THIXOFORGING OF A CuZn-ALLOY. J. Baur. Institute for Metal Forming Technology University of Stuttgart - Germany

Transcription

1 THIXOFORGING OF A CuZn-ALLOY J. Baur Institute for Metal Forming Technology University of Stuttgart - Germany ABSTRACT: The research on thixoforming of aluminum is very general in comparison to the research work done on brass. Within the research work at the Institute of Metal Forming Technology at the University of Stuttgart, the optimal parameters for the forming velocity, the die geometry and the heat treatment were investigated. Furthermore, new possibilities of the forming process were developed. Thus, it is possible to integrate hollow and two opened-ends inserted parts into the workpiece. Under the aspect that the investigated alloy is used for plain bearings and valve guides, torsion test will be done with the forged workpieces. Furthermore, at the institute, a new technique of detecting the liquid phase in the structure without physical contact of the measuring equipment was developed. So, it is now possible to detect the phase quotient during the heating process. The used principle of the detection is the measurement of the effective current in the secondary circuit of the equipment. This current changes in parallel with the structure of the workpiece when the alloy passes the solid temperature. With this change, the process becomes easy to control during the heating of the raw material. KEYWORDS: Thixoforging, brass, inductive heating, inserted parts. 1 INTRODUCTION The thixoforming of aluminum, for example in the area of automotive parts, is state of the art [1]. The thixoforging of steel [2] is at the moment restrained by the high temperatures of the steel and thereby, resulting in die problems [3]. The thixoforming of brass takes place at temperatures higher than the forming of aluminum but below the temperatures of thixoforming of steel. For this reason, the wellknown technologies and parameters (die, press and process) of the aluminum processing can be easily adapted to the forming of brass alloys. The economic demands on the thixoforging process lead to mass production. Concerning the processing of brass, faucets and bearings are preferred workpieces for high production. The thixoforging process should be used since it provides for both faucets and bearings a very good surface quality of the thixoforged workpieces, an absence of pores and inner porosities and the forming possibilities due to the low necessary mechanical stresses. At the Institute for Metal Forming Technology, the thixoforging of brass has been developed since 1994 [4]. Aim is the optimization of the process parameters under the criteria of good mechanical properties and the possibilities of a wide shape variety.

2 2 CuZn-ALLOY The alloy CuZn40Al2 is preferably used for the production of valve guides and plain bearings. The properties of this alloy result from the structure. The good hot-forming characteristic of this alloy is caused by the β-crystals of this alloy. The high content of manganese silicides leads to a high wear resistance. The 2% content of aluminum is as well a benefit since it avoids a tinder layer on the raw piece surface during the heating process. So, it is possible to heat without a protective nitrogen atmosphere and thus making the heating process more simpler to realize. One of the main advantages of thixoforged brass work pieces is the high quality surface that is able to be attained. The material used is a lead-free alloy, and because of this, it is ideal for producing drinking water faucets. 3 DIE Figure 1 shows an experimental work piece, the so-called meander. The diameter of the whole piece is 185mm and of the arms, 10mm. It can be seen on the right photo that it is possible to get four specimens for tensile tests and two specimens for torsion tests. The specimens Z1 and Z4 for the tensile tests have longer flow paths than the specimens Z2 and Z3. So, it is possible to determine the influence of the length of the flow path. In addition, the influence of a material overflow can be determined by comparing Z1 to Z4. The die used for the detection of the optimal forging parameters was made from hot-working steel. The active parts were mounted into a nitrogen closing device. The borehole in the bottom side of the die can also be used for the fixing of the inserted parts which will be formed during the forging process. Fig. 1: Test Workpiece Meander. In figure 2, the filling behavior of the total filling of the meander is shown. It is obvious that at the beginning of the process, the cavities of the specimens for the torsion test will be first filled since the profile of this cavity is larger. After the complete filling of these cavities, the flow velocity in the area of the specimens for the tensile test will go up to 5,000 mm/s due to a punch velocity of 500 mm/s.

3 Fig. 2: Filling Behaviour of the Die. 4 MECHANICAL PROPERTIES Already, the first experiments showed that the mechanical properties in general were better at lower die temperatures and with a punch velocity of about 500 mm/s. At the same time, it was obvious that the thixoforging of brass leads to high residual stresses. Cracks or fissures were consequently the result of a too large cooling rate of the workpiece. The mechanical properties of these parts were unsatisfying the results for the tensile strength were below 350 N/mm². Because of the high inner stresses in the brass structure resulting from thixoforging, it is necessary to execute an annealing process. The annealing duration lasts on average 4 hours at a temperature of 550 C. Also, during the annealing process, the use of a protective nitrogen atmosphere was not necessary. The aluminum content prevented a tinder layer on the surface of the parts even after an annealing time of 6 hours. It is obvious that the values of the tensile test specimens Z2 and Z3 are better because they have a shorter flow path and thereby, less cooling occurs. In addition, a comparison of the specimens Z1 and Z4 demonstrated the necessity of a material overflow. The following figure represents the characteristic values of the material determined by the workpiece meander CuZn40Al2, WZ 300 C, 550mm/s, 550 C/4h RP0,2 Rm A5 Z1 Z2 Z3 Z

