Experimental and Simulative Analysis of the Machining of Functionally Graded Workpieces (CRC/TR 30 – SP A3)

Production processes of high quality mass-produced articles often contain several successive manufacturing steps. Such manufacturing processes are both costly and time consuming. The research done in the Transregional Collaborative Research Centre SFB/TR TRR 30 entitled “Process-integrated manufacturing of functionally graded structures on the basis of thermo-mechanically coupled phenomena” focuses on shortening the process chain of such products by incorporating the forming and a functional grading of workpieces in one manufacturing step and at the same time reaching high flexibility in shaping the product. The coupling of thermal and mechanical effects on steel, aluminium and plastic workpieces while forming allows creating different material properties in different regions of the workpiece.

The AISI/SAE 6150 steel workpiece prototype for example features different hardness values in different regions (Fig. 1). Therefore a strategy for a combined machining of hard and soft workpiece zones has to be developed for this inhomogeneous workpiece. Due to the fact that a tool change is undesirable while machining workpiece zones featuring property gradients, these zones have to be machined with one tool. Unfortunately an ideal cutting tool material for machining of both hard and soft materials does not exist. For soft zones the low cost, high toughness cemented carbide is recommended. On the other hand more expensive very hard cutting materials like ceramics or cubic boron nitrides (CBN) are more suitable for hard machining. Another challenge next to the choice of the tool is the choice of suitable machining parameter values because the high feeds, cutting speeds and depths of cut used in machining of unhardened steel cannot be used for hard machining due to the high thermal and mechanical load affecting the tool.

Fig. 1: Process steps for manufacturing functional graded materials [1] and measured Vickers hardness numbers

The design of experiments, modelling and optimization is carried out in cooperation with SFB/TR TRR 30 project D5, also situated at the ISF. To analyse the effect of the process parameters on the machining and to identify optimal parameters with a reduced number of experiments, DACE-models are used for the modelling [2]. The statistical design of experiments and the modeling allows determining the factors of influence, their interactions and their effect on the target variable. The advantage of DACE-models is, that besides a prediction of values the uncertainty of the prediction can be calculated. This allows a refining of the model by sequentially adding observations at sites afflicted with high uncertainty. At first the performance of different cemented carbide and mixed ceramic tools when turning the inhomogeneous steel workpiece has been tested, indicating that mixed ceramic tools perform better and allow for higher parameter values [3,4] (Fig. 2).

Fig. 2: Experimental set up for the turning experiments

On the basis of these results six different cutting tools have been compared. One cemented carbide, two ceramics and three types of CBN. The CBN cutting tools show the lowest wear but different kinds of wear were observed for different CBN grades [5]. For example one CBN grade shows cutting edge chipping when machining the soft workpiece zones due to adhesive wear caused by work hardening of the unhardened workpiece material. The best performing CBN is currently being tested to find one parameter value set being optimal for machining a hardened workpiece and one set being optimal for machining an unhardened workpiece. The two parameter sets will be combined to an adapted process strategy taking into account the local hardness of the workpiece.

In addition, based on experimental investigations a predictive finite element model is developed for the machining of the steel workpiece [6]. The aim is to compute the resultant forces as well as to predict the material behavior (especially residual stresses) resulting from the machining process. Therefore a material model is developed, incorporating the functional gradation, i. e. the different hardness values of the workpiece. Furthermore, the optimal parameters to simulate these turning and drilling processes are defined by using the 3D-FEA-software package DEFORMTM. To carry out 2D-FE-simulations, the FE-software ANSYS is used. In this regard, friction laws and mechanisms of wear attract a special interest. The computed machining forces could be approximated to 80 percent of the measured values by fitting the temperature and strain rate dependent material parameters for all experimentally tested cutting parameter values. First simulations concerning the indexable inserts cutting edge radius show a distinct increase of the passive force and the feed force compared to a simulation using an ideally sharp cutting edge in which the forces only represent five percent of the real values. The chip formation is examined in these simulations as well. Figure 3 shows a real chip and a simulated chip for one parameter value set taking into account the measured cutting edge radius.

Fig. 3: Real chip and simulated chip using insert with cutting edge radius. Machining parameter values: Cutting speed vc = 150 m/min, feed f = 0.15 mm, depth of cut ap = 0.1 mm

References

[1]
Weidig, U.; Bergmann, K.; Scholtes, B.; Steinhoff, K. “Functionally Graded Properties by Controlled Thermo-Mechanical Interaction in Metal Forming Processes”; In: Proceedings 8th International Conference on Technology of Plasticity ICTP 2005, S. 397-398; Verona 2005.
[2]
Sacks, J., Welch, W. J., Mitchell, T. J., Wynn, H. P., Design and Analysis of Computer Experiments, Statistical Science 4 (1989), S. 409-435
[3]
Biermann, D.; Zabel, A.; Grünert, S.: Drehen funktional gradierter Werkstücke mit Hartmetall. VDI-Z Integrierte Produktion, 149 (2007) I; S. 26-28
[4]
Biermann, D.; Zabel, A.; Grünert, S.: Machining of functional graded workpieces by turning. In: Proceedings of the 2nd Manufacturing Engineering Society International Conference, CISIF-MESIC 2007, 9.-11. Juli 2007, Madrid, Spanien, digital veröffentlicht auf CD (ISBN 978-84-611-8001-1), 8 Seiten
[5]
Biermann, D.; Zabel, A.; Grünert, S. Sieben, B. Wagner, T.: Machining of Inhomogeneous Metallic Workpieces, In: Proceedings of the 2nd International Conference “Innovative Cutting Processes and Smart Machining”, Intercut 2008, 22.-23. Oktober 2008, Cluny, Frankreich, digital veröffentlicht auf CD, 10 Seiten
[6]
Biermann D., Zabel A., Höhne F., Sieben B.: Experimental and Simulative Investigation of the Machining of Functionally Graded Materials. In K. Steinhoff, editor, Functionally Graded Materials in Mass Production, pages 75-90. Verlag Wissenschaftliche Scripten, Auerbach, Germany, 2009

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