Picosecond laser machining of tungsten carbide

Highlights

• Investigated 300 W average power picosecond laser machining of tungsten carbide material

• Thermal defect free machining can be achieved at 300 W.

• A material removal rate of ~45 mm³/min was observed at 300 W.

• Pulse burst based laser machining does not produce desired results on tungsten carbide machining.

• High power laser machining of tungsten carbide is a viable alternative to conventional machining.

Abstract

Hard materials such as tungsten carbide (WC) are extensively used in cutting tools in high-value manufacturing, and the machining of these materials with sufficient speed and quality is essential to exploit their full potential. Over the last two decades, short (nanosecond (ns)) and ultra-short (picosecond (ps); femtosecond (fs)) pulse laser machining has been evaluated by various researchers and proposed as an alternative to the current state-of-the-art machining techniques for advanced materials like WC, which include mechanical grinding and electrical discharge machining. However, most of the established/existing research on this topic is based on low power lasers, which may not be adopted in industrial production environments due to its low material removal rate. This paper presents the results of a fundamental study, on using a 300 W picosecond laser for the deep machining of tungsten carbide. The influence of various laser parameters on the geometric precision and quality (surface and sub-surface) of the ablated area was analysed, and the ablation mechanism is discussed in detail. Laser pulse frequency and scanning speed have minimal effect on ablation rate at high power levels. The surface roughness of the ablated area increases with the ablation depth. At optimal conditions, no significant thermal defects such as a recast layer, micro crack or heat affected zone were observed, even at a high average power of 300 W. The material removal rate (MRR) seems to be proportional to the average power of the laser, and a removal rate of around 45 mm³ per minute can be achieved at 300 W power level. Edge wall taper appears to be a significant issue that needs to be resolved to enable industrial exploitation of high power ultra-short pulse lasers.

Introduction

Advanced materials such as tungsten carbide (WC) have excellent mechanical [1] and thermal properties, including hardness, wear-resistance and retention of strength at elevated temperatures which make them suitable for a wide range of applications from aerospace to tooling [2]. These properties make the machining of WC challenging by conventional mechanical based machining processes. In conventional machining, WC materials are machined mostly using grinding techniques [3]; however, the grinding processes have several issues and limitations including geometric constraints, grinding tool wear, low material removal rates and the need for high mechanical loads when grinding hard materials. Non-conventional machining processes like laser [4] and electrical discharge machining [5] are nowadays frequently used in the machining of advanced materials due to their capability in processing hard and difficult-to-cut materials.

Of all the non-conventional machining processes, laser machining using short [6] (nanosecond (ns)) and ultra-short [7] (picosecond (ps)-femtosecond (fs)) pulsed laser sources are becoming a standard process for material removal due to their flexibility and scalability. Over the last decade, the power of commercial lasers has increased, and the cost decreased, making it an ideal process for the machining of advanced materials.

Pulsed laser ablation (PLA) by nanosecond lasers is used across a range of industries; however, significant spatter re-deposition, and thermal damage was reported at higher material removal rates (MRR) [6]. The mechanism of ns PLA involves substrate absorption of laser energy followed by melt-pool formation, partial vaporisation and ejection of the melt by vapour induced recoil pressure. In PLA only part of the melted material is vaporised or ejected, and the rest solidifies inside the laser-irradiated zone as a recast layer, which is not acceptable for applications like tooling. Also, a considerable quantity of the ejected material is re-deposited as dross (or spatter) [8] around the edge of the machined region [9], which needs further post-processing. Moreover, in ns laser machining, the material removal rate is inversely proportional to the quality, which limits the scale-up of the process.

Ultra short pulse laser (USPL) machining using picosecond (ps) and femtosecond (fs) lasers are represented as the best tool for the machining [10,11] of exotic materials and have proven to be versatile tools for the ablation of material from macro to nano-scale. As such, machining with USPL sources allows for fast, flexible and accurate [12] control of the surface quality and topography, including the wetting properties of surfaces. The short interaction time of picosecond laser pulses means that any material can be ablated before the material has time to react to the thermal load of the laser [13].

Urbina [14] used a 50 W laser with a pulse duration of 10 ps to study the influence of various laser parameters in the machining of 12% Co-based tungsten carbide (WC). Urbina [14] showed a maximum material removal rate of 1.98 mm³/min. Finger [15] used a 100 W laser at 1–8 ps and showed a material removal rate of 9.5 mm³/min with a surface roughness (Ra) of less than 2 μm in Inconel 718. Finger [15] concluded that the increased material removal rate with ultra-short pulse laser machining is attributed to the residual heating effect at high power and frequency. Eberle [4] of Ewag AG used a 50 W laser that operates at 10 ps to fabricate drill bits made up of polycrystalline diamond and tungsten carbide and demonstrated surfaces with a roughness of less than 0.25 μm Sa and with excellent edge definition. However, it took ~25–30 min to machine all the geometric features of a drilling tool of diameter 0.5–0.8 mm. Hajri [16] used a 50 W, 12 ps laser source to fabricate a ball end nose milling tool of diameter 100 μm at a rate of 15 min per tool and showed tool geometries with acceptable quality. Dolda [17] also used a 50 W laser operating at 10 ps for machining and finishing of PCD cutting inserts and concluded that the time for machining with a laser is slightly better than the conventional grinding process. Recently, Stankevič [18] used an 80 W laser operating at 12 ps to investigate the effect of various laser process parameters on the machining of tungsten carbide, ceramic, metal composites and polycrystalline diamond. Stankevič [18] made a number of interesting findings, including the fact that rough ablation with high pulse energy results in a better material removal rate but lower quality. This trend was reversed with lower pulse energy. Stankevič showed a maximum material removal rate of ~18 μm/scan layer.

As described in the literature, most of the current research on ultrashort pulse laser is based on ps laser machining using lasers with an average power of less than 100 W and at a fluence level close to the material ablation threshold. It is well known that USPL machining can produce excellent quality when machined close to the material ablation threshold; however, high fluence ablation is essential to scale-up the machining process. The use of high power lasers to achieve higher MRR in line with other laser processes is essential to the industrial exploitation of ps laser machining processes. This paper aims to perform a fundamental scientific investigation on high-power picosecond laser ablation of tungsten carbide. Although this paper focuses on the machining of tungsten carbide, the results should be applicable to other materials like ceramics and other ultra-hard materials.