the mold industry, having an initial rough surface obtained by electrical discharge machining. The microstructures of the re-melted layer and heat-affected zone due to the LP process were also studied. Four parameters: the laser spot size, velocity, maximum melt pool temperature and overlapping were investigated via a design of experiments (DoE) approach, specifically a factorial design. The responses were line roughness (Ra), surface roughness (Sa), and waviness (Wa). The surface topography was measured before and after the LP process by white light profilometer or confocal microscopy. DoE results showed that the selected factors interact in a complex manner, including the interactions, and depend on the responses. The DoE analysis of the results revealed that the roughness is mainly affected by the velocity, temperature and overlap. Based on a first DoE model, an optimization of the parameters was performed and allowed to find optimum parameters for the LP of the rough samples. The optimum conditions to minimize the roughness are a spot size of 0.9 mm, a velocity of 50 mm/s, a temperature of 2080 °C and an overlap of 90%. By using these parameters, the roughness could be reduced by a factor of almost 8 from 3.8 µm to approximately 0.5 µm. Observations of the microstructure reveal that the re-melted layer consists of columnar grains of residual austenite. This can be explained by the carbon intake of the electro-machined surface that helps stabilize the austenitic phase.

Development of an Injection Mold with High Energy Efficiency of Vulcanization for Liquid Silicone Rubber Injection Molding of the Fisheye Optical Lens

“…However, the mechanical properties [29] of the LSR injection mold were lower than those of the LSR injection mold fabricated with conventional tool steel [30]. Therefore, improving the mechanical properties of an LSR injection mold by adding some distinctive reinforcing filler [31][32][33][34][35] in the mixture is an important research direction for future study. To elucidate these points, further work must be done.…”

Enhancing Surface Temperature Uniformity in a Liquid Silicone Rubber Injection Mold with Conformal Heating Channels

“…In this study, two different sets of rapid tools were implemented using a mixture comprising of Al powder and epoxy resin. Unfortunately, the mechanical property [28] of the rapid tool [29] was inferior to conventional mold steel [30]. Thus, improving the mechanical properties of the fabricated LSR injection mold [31][32][33] by adding stainless steel or iron powder [34] is an interesting research topic.…”

An Investigation into the Surface Integrity of Micro-Machined High-Speed Steel and Tungsten Carbide Cutting Tools

“…At 370 V and 120 mS/cm of electrolyte conductivity, Schulze et al were able to reduce the surface roughness of tool steel from 300 nm to 67 nm in 240 s using electrochemical pulse polishing [31]. Surface roughness as low as 0.5 µm was achieved by Meylan et al using high-power diode laser polishing at a wavelength of 980 nm [32]. At a translating speed of less than 1 mm/s, Kawanaka and Kunieda were able to create a mirror-like surface smoother than Rz 0.2 mm using a monopolar pulse current with a short duration of 100 ms and a low duty factor under 10% [33].…”