Machinability and Chip Morphology Study of Inconel 740H and Haynes 282 for Advanced-Ultra Supercritical Applications

Abstract

High speed machining of Nickel based alloys has gained a lot of research interest since the last decade owing to their applications in areas like power generation, aerospace, marine propulsion, and nuclear reactors. For the realization of an advanced ultra-supercritical (A-USC) thermal plant, operating at service temperatures of about 700 to 760oC and pressures of around 24 MPa, the use of Nickel-based super alloys is indispensable. Some these parts used for the application may be required to be machined before use, although the machinability of these materials is relatively poor. Chip morphological studies are performed for better understanding of cutting force and surface finish of the alloys. In this work, authors have tried to study the effect of machining parameters like cutting speed, feed rate, rake angle, and nose radius on chip morphology like peak height, valley height, tooth height, tooth pitch, and segmentation frequency. The results of chip morphology studies indicate the need for saw tooth chips for better machining. Accordingly, Inconel 740H is found to have better machinability than Haynes 282 for the used range of machining parameters. Moreover, higher cutting speeds, lower feed rates, and positive rake angles are found to yield optimum conditions for an improved machinability.

Introduction

I. INTRODUCTION

Increasing worldwide restrictions on reducing green-house emissions such as NOx, SOx and CO2, demand an improved thermal efficiency of all coal-fired power plants. The focus is on developing new power plants operating on A-USC technology and renovation of all existing thermal plants incorporating this technology. The key ingredient of this technology is improved boiler efficiency [1]. The range of boiler operating temperature is about 700 to 760oC and pressures of about 24 MPa. High temperature creep strength and high coal ash corrosion resistance are the key requirements for the boiler material. The alloys with high percentage of nickel and chromium have these characteristics at high temperatures and extreme operating pressures [2]. According to Robert et al. [1], the Nickel based super alloys like Inconel 740H and Haynes 282 are equally qualified for the A-USC applications. High Speed Machining of these materials is inevitable for use in such applications since it is the only solution for enabling material removal in the ductile regime. However, these alloys are categorized as ‘hard-to-machine’ materials, owing to their low thermal conductivity and the heat generated due to friction that leads to a progressive hardening [3,4]. Therefore, a proper identification of machining conditions is of top priority for the ease and successful machining of these alloys. Stenberg et al. [5] extensively used numerical tools to model and analyze the machining operations to find the optimal process parameters and machining-induced residual stresses. The cost incurred in doing experiments can be extensively reduced using numerical tools. Moreover, the detailed responses such as deformation, chip removal, stress and temperature distribution, and chip morphology can be predicted in detail only by numerical tools [6].

Table 1 Correlation of Orthogonal machining with turning [6]

Investigation on the effect of process parameters on the machining process is one of the major outcomes of this research work. Full factorial design is used to obtain all possible combinations of process parameters, and Analysis of Variance (ANOVA) is used to analyze the effect of parameters on the cutting responses.

The prime objective of this work is to develop a detailed FE model for high speed machining of Inconel 740H and Haynes 282 alloys, and identify the optimal values of process parameters that would enable machining in a ductile regime.

II. FINITE ELEMENT MODEL

The model is developed using a commercially available FE package - Abaqus/Explicit. The specimen is modeled as 2D deformable and the cutting tool is modeled as analytically rigid. The machining process is treated as a coupled thermo-mechanical problem, with the domain meshed using CPE4RT elements (4-noded plane strain, thermally coupled quadrilateral element, with bilinear displacement and temperature).The primary degrees of freedom are evaluated at the nodes, and secondary and tertiary degrees of freedom are evaluated at the integration points. The elements use reduced integration scheme to lower the computational expensiveness.

The workpiece model (Figure 2) is partitioned in such a way that, a structured mesh is used in areas where the response is measured and a free mesh is used in the remaining part of the domain. This, again, helps in reducing the computational cost.

The tool and workpiece are initially maintained at a room temperature of 200C. Appropriate boundary conditions are applied on the workpiece and the tool, with the bottom and left edges of the workpiece being fixed with respect to all degrees of freedom, and the cutting tool assigned with an appropriate translation velocity derived from the concepts of orthogonal machining. The temperature distribution arising from the cutting operation is computed during the simulation. In order to effectively capture the machining responses at the primary shear zone, a minimum of 10 elements are used along the depth of cut with a good aspect ratio.

A surface to surface interaction is defined between the tool and the workpiece. As per contact criteria [13-14], tool is defined as the master and workpiece is defined as the slave. The contact algorithm used here is, penalty contact with a coulomb friction model.