4 Fig. 3: Mechanical Properties of the Meander. The mechanical properties of the tensile test specimens Z2 and Z3 were similar to the original material. However, the percent elongation was about 40% less. The original alloy had the following properties: YS = 340 N/mm², UTS = 640 N/mm², A5 = 18%. 5 INDUCTIVE HEATING At the institute, a heating installation is used with an inductive coil having a pulsating square voltage. Therefore, it is necessary to change the AC voltage of the power supply system into a DC voltage and to pulse it through a thyristor. Thereby, it is possible to adjust the frequency and the power of the inductive heating through the use of pulse modulations. In addition, the power induced into the workpiece can be modified using a constant frequency over a modulation of the pulse period. With both these variation parameters, an optimization of the heating procedure is easily reachable. Furthermore, the absence of a resonant circuit reduces cost, weight and space of the heating installation. The control of the heating process is realized in the industrial use through several different methods. The most common method is the measurement of the inductive circuit of properties of the raw piece and the input of the electrical power controller that quantifies the power yielded into the workpiece. At the Institute for Metal Forming Technology, a measuring method was developed in which a registration of the temperature over the effective current in the coil is possible. Since there are no values known out of the literature, it is possible to determine the percentage of liquid phase with the use of reference tests if the temperatures at the end of the heating combined with micrographs of the quenched structure are known. Thereby, the measuring principle is by constant effective voltage, the effective current at the transition which changes from solid to semi-fluid state. With an easy to make current measurement and current analysis, the point of melting as well as the actual percentage of liquid phase could be determined. The advantage of this method lies in the easy and wear-free structure of the control equipment. Furthermore, it is possible to continue the heating process after an interruption, for example an installation failure with the same raw part in the heating. This is possible because the measured effective power depends only on the state of the structure and is reproducible.

5 Fig. 4: Circuit Diagram of the Inductive Heating Equipment. In the following figure, the results of a heating process of a AlSi7Mg workpiece is reproduced in principle, the control mechanism is also possible for other alloys which are used for thixoforming. The effective current decreases relatively slow as long as both phases are solid. The moment one of the phases starts to melt, the effective current will decrease significantly. This obvious change can be used for the control of the heating process. A benefit is that this method can also be used when the part was previously heated or heated then cooled and once again reheated, e.g. as in the case where a fault in the production equipment is present. In the following figure, 4 parts were heated up and the curves were compared. It is quite obvious that the curves of the effective current of all four tests versus the temperature are nearly the same. Fig. 5: Effective Current versus Temperature.

6 6 INSERTED PARTS In [5] and [6], different ways of integrating parts are demonstrated by forming them as solids into the semi-solid workpiece during the forming process. In every case, it is necessary that the inserted part has a higher melting point than the thixoforming basic material. In both methods, the integrated part is either massif or hollow in which an unrestrained intrusion of the semi-solid material takes place. At the Institute for Metal Forming Technology, a method was developed in order to have a reduced intrusion of the semi-solid material. In figure 6, a workpiece is shown in which the female thread was completely filled with salt before the forming process and then the salt was consolidated before the forming process. Therefore, the inserted part was placed in a borehole in the lower die. During the thixoforging process, the semi-solid material condenses the salt in the female thread and partially intrudes in the inserted part. This effect leads to an inner counter pressure which holds the thin walls of the inserted parts against the liquid pressure of the thixoforming material. In this way, it is possible to integrate thin-walled inserted parts in the thixoforging workpiece. During the same forming process, the flange of the inserted part was formed into the semi-solid material so that the parts would be joined by the forms. This joining is of high strength. The salt which was in the inserted part during the forming process can be easily removed by water. For aluminum, it is especially no problem to remove the salt during the quenching process. With this method, it is also possible to vary the inner thread or form of the bore, e.g. with inner toothed gearings or undercuts. punch workpiece inserted part female thread (filled with salt before forming process) die Fig. 6: Inserted Part in a Brass Workpiece. 7 CONCLUSION The thixoforging of the alloy CuZn40Al2, in addition with a heat treatment, leads to workpieces with high strength and restrained elongation values. The possibility of integrating inserted parts in the thixoformed workpieces and of controlling the heating process by the supervision of the effective current are both further steps on the way to the mass production of thixoformed parts. ACKNOWLEDGEMENT The author wants to thank the Karl-Goehring-Foundation in Stuttgart-Moehringen (Germany) and the Wieland-Werke AG in Ulm (Germany), for their support of the IFU's research work.

7 REFERENCES [1] Lagemann, J. et al.: Mass production of thixoforming security components for the automotive industry. In: Proceedings the Symposium "Neuere Entwicklungen in der Massivumformung. Editor: Siegert, K.. Fellbach (Germany), May 19 20, 1999, pp [2] Kapronas, P. et al.: Properties ofthixoformed steel. In: Proceedings of the 3 th International Conference on Semi-Solid Processing of Alloys and Composites. Editor: Kiuchi, M.,. Tokio (Japan), June 13 15, 1996, pp [3] Hirt, G. et al.: Thixoforming of steel. In: Proceedings the Symposium "Neuere Entwicklungen in der Massivumformung. Editor: Siegert, K.. Fellbach (Germany), May 19 20, 1999, pp [4] Thixoforging einer Messing Knetlegierung. Tagungsband Neuere Entwicklungen in der Massivumformung Herausgeber: Siegert, K., S Fellbach, Juni [5] Kiuchi, M. et al.: Mashy-State Joining, a new process for joining materials together. In: Proceedings of the 5 th International Conference on Semi-Solid Processing of Alloys and Composites. Editors: Kumar Bashin, A. et al. Golden (Colorado/USA), June 23 25, 1998, pp [6] Dendo, T. et al.: An attempt for fabrication of clad parts through semi-molten processing. In: Proceedings of the 5 th International Conference on Semi-Solid Processing of Alloys and Composites. Editors: Kumar Bashin, A. et al. Golden (Colorado/USA), June 23 25, 1998, pp

8 AUTHOR Jens BAUR Holzgartenstrasse Stuttgart phone: ++49 (0) fax: ++49 (0) mobil 0170/ baur@ifu.uni-stuttgart.de