FEM MODEL AND EXPERIMENTAL PRODUCTION OF TITANIUM RODS USING CONFORM MACHINE. Josef HODEK, Tomáš KUBINA, Jaromír DLOUHÝ

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1 FEM MODEL AND EXPERIMENTAL PRODUCTION OF TITANIUM RODS USING CONFORM MACHINE Josef HODEK, Tomáš KUBINA, Jaromír DLOUHÝ COMTES FHT a.s., Prumyslova 995, Dobrany, Czech Republic, Abstract Continuous extrusion of metals using Conform machine is a well-known process which has been used in industry for several decades as a technique for extrusion of soft metals (Al, Cu) and their alloys. New developments in continuous extrusion of metals include its use for high-strength materials and a utilization of its beneficial side effect, which is the microstructure refinement. This work describes a DEFORM-3D FEM model and experimental continuous processing of titanium rods. The model traces temperature and strain fields which are then compared to experimental data obtained from continuous extrusion of titanium. The temperature of the wheel, the temperature of the input titanium feedstock and the circumferential speed of the wheel have substantial impact on the entire forming process. Key words: finite element method, continuous extrusion, titanium, SPD, Conform, Deform 1. INTRODUCTION It has been found that forming in Conform equipment increases the strength of and refines grain in titanium feedstock. Materials processed in this fashion become ever more common in implantology thanks to their enhanced mechanical properties and equal biocompatibility. The side effect of the processing, i.e. grain refinement, is reported by some sources [1], [2] to improve the biocompatibility of the material even further. Continuous extrusion of titanium using the Conform process poses new requirements for the entire technology in contrast to the processing of standard soft materials (aluminium and copper alloys). The die materials are required to sustain multiple times higher mechanical stress at higher temperature. The standard Conform 315i equipment by BWE [3], which is used for the experimental production, comprises a die chamber with a material reservoir pocket. The design was modified to reduce the size of the reservoir pocket, this making the process closer to the ECAP procedure. Forming takes place at a reduced temperature, at which the titanium feedstock does not recrystallize [4]. Titanium feedstock TiGr2 is driven into the wheel groove and is carried by the wheel for several seconds. Once it reaches the die chamber and hits the abutment, it undergoes shear deformation. Intensive heat transfer takes place between the titanium workpiece and the wheel (see Fig. 1). The present paper describes the use of DEFORM 3D software [5] for FEM modelling of the Conform process in experimental production of fine-grained titanium. Fig. 1 CONFORM process description

2 2. Simulation Model The model of the process is shown in Fig. 2. Thanks to its plane symmetry, only one half of the equipment and feedstock were could be used for computation. The resulting strain-temperature field variation with time was analysed mathematically. At each iteration step, strain was computed, which was corrected with respect to temperature, until both strain and temperature fields changed. In the deformation zone, titanium was considered to be plastic, whereas the other parts of the model were deemed rigid. The friction between the wheel and titanium plays a significant role in the mathematical model. Shear and Coulomb types of friction were tested in the model. The best results were achieved using Coulomb friction with and a Fig. 2 FEM model of the Conform process constant value with the compression separation criterion. The temperature fields have only been examined within the titanium feedstock. The temperature of the wheel in the model was set to a constant value C. The heat transfer coefficient between the titanium feedstock and the wheel was. The impact of the environment was neglected due to the rate of temperature changes. About elements were created within the titanium feedstock. The chamber die and its surroundings were fine meshed. For more information of the FEM model see [6]. Temperaturedependent materials characteristics of TiGr2 were obtained by calculation in JMatPro on the basis of the material s chemical composition [7]. Fig. 3 Effective strain field within the workpiece Fig. 4 Temperature field within the workpiece Fig. 5 Effective strain rate field within the workpiece Fig. 6 Total velocity field within the workpiece The strain distribution within the titanium feedstock exiting the chamber die is uniform throughout, with equal values both on the section plane and on the surface (Fig. 3). The temperature field (Fig. 4) shows that heat is generated during forming. The strain rate distribution (Fig. 5) clearly shows that shear deformation of titanium

