A process modeling approach for micro drilling of aerospace alloys with a waterjet guided laser system

https://doi.org/10.1016/j.optlastec.2021.107682Get rights and content

Highlights

  • A process input flowchart is presented for a holistic material removal model.

  • Significant process parameters affecting the material removal are determined.

  • The methods are presented to determine the in-process unknown variables.

  • The model can estimate the process time and/or hole depth.

Abstract

Laser drilling is a preferred method to make cooling holes on gas turbine parts. Although it is a fast process, there are some quality issues. Waterjet guided laser (WJGL) is a hybrid process, in which a laser beam is coupled with and guided through thin cylindrical waterjet. Pressurized water in this novel process provides focusing, cooling and cleaning of the machining zone, eliminating undesired side effects of the laser. The process can be conveniently used for micro drilling operations on aerospace jet engine parts. However, more research on the process is required to understand the effects of the variables on material removal rate. The drilling time varies for each material and geometry. Optimum parameters should be adjusted for each case. In this study, the material removal mechanism of WJGL is investigated and a holistic modeling approach is developed including the necessary parameters for drilling of different aerospace materials and hole geometries.

Introduction

Jet engines used in the aerospace industry are efficient machines that produce propulsion and power. In a jet engine, air is sucked in, compressed and then expanded by igniting fuel in a combustion chamber. The high-pressure hot gases rotate the turbine and then exit from the exhaust section. Turbine blades are the first rotating components subject to the heat conveyed from the combustion chamber. The temperature can be as high as 1500 °C [1]. There are also rotational forces on the blades. Thus, the blades should bear high mechanical loads and temperatures within a reasonable life span. In order to endure the harsh conditions, these parts are generally made of nickel-based superalloys. These special materials maintain their mechanical strength even at high temperatures [2]. Another measure generally taken for the design and manufacturing of these parts is to apply cooling technologies. Micro holes on the parts facilitate cooling by letting the air pass through the internal channels. The cooling holes increase the allowable working temperature, as well as the service life of the parts [3]. The optimum hole diameters are less than a millimeter; generally about 0.3–0.5 mm [4]. Since the holes are small and the wall thickness is considerably large, the best method to drill these holes is to use non-traditional manufacturing processes [5]. Electrochemical machining (ECM), electrical discharge machining (EDM) and laser beam machining (LBM) are the most common methods used in the industry [6]. Process speed and quality are the most important factors. ECM and EDM methods are more costly and slower compared to the LBM based approaches. On the other hand, lasers have quality issues associated with high heat input on the workpiece, such as spatter, taper, dross formation, higher surface roughness, recast layer, heat affected zone (HAZ), etc. [7], [8].

Many of these adverse effects can be overcome by the waterjet guided laser (WJGL) technology [9], [10]. In this hybrid process, the laser beam is guided through a thin cylindrical waterjet. The pressurized water provides focusing, cooling and cleaning on the machining region, eliminating the undesired side effects of the conventional dry LBM. This technology is used for various applications in different industries, including micro drilling operations on aero engine parts. However, WJGL drilling studies are highly limited and mostly focus on the quality aspects. In one of these studies, Rashed et al. [11] drilled 180 µm diameter holes with a spiral toolpath on stainless steel fuel injector nozzles using a WJGL system. Surface roughness results were compared with EDM drilling. It was concluded that the WJGL has repeatable results and the surface roughness is three times less. Wang [12] investigated WJGL diffuser shape hole drilling on thermal barrier coated aero engine superalloys. Laser energy density was empirically modeled for given laser parameters and then material removal at different machining conditions were studied. The optimum parameters were found for faster process time and better quality, in terms of delamination and recast layer. Gurav et al. [13] drilled less than 1 mm diameter holes on CMSX-4, an aerospace nickel-based superalloy, using different machines. Many experiments were conducted in order to compare the results between WJGL drilling, conventional dry LBM drilling and EDM drilling. Dimensional, surface quality and metallurgical inspections were performed on the holes. It was observed that the WJGL quality is better. Subasi et al. [14] performed a multi objective optimization in terms of process time and quality for the 400 µm diameter holes drilled on Inconel 718 material using WJGL. Some of the significant factors of the process were identified and it was shown that it is possible to obtain better quality and shorter process time by adjusting the input variables.

The material removal mechanism was not properly addressed for the aforementioned studies. Nevertheless, there are some modeling studies dedicated to WJGL grooving and cutting, in which predictions are made for depth of material removal. Li et al. [15] presented a model for WJGL grooving of silicon, in which laser energy, waterjet cooling effect and melting of silicon were taken into account. The model was validated by comparing simulations with the experimental results. The shape and the width of the groove cross-section was successfully predicted for specific cutting speeds. It was also found that the maximum depth is reached with the first pulses of the laser and the depth of the groove is not steady especially for higher speeds. Yang et al. [16] came up with a numerical model estimating the temperature field on the cutting area for WJGL micromachining. It was stated that the temperature field of WJGL is less than the traditional dry LBM. Both conduction and convection heat transfer were considered and removed material volume was calculated. Validation experiments were performed on steel and silicon samples. It was concluded that the developed model is satisfied, but parameter optimization is needed for different materials. Adelmann et al. [17] conducted an experimental study on WJGL cutting, showing the effect of using different process parameters on the cutting depth for different materials. It was found that the cutting depth increases with higher laser power and lower frequency. It was also observed that there is a dimensional depth limit for the WJGL cutting and the material removal rate (MRR) decreases as it gets deeper into the slit, independent of the workpiece material. Diboine et al. [18] performed WJGL pocketing experiments on aerospace nickel alloys. A semi-empirical model was developed using energy balance approximation and 1D thermal diffusion. Prediction of depth per layer was done and validated by further experiments. Accumulation effect of water was reported to adversely affect the material removal as the toolpath gets closer to the walls.