III.  RESULTS AND DISCUSSIONS

In the presented work, a detailed finite element model is developed for high speed machining of advanced-ultra super critical alloys (Inconel 740H and Haynes 282). Initially, the model is validated for the titanium alloy (Ti-6Al-4V) by comparing the obtained results with the numerical results available in literature [6]. The validated FE model is then extended for the case of A-USC alloys.

A. Analysis of Cutting Force and Temperature Distribution

1. Results for Titanium Alloy (Ti-6Al-4V)

The model results indicate that the maximum value of effective stress will be obtained at the primary shear zone (tool-workpiece interface) as shown in figures 3. The type of chip formed is discontinuous in nature for the cutting conditions mentioned. The results indicate a peak value of Von-Mises stress of around 1500 MPa instantaneously developing at the primary shear zone.

The contour plots for Von-Mises stress distribution in Haynes 282 are shown in figures 5. The trend in the effective stress distribution is found to be similar as in the case of Ti-6Al-4V. However, a reduction of about 22% in the peak value is observed in Haynes 282 compared to the titanium alloy. It has to be understood that, a much higher value of cutting speed is used for the case of Haynes 282.

The trend in the temperature distribution is also found to be similar as in the case of the titanium alloy. Figure 6 shows the temperature distribution for varying process parameters. The peak value of temperature at the secondary shear zone is found to vary in the range of 600oC to 640oC.

Cutting speed is found to have a direct influence on the heat generated at the tool-chip interface. Temperature at the secondary shear zone is found to increase with the increase in cutting speed. This may be due to the reduction in the percentage of heat conducted to the work piece as a result of insufficient time for heat transfer. This, hence, reduces the chance for material hardening.

Contrary to the type of chip formation in Haynes 282, serrated type chips are formed in Inconel 740H. This clearly indicates the ease of machining of Inconel 740H compared to Haynes 282. A detailed chip morphological study is also undertaken for further understanding on the machinability of the alloys (discussed in section 5).

B. Comparison of Results – Inconel 740H Vs Haynes 282

1. Effect of Machining Parameters on Cutting Force

The influence of machining parameters on the cutting force developed is investigated. The numerical results indicate feed rate to have the largest influence on increasing the cutting force compared to the remaining parameters. This can be attributed to a larger volume of the workpiece material getting engaged with the tool in unit time as the feed rate is increased. Cutting speed is found to have no significant influence on cutting force developed in either of the alloys. This may be due to the fact that since the cutting operation is already performed in the high speed range, the initial inertial resistance offered by the workpiece material is already taken care of. An increase in nose radius, on the other hand, is found to increase the cutting force in either of the alloys. This points to the larger resistance offered by the workpiece against a relatively blunt tool. Positive rake angles are found to increase the ease of machining of the alloys. This is due to a reduction in the ‘Ploughing effect’ at the primary shear zone on increasing the positive rake angle. As Inconel 740H and Haynes 282 alloys are equally qualified for the A-USC applications, the next task is to determine the comparative machinability of each material. It is observed that, for a cutting speed of 90 m/min (950 rpm) with all other cutting parameters kept constant (Figure 9), Haynes 282 alloy required around 62% higher cutting force than Inconel 740H. However, increasing the cutting speed to a range of 180 - 250 m/min (1900 -2650 RPM) is found to further increase the cutting force by about 65 - 70 % in Haynes 282 compared to Inconel 740H.  

On the other hand, feed rate is found to have a significant influence on the cutting force developed in either of the alloys. For a feed rate ranging from 0.05 – 0.15 mm/rev with all other parameters kept constant, the cutting force required for machining Haynes 282 is found to increase by a range of 60 - 75 % compared to Inconel 740H (Figure 10).

Nose radius and rake angle are found to have a similar effect on the increasing cutting force requirement of Haynes 282 compared to Inconel 740H. When the nose radius is varied from 0.01 to 0.025 mm, a corresponding increase of about 69 to 75% in cutting force is observed in Haynes 282 compared to Inconel 740H. When the rake angle is varied from +8o to -5o, the cutting force is found increase from 69 % to 78 % more in Haynes 282 in comparison with the other alloy.

From the above table, it is evident that the percentage of deviation in results between the (n+1)th and the nth levels of refinement is less than 5%. This indicates that the results have become independent of the element size, and hence, the nth level of mesh refinement is indeed qualified for running simulations.

D. Experimental Validation (secondary) of Numerical Results for Haynes 282

The numerical results obtained for the case of Haynes 282 are correlated with the experimental results from Marcos Rodriguez-Millan et al. [16], corresponding to a cutting speed of 180 m/min and a depth of cut of 2 mm. Table 5 demonstrates the validation for two different cases of feed rates. The results indicate a percentage of deviation of less than 5, which further emphasizes the accuracy of the model.

TABLE 5

Secondary validation of the FE model for Haynes 282

IV.  RESULTS FOR CHIP MORPHOLOGY

Chip morphology gives an additional accurate indication on the machinability of materials. High speed machining of hard-to-machine materials like Nickel based A-USC alloys, results in the formation of serrated chips, provided the machining is performed in the ductile regime. Figures 11 to 14 (Section 3.1.3) indicate the formation of serrated chips for the case of Inconel 740H for the given range of cutting parameters. However, for the range of parameters shown in the corresponding Figures 7 to 10 (Section 4.1.2) for Haynes 282 do not correspond to such a chip formation.    

Figure 22 displays the various parameters that define chip morphology, viz., tooth pitch (Pc), peak height (tP), valley height (tV), tooth angle (localized shear (Øseg) and bulge (ρseg)), and cut chip length (Lc) (used to evaluate segmentation frequency).

wherein, V is the chip speed and Pc is the tooth pitch. Chip speed is evaluated using cutting ratio; r = Lc/L and cutting speed Vc.

Figure 23 shows the variation of peak height with respect to cutting speed for Inconel 740H and Haynes 282. The results obtained indicate that, peak height slightly reduces with the increase in cutting speed, which is desirable. As the peak height reduces, the chip serration mechanism remains nominal. Therefore, the machinability can be improved by increasing the cutting speed.

V.  ACKNOWLEDGEMENT

Authors extend a very profound gratitude to the Chair of the Department of Mechanical Engineering and Dean-Engineering at Amrita School of Engineering, Coimbatore, India, for their wholehearted support and contributions for successfully completing the research work.

VI.  CONFLICT OF INTEREST

Ramakrishnan P A, Ajith Ramesh, and C S Sumesh declare that they have no conflict of interest.

Conclusion

The paper presents a detailed finite element model for simulating high speed machining of A-USC qualified alloys: Inconel 740H and Haynes 282. The developed model is primarily validated for Ti-6Al-4V alloy using the numerical results from available literature. The validated model is then extended for the case of Inconel 740H and Haynes 282. These results are then further validated with the experimental results available from relevant literature for the corresponding alloys. Once the finite element model is successfully developed, optimization studies are performed to understand the influence of the cutting parameters like cutting speed, feed rate, nose radius, and rake angle on the output responses of cutting force, surface finish, and Material Removal Rate (MRR). Main effect plots indicate feed rate to have the most significant influence on cutting force and surface finish when compared to all the remaining parameters. Haynes 282 alloy is found to require about 60 – 80% larger cutting force than Inconel 740H for varying cases of process parameters. Similarly, results also indicate Inconel 740H alloy to have a significantly better surface finish than Haynes 282 in the given range of parameters. For the case of MRR, on the other hand, cutting speed and feed rate are found to have almost equal influence for both the alloys. MRR is found to increase appreciably on increasing the cutting speed and feed rate. However, nose radius is found to have an inverse effect on the MRR. Similarly, positive rake angles are found to increase the MRR of either material. It is interesting to note that, although Inconel 740H is found to have a much lower cutting force and significantly better surface finish, Haynes 282 has a relatively higher material removal rate (6 – 17%). A multi-objective optimization is also performed for Inconel 740H considering minimization of cutting force and maximization of MRR, simultaneously. The corresponding optimal values for the cutting parameters are thereby obtained. Chip morphology studies are also performed in order to get an additional leverage towards the ductile regime machining of the A-USC alloys. The influence of cutting parameters on chip morphology is also investigated. It is found that a serrated form of chips is a direct indication towards better machinability of these alloys in the high speed range. The results indicate a lower peak height, lower tooth pitch, and a higher segmentation frequency to be corresponding to larger cutting speeds and lower feed rates. Such a combination of parameters is found to result in a nominal chip serration and towards a reduction in the chances of chip breakability. On the overall, it may be concluded that for the given range of cutting parameters, Inconel 740H has a relatively better chip serration than Haynes 282. Therefore, considering all the numerical results obtained from the investigation, it may also be concluded that Inconel 740H has a relatively better machinability than Haynes 282 in the considered range of parameters.

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Copyright

Copyright © 2022 Ramakrishnan. P A, Ajith Ramesh, C. S. Sumesh. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.