3 takes place in the chamber die. The velocity field (Fig. 6) reveals that the velocity changes in the chamber die. It decreases locally due to an increase in the cross section area. An important outcome of FEM simulation is the dependence of the highest temperature within the formed piece during forming on the feedstock input temperature and on the speed of the wheel. By this simulation, forming parameters were found, at which the temperature in the die chamber does not exceed 450 C. (see [4]). 3. Experimental Production The die chamber of the Conform 315i equipment by the BWE company (see Fig. 7) includes a reservoir pocket for workpiece material. Design modification (see Fig. 8) was used to reduce its capacity. The modification made the velocity profile more uniform and increased the proportion of shear strain within the die chamber. Fig. 7 Original cross section of the die chamber with the pocket Fig. 8 Adapted die chamber Experimental production is started after the forming process starts up and the temperatures of the wheel and the chamber die become stable. The forming parameters (input feedstock material, rotation speed) are set in accordance with results of FEM simulation for the maximum permitted forming temperature. In this fashion, the titanium feedstock can be processed repeatedly. Fig. 9 shows the process of feeding the stock into the equipment through the pre-heating inductor coil. At the back, a temperature measuring device and a camera can be seen. Fig. 10 shows the exit from the die chamber with thermocouples for measuring the die chamber and abutment temperatures.

4 Fig. 9 Entry of titanium feedstock Fig. 10 Exit of titanium feedstock 4. Experimental Results =200 µm; Map1; Step=0.75 µm; Grid600x500 =5 µm; Map1; Step=0.04 µm; Grid400x400 Fig. 11 Initial state average grain size 28μm Fig. 12 The third pass average grain size 0,75μm The first pass through the die chamber substantially changes the feedstock microstructure (see Fig. 11). Upon the second and third passes, the proportion of coarse distorted grains decreases in favour of the finegrained matrix. Grain sizes of the fine titanium matrix upon the first, second and third pass are approximately 2.5 μm, 1 μm and 0.75 μm, respectively (see Fig. 12).

5 Fig. 13 Stress-strain plot for TiGr2 material (initial condition and third pass) The strength of the titanium product increases with the number of passes: from the as-received material strength of 470 MPa to 550 MPa upon the first pass,.660 MPa upon the second and 715 MPa after third pass. Elongation decreases from the value of 28% in the as-received material to 15%. 5. Conclusion The aim of this investigation was to describe experimental production of fine-grained titanium denoted as TiGr2 and the FEM simulation of the forming process. The FEM model was constructed using DEFORM 3D software and validated against experimental data. The FEM model allows the conditions within the die chamber to be predicted. With this information, appropriate forming parameters may be identified (preheating temperature and wheel speed), at which the forming temperature remains below the recrystallization temperature. Experiments were conducted in standard Conform 315i machine manufactured by the company BWE. However, the machine s die chamber was adapted in order to increase the proportion of shear deformation during forming. Selected forming parameters, namely input temperature and die chamber and abutment temperatures, were monitored. Titanium feedstock was processed using three passes. The average grain size in the material decreased from the initial 28 um to 0.75 um, whereas the strength rose from 470 MPa to 714 MPa. Additional passes were not evaluated, as cracks developed in the feedstock surface. Their size and number increased with every additional pass. Future development will be oriented towards two efforts. The first will involve further optimisation of the FEM model, namely the relationship between the strain magnitude, number of passes and grain size, whereas the second will focus on optimisation of the die chamber shape in order to suppress formation of surface cracks. ACKNOWLEDGEMENTS This work was supported by the grant MSM/ED2.1.00/ REFERENCES [1] VALIEV R. Nanostructured Titanium for Biomedical Applications, Advanced Engineering Materials 2008,10, No. 8, Weinheim [2] PETRUŽELKA, et al, Nanostrukturní titan nový materiál pro dentální implantáty, Česká Stomatologie, Vol. 106, 2006, No. 3, pp In Czech

6 [3] Website presentation, On-line: [4] KUBINA T., Preparation and Thermal Stability of Ultra Fine Grainned Commercially Pure Titanium Wire, Comat 2012, 2 nd International Conference, Pilsen [5] Website presentation, On-line: [6] HODEK J., FEM Model of Continuous Extrusion of Titanium in Deform Sofrware, Comat 2012, 2 nd International Conference, Pilsen [7] Website presentation, On-line: