Objective Poor rigidity of micro milling tools and a high milling force are the main causes of low machining efficiency, poor surface integrity, and severe tool wear in micro milling TiAl intermetallic alloys. In this study, an innovative hybrid machining process comprising laser-induced controllable oxidation assisted micro milling was proposed to address these problems. In the proposed process, a controllable oxidation reaction occurs in the cutting zone, and loose oxides, which are easy to cut, could be synthesized during the hybrid machining, thereby decreasing the milling force and achieving a mass removal rate. Subsequently, micro milling would be applied to the subsurface materials and high quality microstructures would be manufactured. Most importantly, in this study, nanosecond pulse laser-induced oxidation of TiAl intermetallic alloys was studied, and the influence of laser machining parameters together with an assisted gas atmosphere on the oxidation behavior was investigated. The micro-zone oxidation mechanisms of workpiece materials under both laser irradiation and oxidizer were investigated in detail, and the forming mechanisms of loose oxidation were studied. A control strategy of loose oxidation was proposed; then, the oxidation behavior was adjusted subjectively. The results of this study will provide both theoretical and technical supports in micro milling of TiAl intermetallic alloys.

Methods TiAl intermetallic alloys were used in this work (Fig. 1). Laser-induced oxidation experiments were performed with high precision nanosecond (ns) pulsed laser equipment composed of a pulsed ytterbium fiber laser (YLP-F20, IPG Photonics Corporation) and CNC air floating platform. The laser spot diameter and pulse repetition frequency were fixed at 57 μm and 20 kHz, respectively. Laser-induced oxidation experiments were performed in a 99.5% pure oxygen-rich atmosphere and an injection velocity of 5 L/min. The laser energy density was varied from 6.86 J/cm ² to 11.76 J/cm ², and the laser scanning speed was 1 mm/s, 3 mm/s, 6 mm/s, and 12 mm/s (Table 3). The oxidation behavior in the atmosphere of air, argon (Ar), and nitrogen (N₂) under the same laser parameters was studied. A scanning electron microscope (SEM, Hitachi S-4800) was used to observe the morphologies and cross-sections of both the oxide layer and sub-layer. The hardness of TiAl alloys before and after laser-induced oxidation was measured with a Vickers diamond pyramid indenter (HVS-50) with a static load of 196 N and a loading time of 15 s. The phase compositions with the laser energy density after laser irradiation were detected by X-ray diffraction (XRD, Bruker D8). Cu-K(α) radiation with a scanning step of 0.02° and a sweep speed of 6 (°)/min were used.

Results and Discussions At the fixed laser pulse repetition frequency and laser spot diameter, the absorbed energy of the irradiated surface increased as the laser energy density increased. When the laser energy density was greater than the ablation threshold of the irradiated material, the oxidation reaction between the irradiated material and oxygen-rich atmosphere occurred, producing the titanium oxides. However, when the laser energy density was too high, the thermal effect accumulated on the surface of the irradiated material ablated the generated oxide (as shown in Fig. 5). The varied laser energy density significantly influenced the topographies of the sub-layer. At low laser energy density, the subsurface was flat, and residual oxides as well as micro-cracks existed. At lower laser energy density, the oxide layer primarily included low valent titanium oxides, such as TiO₂ and Ti₂O₃, as well as Ti₃O₅ and Al₂O₃. As the laser energy density increased, stable and high valent titanium oxides were produced, and the phase compositions primarily consisted of anatase TiO₂, rutile TiO₂, and Al₂O₃ (Fig. 6). At high laser energy density, the subsurface had a recasting-layer and many tiny micro craters together with large cracks (Fig. 7). In addition, the thickness of the oxide layer and sub-layer increased as the laser energy density increased (Fig. 8). Moreover, the low laser scanning speed produced better oxidation results compared with the results produced under high scanning velocity at the fixed laser energy density and repetition frequency (Fig. 9). It was noted that at low scanning speed, the thickness of the oxide layer was better than that at high scanning speed (Fig. 10). Furthermore, the irradiated material had better oxidation results under the oxygen-rich atmosphere, compared with other assisted gas atmospheres (Fig. 11).

Conclusions In this paper, the oxidation behavior of the irradiated material was studied under changing laser energy densities. All other laser parameters remained unchanged. In the oxygen-rich environment, the accumulated energy absorbed by TiAl material increased gradually as the laser energy density increased, which further promoted the oxidation reaction. In addition, the thickness of the generated oxide layer gradually increased. However, when the laser energy density was more than 9.80 J/cm ², the produced oxides started to melt and a dense recast layer was formed. The heat-affected zone generated by thermal diffusion expanded rapidly and the thickness of sub-layer increased dramatically. At high laser energy density, the oxide layer was primarily composed of anatase TiO₂, rutile TiO₂, and Al₂O₃. For the varied range of laser parameters, the oxidation result was better at a lower laser scanning speed. However, the laser scanning speed and assisted gas atmospheres other than the oxygen-rich environment had no effect on the thickness of the sub-layer. Overall, at laser energy density of 8.82 J/cm ² and laser scanning speed of 1 mm/s, as well as in an oxygen-rich environment, TiAl intermetallic alloys had better oxidation results, where the thickness of the oxide layer and sub-layer was 66 μm and 22 μm, respectively. After laser irradiation, the hardness of the sub-layer (200 HV) was lower than that of the substrate (365 HV, Table 1), which indicated that the laser-induced oxidation can improve the micro machinability of TiAl intermetallic alloys and promote the service life of micro end mills.