Dipl.-Ing. Andreas Wolf; Dipl.-Ing. Jens Baur Institut für Umformtechnik (IFU) der Universität Stuttgart

Wolf, Baur, Gullo Thixoforging 1 Thixoforging Dipl.-Ing. Andreas Wolf; Dipl.-Ing. Jens Baur Institut für Umformtechnik (IFU) der Universität Stuttgart Dipl.-Ing. Gian-Carlo Gullo Institut für Metallforschung der ETH Zürich Abstract Thixoforging is the process of forming metals which have a globular and fine-grained microstructure. The forging of parts with an easily reproducible good quality leads to the necessity of optimizing the die and press technologies and also to the necessity of determining in advance microstructures, induction heating processes and quenching as well as the thermal treatment following the forming process. This paper focuses on describing an optimized induction heating unit with an integrated temperature control and also on the fundamental elements of a press used for the thixoforging process. Moreover, different examples of real parts are discussed. These parts are formed by using the thixoforging process. With these parts, the simulation of the filling of the die cavity is demonstrated. Furthermore, the results of a metallographic look into the microstructure and of the effect of squeezing on the formed part are presented. Finally, the mechanical properties of parts of different thermal treatments are discussed. 1 Induction Heating of Parts for the Thixoforging Process In industry and research laboratories only induction heating units are used to obtain heated parts within the range of temperatures between the solidus and the liquidus state with a semi-solid microstructure. Through the use of induction heating it is possible to induce high energy in the part, to obtain heated parts at the right temperature in a short time. Considering the possible output of a thixoforging process, the time for the heating process is also important. Furthermore, the duration of the heating process influences the quality of the microstructure. Research done in the area of induction heating aims at optimizing the heating process. The whole part has to have the same temperature. The temperature of the part must be

2 Thixoforging Wolf, Baur, Gullo between the solidus and the liquidus temperature. Then both the solid and the liquid phase coexist in the microstructure. The temperature range is always very small, for some alloys it is only a few Kelvin. During the heating process, the whole part has to be heated homogeneously up to the temperature needed for the process. It is important that the induction current, conditioned by the working frequency of the induction heater, heats up only the outer part. This effect is called "skin effect". The necessary homogenization of the temperature of the whole part is achieved by heat conduction. Therefore some time is needed in order to reach the same temperature level. Even after this time, there is a radial temperature gradient. This gradient can be minimized by optimizing the parameters of the heating process. The heating of brass and steel parts is made easier by the fact that the surface of the parts are cooled down more by radiation and convection than is the case with aluminum. This effect reduces the difference in temperature between the surface and the inner part. With regard to the time needed for the induction heating process, it is necessary to have several heating units for one forging press. 1.1 Thixoforging Facilities at the IFU Heating units with an oscillating circuit are usually used in industry and research laboratories /1/. In contrast to this technique, the induction heater at the IFU works with a pulsed direct current in the induction coil. The pulsing is realized by a thyristor. The wiring diagram of the heater used at the IFU is shown in Fig. 1. As can be seen in Fig. 1, the supply voltage is rectified. By using a thyristor, a pulsed direct current causes the induction in the coil. A medium frequency transformer connects the thyristor and the coil. The measuring unit, which is also shown in Fig. 1, is described in chapter 1.2. The working frequencies of the Institute's induction heater range from 1. Hz to 4. Hz. The frequency of the pulsed voltage in the coil can be optimized by changing the thyristor's frequency. In contrast to the induction heater at the IFU, the heating frequency of oscillating circuit units is close to the resonance frequency. The adjustment of the working frequency is realized by connecting and disconnecting capacitors and coils. This leads to complex circuits, and for each new coil the circuit has to be adjusted to the resonance frequency /2/.