Subasi et al. [19] correlated material properties to process time and quality for WJGL micro drilling with a spiral toolpath. It was found that material removal with only evaporation assumption is a good approach and it is possible to predict the drilling time provided that the machining efficiency and material properties are known. It was also stressed that the efficiency of the drilling depends on the laser energy level and hole dimensions, but it doesn’t depend on material properties within the same family of alloys.

Although its success is demonstrated many times compared to the traditional dry LBM in terms of quality, the WJGL technology is still not widely elaborated, as there is only limited amount of research on the subject so far. A holistic material removal model including the effect of process parameters, material properties and hole geometry for the process is needed. There are some difficulties in establishing a valid process model for WJGL micro hole drilling. Firstly, the constantly flowing pressurized water splashing back from the machining area is a problem. Reflection of the water might disrupt the waterjet for some cases and generally becomes an issue as the hole gets deeper. Thus, it is not possible to use the common percussion drilling method, where the laser spot remains stationary on the surface. The volume should be removed gradually using a specific (spiral) toolpath, by which the laser spot always moves forward leaving the splash back zone behind. Then, the material removal efficiency is adversely affected by the water cooling effect, since the heat does not build up rapidly on the machining region. It is known that the processing time is longer with a WJGL machine, when compared to a conventional dry LBM system [20], [21], [22]. Furthermore, there are many dependent and independent variables in the process, which affect the MRR and the quality of the cuts. The machining parameters and their correlation to the material and geometrical properties are still not fully understood. The physics of the material removal is highly complicated due to the scattering and reflection losses at the surface, with heat diffusion causing phase change, melting, and/or vaporization. Consequently, there is a potential for investigating the relation between the variables and the input–output interactions of the process.

The main motivation of this study is to look deeper into the process and to come up with a useful material removal model, especially validated for drilling of aerospace materials. Different methodologies including machine learning, empirical (statistical) modeling and mathematical modeling are used, together with data collection and experiments. The result is a simplified model for a super complex process. The authors believe this study will be a reference for future process modeling studies and open up new research prospects for WJGL micromachining as well.

In the following sections, the related variables affecting the outputs of WJGL, the modeling approach and material removal theory are presented. Then, experiments are explained and finally the results are discussed.

Section snippets

Waterjet guided laser technology

In the WJGL system, the laser beam is focused on a waterjet nozzle, so that the laser energy can be carried by the total internal reflection within the pressurized jet. The working principle of the technology is shown in Fig. 1.

Modeling methodology

The aim for parameter modeling is to decrease the trial and error steps while deciding on the inputs for the optimized results. The related process parameters for the process time and quality can be seen in Fig. 6. At the beginning, the workpiece variables are generally known, such as material type, part geometry, hole diameter, etc. The nozzle diameter is almost always chosen as 50 µm, since it is an ideal size by experience for making the required micro hole dimensions. Other process

Laser ablation mechanism for WJGL

Laser ablation is the removal of material from a substrate by direct absorption of laser energy [34]. The main mechanism of material removal is vaporization. WJGL material removal mechanism is similar to the conventional laser machining. The laser beam interacts with matter and is absorbed by the surface. The main difference is water cooling between the pulses. Thus, the material removed by each pulse can be evaluated independently [35].

Considering the ablation process, a water film is formed

Experimental design and setup

The next step is to determine how the MRR, MRP and thus the machining efficiency, are changing with respect to the process parameters given in Fig. 6. Understanding about the process efficiency would help predict the material removal for future cases, as explained in the previous section.

In order to understand the material removal behavior, a blind hole drilling experiment is conducted. For all the trials, the waterjet nozzle diameter is selected to be 50 µm, as usual. Spiral toolpath shown in

Results & discussion

The effects of the factors can be seen in Table 8. MRP/MRPth is the machining efficiency, as described in Section 4. The plots of efficiency for certain factors are provided in Fig. 15.

Considering Table 8 and Fig. 15, there are numerous conclusions that can be made. The waterjet parameters (water pressure and gas flow) change the effective cutting length, but as long as the laser energy reaches the surface, they do not play an important role in material removal.

Since the processed materials are

Conclusions

In this study, the material removal mechanism of WJGL is investigated and a holistic model is developed including the necessary parameters for micro drilling of nickel-based superalloys. Different methodologies including machine learning, empirical (statistical) modeling and mathematical modeling are used, together with data collection and experiments. A simplified model is obtained for the process. The outcome and significance of the study are as follows.

  • Safe working range of the laser

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by The Scientific and Technological Research Council of Turkey (TÜBİTAK) Directorate of Science Fellowships and Grant Programmes (BİDEB) 2211-A [grant number 1649B031804554]. The authors would also like to thank to Synova SA for their technical support, TEI for their permission to use the facilities and colleagues from TEI (Tusas Engine Industries) who made this study possible.

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