High-power diode laser assisted hard turning of AISI D2 tool steel

Laser technology is being employed at an increasing rate in many industrial applications. The increasing demand for new engineered materials including ceramics, composites and hardened steel require manufacturing technologies alternative to traditional ones. The use of lasers in hot machining processes is one of them. The current research presents a study on laser assisted turning of hardened AISI D2 tool steel (≈ 60 HRC), widely utilized in the tool making industry, which poses problems with respect to the current state of machining technology due to hard chromium carbide particles present in its microstructure. This research work relates to the application of an analytical model to predict the rate of heating and cooling of the surface of the workpiece material subject to laser heating. Experimental temperature measurements were performed using an infrared thermometer and a thermocouple in order to calibrate and validate the temperature model. The predicted temperature evolution was then used in designing the laser assisted turning process with respect to cutting parameters and kinematics. Cutting tests were performed on a Nakamura Tome-450 CNC lathe on which a 2 kW diode laser (Laserline LDL 80-2000) was integrated. Two machining configurations: grooving and longitudinal turning were evaluated using carbide tooling with an emphasis on tool life, cutting forces, mechanism of chip formation, workpiece surface temperature, and surface integrity. Cutting tests performed on AISI D2 tool steel when using laser assist showed that an average temperature of about 300◦C in the uncut chip thickness is sufficient for proper LAM. The main mechanisms of tool wear identified during both conventional and laser assisted grooving were cutting edge chipping, flank face abrasion, and adhesion. Built iii up edge (BUE) was invariantly present during LAM, which was very stable for low cutting speed (20 m/min) and became unstable with an increase in cutting speed to 30 m/min. The use of laser assist enabled the cutting up to a speed of 30 m/min in the grooving cutting tests with good tool performance which was not possible without the use of laser. LAM also significantly reduced chatter, which was consistently noticed during conventional machining. The use of laser assist in grooving changed the cutting to thrust force ratio Fc/Fp from ≈ 0.5 in conventional cutting to ≈ 1 during LAM, indicating material softening. Chip thickening was noticed when using LAM, which suggests a decrease of about 10◦ in the shear angle. No thermal damage was found in the generated subsurface for the grooving experiments. Longitudinal turning tests were performed in two LAM configurations corresponding to two different laser beam orientations: spot slow axis parallel to the workpiece axis (LAM ‖) and spot fast axis parallel to the workpiece axis (LAM ⊥). Chipping and abrasion were identified as the main mechanisms of flank wear for both conventional and LAM tests. During both LAM tests (LAM ‖ and LAM ⊥) chipping was reduced and the tool life improved by about 100% compared with conventional turning. Chip analysis revealed that the segmented chips characterized both conventional and LAM ‖, while for LAM ⊥ chips transformed into continuous chips. This observation together with the measured surface temperature in front of the cutting edge (≈ 400◦ C for LAM ‖ and ≈ 600◦ C for LAM⊥) showed that LAM⊥ is the proper LAM configuration for longitudinal turning. No thermal damage was identified in the generated subsurface and surface roughness increased with the increase of temperature in the uncut chip thickness.