High strength materials (HSM), particularly nickel base Inconel 718 superalloy and 300M low alloy steel, are widely used in critical applications for aerospace industry. Inconel 718 and 300M steel are used respectively for manufacturing of critical parts such as turbine disks and aircraft landing gear. These materials have superior mechanical properties compared to most of the conventional materials. Critical parts are manufactured largely using conventional manufacturing processes such as hard turning. These critical parts require a high life service. However, the machining of these parts has an impact on surface integrity in terms of roughness, residual stresses, microstructure and damages which lead to reduced service life particularly with regard to the fatigue life.
The main goal of this thesis is to study the effect of hard turning of Inconel 718. More precisely, the impact of hard turning on microstructure evolution and on subsurface damage is studied. The secondary objective of this project is to study the effect of cutting parameters on the surface quality of 300M steel and increasing productivity by using finishing parameters. Experimental investigations are based on four (4) main stages:
1. A critical analysis of the literature allowed us to identify the lack of information on which the originality of this project was developed. Previous researches have shown the absence of accurate characterization methods for subsurface damage quantification. Similarly, the literature presents a lack of reliable and accurate empirical models for predicting the evolution of microstructural damage. In addition, literature presented a deficiency regarding responsible mechanisms for carbides cracking and the appearance of softened layer beneath the machined surface in the case of Inconel 718.
For 300M steel, previous researches presented few data regarding the impact of machining on surface integrity and increasing productivity for finishing regime.. Moreover, the majority of the studies were concentrated on similar steels for example AISI 4340 and AISI 52100.
2. A precise characterization method has been developed in order to quantify the impact of machining on the microstructure and distinguish between the polishing and machining defects. This method consists in protecting the surface with a thin layer of pure nickel deposited by electrolysis. It allowed to precisely quantifying the subsurface damage, especially the thickness of the deformed layer (DL) by machining and many types of machining defects. Among the other advantages of the developed method is to avoid the detachment of carbides on the surface, to avoid the deformation of the edge of the sample and to adequately quantify the percentage of cracked carbides (ACC) on the subsurface layers within the limit of the affected layer which is in the order of 30 microns. Advanced characterization techniques have been used such as laser/optical microscopy, scanning electron microscopy (SEM), In-situ nanoindentation and Electron backscatter diffraction (EBSD) techniques. The microstructural characterization of Inconel 718 machined parts with different conditions showed the appearance of a hardened layer under the effect of work hardening and severe plastic deformations comprised between 10 μm and 15 μm below the machined surface. In addition, the appearance of a softened layer between 15 μm and 25 μm is also there depending on the machining conditions.
3. A reliable and cost-effective experiment plan, Roquemore 311B, has been selected to plan the machining tests. Thus, DL and ACC were quantified for each cutting condition. The results show that in the selected finishing domain, values of DL are comprised between 6 μm and 15.5 μm and values of ACC varied between 8.3% and 20.5%. The analysis of the results using the analysis of variance ANOVA allowed the determination of the impact of each parameter on the subsurface damage. In addition, the results showed that the cutting speed (Vc) and the feed rate (f) are the most significant parameters. In order to simplify the correlations between the cutting parameters and the subsurface damage, Vc and f have been transformed into quantities such as the deformation energy (E) and material removal rate (MRR). Thus, direct and strong correlations were found between the cutting parameters and microstructure damage. The best fit used to model the correlation is a square root function. The results showed that the thicknesses of DL and ACC increase with increasing MRR and E. In addition, high values of f and low values of Vc lead to high DL and ACC.
The accuracy of the developed models was validated using machining conditions other than those planned in the experimental design especially in the case of Inconel 718. The proposed models for predicting DL and ACC for Inconel 718 have maximum errors equal to 12% and 28% for the center and extremity of the domain study respectively at 95% of confidence interval.
4. The study investigated two main phenomena which are the appearance of cracked carbides independently of the relative distance to the machined surface and the appearance of a softened layer just after the hardened layer. In order to further analyze the responsible mechanisms for (Nb, Ti) C carbide cracking, EBSD study was conducted using Schmid factor distribution maps and Kernel Average Misorientation maps (KAM). The results showed that the preferential locations of the crack initiation in carbides were located at the interface of grains that present a gradient of Schmid factors. In addition, it can be observed that the carbides located at the grain boundaries are those which are the most easily cracked than the carbides located in the grains. In the same context, the results of the KAM maps confirmed that the initiation of cracks in (Nb, Ti) C carbides started where there was a concentration of dislocation density at grain/carbide boundaries. Also, optical microscopy characterization results showed that the carbides started to crack from a critical size equal to 3 μm. Regarding the appearance of the softened layer, the results showed that the cracked carbides were located between 10 and 25 microns of the machined surface which coincides with the depth of the softened layer. The appearance of this layer has been associated with the shearing of the hard particles in the sublayers.
Regarding 300M steel, experimental results have shown that the selected machining parameters do not have impact on microstructure evolution and subsurface damage in terms of hardness, defects and affected layer by machining. The characterization of the machined samples confirms that the layer affected by machining has a maximum thickness of 1.7 μm for the most severe conditions. In addition, SEM analysis did not reveal the presence of a white layer on the surface. This result remains beneficial for our industrial partner Héroux Devtek, because it allows it to increase their productivity until 19 % for semi finishing and from 50 until 122 % for finishing without degradation of the surface quality.
Investigations performed in the frame work of this thesis, showed that microstructure evolution during the machining of 300M, did not reveal a significant impact. For this reason, it is recommended to increase the range of machining parameters for 300M steel in order to deeply study the effect of cutting parameters on the evolution of the microstructure. Thus, we can correlate the effect of microstructural damage on the fatigue life of the machined parts.
Regarding the Inconel 718, it is recommended to correlate the percentage of cracked carbides with the fatigue life of the machined parts. It should be noted that an investigation was carried out into the effect of cracked carbides on the fatigue life of Inconel 718. The first step was to test the effect cracked carbides on fatigue life. Results showed that the proposed model was successfully applied to a three fatigue samples for which the machining conditions and fatigue life were not provided. In the second step, the model was tested on different machined diameters (0.5 and 2 inches) in Inconel 718 and results showed that when the percentage of cracked carbide increases, the fatigue life decreases significantly.
| Date | 24 Apr 2018 |
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| Original language | French |
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| Awarding Institution | - École de technologie supérieure
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| Supervisor | Mohammad Jahazi (Supervisor) & Philippe Bocher (Co-supervisor) |
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Touazine, H. (Author),
Jahazi (Supervisor) &
Bocher (Co-supervisor),
24 Apr 2018Student thesis: Doctoral thesis › Doctorate in Engineering: Engineering