A laser assisted hybrid process chain for high removal rate machining of sintered silicon nitride
Introduction
In recent years, silicon nitride ceramics (Si3N4) have found increasing use in several industries, ranging from aerospace to automotive, thanks to the unique properties of Si3N4: high temperature wear resistance, chemical inertness and high strength-to-weight ratio [1].
Unfortunately, the preservation of high hardness at elevated temperatures makes Si3N4 very difficult to machine and, for this reason, silicon nitride components are usually sintered to near net shape and successively ground to achieve the required dimensional accuracy. This last process, performed by diamond grinding, makes machining very expensive and time consuming.
In order to increase productivity and reduce cost, hybrid manufacturing that combines chip removal machining and thermal treatment has been proposed [2], [3]. Usually, an external thermal source such as a laser provides the energy/power to soften the ceramic, making the subsequent material removal process easier as it is completed while the material is still at high temperature [4]. Modification of the mechanical and thermal properties of Si3N4 with temperature, as well as the type of laser source and the interaction between material and laser, play a fundamental role in the laser assisted hybrid process.
Several investigations, carried out using different laser sources, propose models to predict the absorbed energy, thermal field and constitutive equation to describe the mechanical behaviour of Si3N4 cut at high temperature [5], [6]. Laser irradiation is done by using CO2, solid state fibre, Nd:YAG, and diode lasers [7], [8]. Although these studies provide very interesting data and results on the softening induced by means of laser heating, none of them address crack propagation with the exception of Ref. [5] that focuses on micromachining.
The present paper extends the results presented in Ref. [5] for Si3N4 micromachining to the fabrication of macro components. The proposed process is based on an initial phase in which surface layer cracks are generated by means of thermal shock induced by exposure to a high intensity continuous wave diode laser, followed by removal of the cracked layer by diamond grinding. The process yields a final surface that provides the required accuracy and surface quality.
In contrast to Ref. [5], this paper presents a laser-material interaction model developed in order to correlate the laser parameters including power, intensity distribution, wavelength and scanning speed, to the thermo-mechanical properties of the target material. This model permits the prediction of crack depth due to laser exposure and estimation of the number of grinding passes per laser pass to remove macro quantities of Si3N4.
Section snippets
Process model
Sintered Reaction Bonded Silicon Nitride (SRBSN) ceramics have properties that depend on the sintering process, the quality of the powder and the additives employed during sintering.
Due to the presence of oxide additives to promote densification at the sintering temperature, SRBSN has an intergranular glassy phase that surrounds the β-Si3N4 grains. The glassy phase plays a fundamental role in conventional laser assisted machining since its yield strength drops rapidly at temperatures greater
Laser setup
A continuous wave IR diode laser (NUVONYX ISL-1000 M) with a maximum power of 1000 W and wavelength of 808 nm was used. The complete experimental campaign is summarized in Table 1. The laser head was mounted on two linear stages that enabled the positioning of the laser and variation of the spot dimensions (Fig. 1). Two other stages were mounted in front of the laser head (perpendicular to one another) to enable the positioning of the laser on the plate surface and movement of the sample during
Numerical modelling results
The simulation model, developed in COMSOL according to the theoretical models described in Section 2, considers all laser parameters involved in the process: power, intensity distribution, wavelength, physical and optical properties of the target material, scanning speed and overlap. In particular, the model is able to predict the temperature and stress evolution in the Si3N4 workpiece during laser treatment.
The severity of the thermal cycle was estimated for the laser parameters adopted in the
Conclusion
The paper presents a new hybrid process for silicon nitride component fabrication. Beginning with sintered components, the process combines a laser source, which generates surface cracks due to thermoelastic stresses that exceed the fracture strength of the ceramic and subsequent material removal by grinding. Provided that material removal does not exceed the crack depth, a significant force reduction can be obtained.
The reduced cutting force leads to an increase in tool life and/or an increase
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