Wolf, Baur, Gullo Thixoforging 3 Fig. 1: Wiring diagram of the induction heater used at the IFU. According to the following equation, different depths of penetration can be easily achieved by changing the frequency in the induction coil: ρ 7 δ = 1 1 2π f µ The lower the frequency is, the deeper is the penetration of the induced heating current. The power output of the induction heater used at the IFU can be changed by modulating the width of the pulses of the direct current at constant working frequencies. At the beginning of a pulse period, the thyristor's output voltage is the rated voltage at all pulse widths. At the maximum pulse width the rated voltage ends after the first half of the pulse period. During the other half of the pulse period the voltage equals zero. In order to be able to control the induced power, the phase during which the voltage in the induction coil is the rated voltage is shorter. Therefore, to keep the heating frequency constant, the time during which the voltage equals zero is longer. In Fig. 2 examples of the pulse widths 154 and 254 are given. These widths are relative, the pulse width "" is the shortest possible width, the pulse width "254" represents the maximum duration of the pulse.

4 Thixoforging Wolf, Baur, Gullo thyristor output in relation to the rated voltage U/Urated 1 PWM 254 PWM 154.5 1 1.5 2 2.5 3 3.5 thyristor frequency in relation to the time f/t in 1/s² Fig. 2: Pulse width modulation. By using different heating frequencies and by modulating the pulse width, an optimization of the heating process can be achieved. Depending on the alloy and the size of the part, the time for the heating process ranges from 6 s when heating small brass parts (.2 kg) to 9 s when heating large aluminum parts (.55 kg). The cycle time of the forging process (inserting the part into the die, closing the die, forging, opening the die, removing the workpiece) and also the time which is needed to achieve a constant temperature in the part condition the number of induction coils. 1.2 Controlling of the Heating Process The thixoforging process is carried out within a small temperature range between the solidus and the liquidus temperature. Therefore, an exact controlling of the heating process is necessary which can be realized by different methods. All of these methods use the part's temperature as an indicator for the proportion of the liquid phase to the solid phase in the microstructure.

Wolf, Baur, Gullo Thixoforging 5 The most common method is measuring the inductivity changes of the heated part. The changes of the magnetic properties of the part are measured by an additional coil. For this method, it is necessary to keep the distance between the part and the additional coil as short as possible. With most heating units, this is realized by integrating the additional coil into the pedestal of the induction heater. This causes relatively high thermal loads in the additional coil. Furthermore, it is necessary to have additional coils which are especially adjusted to the geometry of each part. As an alternative, an electrical input indicator can be used to measure the induction energy. This method has the following disadvantage: The losses of heat energy through radiation and convection can be measured only with difficulty /3/. Therefore, a new method of measuring the proportion of the phases in the part was developed at the IFU. This method is based on the fact that the inductivity changes when the microstructure of the part changes. The method which was developed at the IFU is based on the changes of the magnetic and electric properties that occur during the heating process /4/. These changes occur both in the solid and in the semi-liquid state. The unit at the IFU works with a constant coil voltage. The effective current changes when the temperature changes. This change of the current can be measured with the help of tests that are being done before series production starts. During these tests, the temperature as well as the effective current are measured during the heating process. After this, during the series production, it is possible to use the recorded changes of the effective current to indicate the temperature during the heating process and also to indicate when the material is in the semi-solid state. The beginning of the partial melting process is clearly indicated by a considerable current drop. It is also relatively easy to determine the proportion of the solid phase to the liquid phase because the effective coil current also changes in the semi-solid state of the microstructure. The proportions of the phases in the microstructure can be measured by means of metallographic intersections. Thus, the effective coil current is conditioned by the microstructure. The effect the microstructure has on the coil current can be proven by examining cross sections of workpieces which are heated, quenched and then examined for their proportion of the solid phase to the liquid phase. The determined proportion of the phases can be related to the effective coil current which is measured at the end of the heating process. Thus, it is possible to indicate online the proportion of the liquid phase in the microstructure during the heating process.

6 Thixoforging Wolf, Baur, Gullo Fig. 3 shows the effective coil current versus the temperature while 4 AlSi7Mg parts with the same geometry were heated. In all 4 tests the 4 curves differed only slightly from each other. This is negligible, considering the absolute quantity of the current. It can be inferred from the tests that this method is suitable for series production. effective current in A 615 61 65 6 595 test 1 test 2 test 3 test 4 y = -.13x + 61.26 part - AlSi7Mg - diameter = 59 - length = 24.5 y = -.74x + 993.55 59 1 2 3 4 5 6 temperature in C Fig. 3: Effective current versus temperature in the part during the heating of 4 AlSi7Mg parts ( 59mm, Length 24.5mm). 2 Press Needed for the Thixoforging Process 2.1 Punch Forces during the Forging Process In Fig. 4 two punch-force punch-stroke curves are shown. Both were recorded during forging processes for which the same press and the same die were used. The temperatures of the die were in both cases 55 C, the punch velocity was 1 mm/s. Only the temperatures of the part varied: When a traditional forging technique was employed to form the metal, the temperature of the part was 82 C; during the thixoforging process the temperature of the part was 87 C. Because of the shear-rate dependent viscosity of the metal, the thixoforging technique requires extremely low punch forces at the beginning of the forging process. Only towards

Wolf, Baur, Gullo Thixoforging 7 the end of the process is a higher punch force needed, because the material cools down to temperatures which are closer to the solidus temperature and also because the surface of the workpiece gets larger. The punch force of the traditional forging process, however, increases during the whole punch stroke. After the cavity has been filled, the press works with a constant force over a certain period of time during which the workpiece has to solidify completely. The given punch force has to produce pressure within the workpiece in order to avoid inner porosities in the workpiece. These porosities could be caused by the shrinkage of the material while it changes from the semi-solid to the solid state and also while it cools down to room temperature. punch force in kn 6 5 4 3 2 1 impact die-workpiece thixoforging at 87 C traditional forging at 82 C thixoforging with a given punch velocity profile during the punch stroke constant force over a certain period of time bottom dead center (BDC) Fig. 4: Punch-force punch-stroke diagram of a traditional forging process and a thixoforging process. 2.2 Demands the Press has to Meet during the Thixoforging Process Immediately upon inserting the heated raw part into the die, the punch stroke starts. The punch velocity should be as high as possible in order to close the dies as fast as possible. This is necessary because the differences in temperature of the material and the die cause a quick cooling of the workpiece. The punch velocity needed to close the dies is approximately 8 mm/s. The stroke of the punch should be as short as possible because the cycle time should also be kept short. The minimum length of the stroke is conditioned

8 Thixoforging Wolf, Baur, Gullo by the fact that an insertion of the part and a removal of the workpiece after the stroke has ended have to be ensured. The impact velocity of the upper die when it meets the heated part should be reduced significantly to avoid a bursting of the semi-liquid material. If closing devices are used, the impact of the upper die when it meets the lower die at a reduced velocity will also reduce the increase of pressure in the nitrogen spring system. The impact velocity ranges from 5 to 2 mm/s. During the following forging process the chosen punch velocity depends on the alloy and on the geometry of the workpiece. Attention has to be given to the fact that the workpieces cool down very quickly - depending on the relation between the part's surface and its volume. To avoid "freezing" of the material during the forging process, the punch velocity has to be optimized in such a way that the forging process is finished before the material solidifies. By increasing the temperature of the die, an extension of the time during which thixoforging is possible could be easily achieved, yet this would extend the cycle time because the forged part should solidify as quickly as possible within a very short period of time. In order to meet this demand, the temperatures of the material and the dies should differ greatly. Another reason for using relatively low temperatures is the thermal load to which the lubricants are subject. Still another reason for thixoforging with optimized punch velocities is to fill the cavity with a laminar flow of the semi-solid material. If there are large variations in the cross sections of a workpiece the punch velocity will have to be adjusted during the filling process. After the cavity has been filled, the material of the workpiece should solidify completely under a pressure load of approximately 1. bar. This pressure is needed to ensure a microstructure without inner porosities. This is to achieve a better quality of the workpieces. When the material is solid, the workpiece has to be cooled down to a temperature level at which the workpiece can be removed from the die without any plastic deformation. In Fig. 5 a typical punch-force punch-stroke diagram is shown. The die used here was a die for thixoforging without flash.

Wolf, Baur, Gullo Thixoforging 9 1 1 closing the dies forging punch velocity in mm/s 8 6 4 punch velocity impact upper die-lower die punch force 8 6 4 punch force in kn 2 2... switching point 5 1 15 2 punch stroke in mm Fig. 5: Punch-velocity/punch-force punch-stroke diagram. In Fig. 6 values typical of the cycle time proportions during the thixoforging process are shown. An operating cycle lasts approximately 25 seconds. The values were recorded while thixoforging a brass workpiece with a weight of.8 kg. The values for the required punch velocity and the required punch force are very rarely conditioned by the size of the part. Only workpieces that have got a large surface and a small volume cause problems during the process. The cooling time of the workpiece depends upon the relation between surface and volume. That is why a solidification under pressure, which is exerted up to 8 seconds, may be necessary. This is important for the quality of the workpiece. The time needed for the opening process does not have a negative effect in terms of productivity, since the workpiece can already be lifted out of the die by the ejector pins while the punch moves towards the top dead center.

1 Thixoforging Wolf, Baur, Gullo induction heating 85 lubricant application 5 inserting the heated part 1 stroke-release acceleration of the punch.4 deceleration of the punch.3 closing the dies impact upper die-workpiece.6 forging.2 decelaration.1 dies closed during solidification 2 opening of the dies 3 ejection of the work piece 4 removal of the part 4-1 -5 5 1 15 2 25 time in s Fig. 6: Values typical of the cycle time proportions during the thixoforging process. 2.3 Press Used at the IFU for the Thixoforging Process As was mentioned in 2.2, the thixoforging process requires high punch velocities and low punch forces at the beginning of the punch stroke. Towards the end of the punch stroke, however, it requires high punch forces. The hydraulic single-action press used at the IFU for the thixoforging process has a nitrogen accumulator drive. This drive makes it possible to control punch velocities of up to 8 mm/s during the punch stroke. The nitrogen accumulator ensures a large oil volume flow which makes it possible to work with high punch velocities. A flow control valve between the accumulator and the press cylinder controls the punch velocity.

Wolf, Baur, Gullo Thixoforging 11 1... tank 5... hydropneumatic accumulator 2... pump with motor 6... 4/3-way valve 3... pressure relief valve 7... throttle valve, adjustable 4... check valve 8... hydrocylinder of the press Fig. 7: Schematic of a hydraulic single-action press with accumulator drive. At the IFU, the oil volume of 7 l which is necessary for a punch stroke of 3 mm is achieved by placing the piston accumulator on the press head. The nitrogen volume of the piston accumulator is expanded through an additional nitrogen accumulator. The piston accumulator is mounted directly onto the main valve unit which is mounted directly on the cylinder. Thus, it is possible to keep the inertia of the oil volume as low as possible and to minimize losses caused by piping. The single-action press used at the IFU has a nitrogen pressure accumulator. The technical data of the Institute's press are shown in Fig. 8.

12 Thixoforging Wolf, Baur, Gullo max. press force 4.4 kn max. ram velocity 8 mm/s ram stroke 3 mm press table dimensions 9 x 9 mm max. height of the die 1. mm Fig. 8: Hydraulic single-action 5.kN-press for the thixoforging process.

Wolf, Baur, Gullo Thixoforging 13 3 Example of Manufacturing the Steering Knuckle The Steering Knuckle component is originally forged, and a lot of refinishing operations are necessary. These refinishing operations can be reduced by thixoforging. Borings by cores are possible, comparable to pressure die casting. The material flows around the cores during the forging process. To avoid oxide skins and impurities at the place, where the material fronts flow together, an overflow is necessary. The geometric cut from the part to the overflow affects the die-filling. 3.1 Die-filling and Deformation Parameters The simulation of the die-filling shows that the slug is flattened first (left side of Fig. 9). Afterwards, the region of the component near the rear bar has to be filled. The hole is almost finished before the thixotropic material flows around the front bar. It is evident that the material fronts do not collide exactly in the middle where the cut to the overflow is located. The simulation allows it to be assumed that the overflow has been partially filled by the upward material flow before the downward material flow reaches the point of overflowing. In this case, the overflow is ineffective in the absorption of impurities and oxide skins.

14 Thixoforging Wolf, Baur, Gullo bar 3 bar 6 bar 9 bar 12 bar 15 bar 18 bar Fig. 9: Comparison of the die-filling (steering knuckle; left side: simulation with FLOW 3D /5/, shown is the internal pressure of the die; right side: step-shooting experiments).

Wolf, Baur, Gullo Thixoforging 15 Fig. 1: Geometric cut to the overflow (left: cut geometry of the simulation; right: cut geometry of the thixoforged parts). Due to the results of the simulation, the cut geometry to the overflow has to be slightly changed (Fig. 1). The new cut geometry is not symmetrical and the downward material flow has to be supported whereas the upward material flow has to be choked. By completing step shooting experiments, the effect of the new cut geometry can hardly be reconstructed (right side of Fig. 9), because the die-filling occurs extremely fast and the interruption of the filling process at this time is not possible. But step 4 clearly shows a complete filling around the front bar and an incomplete filling in the overflow. The simulation of the spherical head relief is shown in Fig. 11. A laminar filling of the die in this area is apparent. The comparison of the simulation with the step-shooting experiments shows a good compatibility in reference to the die-filling. step 1 step 2

16 Thixoforging Wolf, Baur, Gullo step 3 step 4 step 5 Fig. 11: Comparison of the die-filling (spherical head relief; left: simulation; right: step-shooting experiments). 3.2 Metallographic Experiments The change of the cut geometry to the overflow leads to homogeneous die-filling of the upward material flow and the downward material flow around the hole. Metallographic cuts of this area show two material fronts welded in the middle (right side of Fig. 12).

Wolf, Baur, Gullo Thixoforging 17 Tote dead Zone zone Verschweißung zweier Materialfronten im Bereich welding des of Überlaufs two material fronts near the overflow Fig. 12: Comparison of the simulation with metallographic cuts of thixoforged parts with optimized overflows. Only near the bars a dead zone is located. This dead zone may be contaminated by oxide skins. In the dark areas in the metallographic cut that identify the material flow, there are no pores or oxide skins; there are, however, areas with a higher eutectic phase amount. These areas with a higher eutectic phase amount indicate where a relative movement of the material takes place. This is near the surface of the die where the thixotropic material solidifies because of the colder die temperatures and hotter thixotropic material which continues to flow. This effect is also seen in the overflow of the component where the upward and downward material flow is welded together. The velocity of the flow of both materials is not identical.

18 Thixoforging Wolf, Baur, Gullo 25 mm 25 mm porosity 15 mm 15 mm porosity Fig 13: Influence of the punch velocity on different areas of the component (left: 4 mm/s; right 2 mm/s). A strong dependence of the die-filling and of the component quality on the punch velocity is apparent. Components that are thixoforged with a high punch velocity of 4 mm/s show a high porosity near the spherical head relief (above left in Fig. 13) as a result of the non-laminar die-filling. The porosity in this area is reduced by a slower punch velocity (2 mm/s) and a laminar die-filling (above right in Fig. 13). Near the bars, the different punch velocities only have a slight influence on the porosity. There is neither porosity for a high punch velocity (4 mm/s) nor for a slow punch velocity (2 mm/s) recognizable in the component (below left in Fig. 13).

Wolf, Baur, Gullo Thixoforging 19 4 Mechanical Properties The following conditions have to be examined: I. Natural ageing without a solution heat treatment (T1) /6/: The material has been aged 2 days at room temperature at a stable state directly after deformation. II. Solution heat treatment, quenching and natural ageing (T4) /6/: The material has been subjected to a solution heat treatment after deformation (for one hour), quenched in water, and naturally aged for 2 days at room temperature. III. Solution heat treatment, quenching and artificial ageing (T6) /6/: The material has been subjected to a solution heat treatment after deformation (for one hour), quenched in water, and artificially aged at 18 C. 4.1 Mechanical Properties of the Casting Alloy AlSi7Mg For a homogeneous heating and an equally distributed liquid phase amount in the cylindrical slugs, it is necessary to hold the slug at the thixoforging temperature (578 C) during isothermal conditions. But it has to be examined if there is any influence on the mechanical properties from the holding time. Because of this, components with various holding times between 12 and 39 seconds were used in the experiments. The results show that there was no effect of the mechanical properties in the state T1 on the holding time (Fig. 14). strength in N/mm 2 3 25 2 15 1 5 YS UTS A 5 Fig. 14: Dependence of the 3 25 2 15 1 5 fracture elongation A 5 in % mechanical properties on different holding times (alloy: AlSi7Mg; state T1; thixoforging temperature: 578 C). 13 seconds 295 seconds 315 seconds The mechanical properties in the state T4 show a similar behaviour (Fig. 15). There is also no apparent influence on the holding time between 13 and 315 seconds. But the higher fracture elongation of the state T4 in comparison with the state T1 is remarkable. The origin seems to be most likely the internal mechanical stress in the state of T1 that

2 Thixoforging Wolf, Baur, Gullo leads to premature failure under the effect of external stress. By using the solution heat treatment of the state T4, the internal mechanical stress was reduced and the fracture elongation accumulated although the strength increased. strength in N/mm 2 3 25 2 15 1 5 3 Fig 15: Dependence of the YS UTS A 5 25 2 15 1 5 fracture elongation A in % 5 mechanical properties on different holding times (alloy: AlSi7Mg; state T4; thixoforging temperature: 578 C). 13 seconds 295 seconds 315 seconds For an artificial ageing to take place in the T6 state, it is necessary to know the exact ageing time to avoid over ageing the alloy. For this, a specimen has to be treated in a solid solution (for one hour at 53 C), quenched in water, and artificially aged at 18 C. To determine the maximum strength, Brinell tests were made during the ageing time to test hardness. The results of these experiments show that the maximum strength is reached after an aging time of 5 hours. Specimens for tensile tests show no dependence on the holding time during the heating process in the T6 state (Fig. 16). This is similar to the reactions observed in state T1 and T4. strength in N/mm 2 3 25 2 15 1 5 YS UTS A 5 Fig. 16: Dependence of the 3 25 2 15 1 5 fracture elongation A 5 in % mechanical properties on different holding times (alloy: AlSi7Mg; state T6; thixoforging temperature: 578 C). 12 seconds 285 seconds 39 seconds The determined mechanical properties in the states T1, T4 and T6 show no dependence on the holding time during the heating process. Therefore, for the casting alloy AlSi7Mg,

Wolf, Baur, Gullo Thixoforging 21 it is better to heat the cylindrical slugs several minutes under isothermal conditions at the thixoforging temperature to get a homogeneous heat penetration. 4.2 Mechanical Properties of the Wrought Alloy AlMgSi1 Comparable to the casting alloy AlSi7Mg, the wrought alloy AlMgSi1 must be heated isothermally for a special amount of time at the thixoforging temperature of 635 C to get a homogeneous heat penetration and an equally distributed liquid phase amount in the cylindrical slug. The holding time is between 12 and 6 seconds. Fig. 17 shows the mechanical properties of specimens in the T1 state. The results show that there is a dependence on the holding time of the wrought alloy AlMgSi1. As a result of a longer holding time, the strength decreases and the fracture elongation increases. strength in N/mm 2 35 3 25 2 15 1 5 YS UTS A 5 Fig. 17: Dependence of the 35 3 25 2 15 1 5 fracture elongation A 5 in % mechanical properties on different holding times (alloy: AlMgSi1; state T1; thixoforging temperature: 635 C). 12 seconds 3 seconds 6 seconds The T4 state apparently requires a holding time between 12 and 6 seconds (see Fig. 18). The strength decreases and the fracture elongation shows the maximum value at the longest holding time. strength in N/mm 2 35 3 25 2 15 1 5 YS UTS A 5 Fig. 18: Dependence of the 35 3 25 2 15 1 5 fracture elongation A 5 in % mechanical properties on different holding times (alloy: AlMgSi1; state T4; thixoforging temperature: 635 C). 12 seconds 3 seconds 6 seconds

22 Thixoforging Wolf, Baur, Gullo To avoid too much ageing in the T6 state and thus obtaining undesirable mechanical properties, the ageing time has to be known. The time can be determined by Brinell tests of hardness at solution heat treated, quenched, and artificially aged specimens. The maximum strength is reached after 8 hours of ageing at 18 C. Thixoforged Components in the T6 state show similar to those in the T1 and T4 state the importance of the holding time. Figure 19 shows the average mechanical properties of the examined specimens for tensile tests. strength in N/mm 2 35 3 25 2 15 1 5 YS UTS A 5 Fig. 19: Dependence of the 35 3 25 2 15 1 5 facture elongation A 5 in % mechanical properties on different holding times (alloy: AlMgSi1; state T6; thixoforging temperature: 635 C). 12 seconds 3 seconds 6 seconds The mechanical properties show in the T1, T4 and T6 state the strong dependence on the holding time during induction heating. At the high temperature of isothermal heating (thixoforging temperature of 635 C), a strong grain growth most likely occurs in the time interval between 12 and 6 seconds. Because of this, the grain growth has to be metallographically examined in relation to the holding time. The average grain size has to be determined as the mean value of the grain size distribution. The metallographic examination of the grain size and the comparison with the mechanical properties demonstrate (Fig. 2) that grain growth happens during the heating at isothermal conditions. Thus the grain size increases from 89µm (holding time: minutes) to 132µm (holding time: 1 minutes). This means a grain growth of 48%.

Wolf, Baur, Gullo Thixoforging 23 YS UTS A 5 16 4 16 14 35 14 average grain size in µm 12 1 8 6 4 3 25 2 15 1 strength in N/mm 2 12 1 8 6 4 fracture elongation in % 2 5 2 2 4 6 8 1 12 isothermal holding time in minutes Fig. 27: Comparison of the mechanical properties (state T6) at different holding times and the metallographically examined grain size of the microstructure. As a result of the experiments, it can be concluded that grain growth in the cylindrical slugs during the isothermal heating occurs at the thixoforging temperature. Therefore, the isothermal holding time should be as short as possible to achieve a high level of strength in the naturally or artificially aged alloy. 5 Summary The experiments have shown that it is possible to inductively heat raw materials for thixoforging and therefore measure the temperature without direct contact. Through the application of the proposed press technology, manufacturing examples with complex geometrical forms are possible. The die-filling of the proposed manufacturing examples Steering Knuckle was simulated and the results were allowed for the construction. The example was metallographically examined. It turned out that welding of two material fronts is possible around bars and cores when the absorption from the overflow of impurities is integrated. Despite this, the formation of small dead zones with impurities cannot be completely eliminated. The examination of the mechanical properties of thixoforged components made of AlSi7Mg and AlMgSi1 showed that, depending on the heat treatment, good results can be obtained; however, the wrought alloy has to be preferred to the casting alloys. Indeed, it is apparent that the wrought alloy AlMgSi1 is strongly dependent on the heating at the thixoforging temperature due to the grain growth. Therefore, the casting alloy AlSi7Mg should be kept longer at the thixoforging

24 Thixoforging Wolf, Baur, Gullo temperature to create a homogeneous heat penetration, while the wrought alloy AlMgSi1 should be heated a short time at the thixoforging temperature to avoid grain growth. Literature /1/ Niu, X. P. Semi-Solid Forming of Cast and Wrought Aluminium et al. Alloys. In: Proc. of the 5 th International Conference on Semi-Solid Processing of Alloys and Composites, Golden (Colorado)/USA, June 23 rd 25 th, 1998, Editors: Bhasin, A. K. et al, pp. 141 148. /2/ Hirt, G. Experience with Sensor Controlled Induction Heating for et al Semi-Solid Forming. In: Proc. of the 5 th International Conference on Semi-Solid Processing of Alloys and Composites, Golden (Colorado)/USA, June 23 rd 25 th, 1998, Editors: Bhasin, A. K. et al, pp. 157-164. /3/ Gräf, T. Controlled induction heating for thixotropic materials into et al. the semi-solid state. In: Proc. of the 6 th International Conference on Semi-Solid Processing of Alloys and Composites, Torino/Italy, September 27 th 29 th, 2, Editors: Chiarmetta, G. L., Rosso, M., pp. 667-673. /4/ Baur, J. Thixoforging of a CuZn-Alloy. In: Proc. of the 6 th International Conference on Semi-Solid Processing of Alloys and Composites, Torino/Italy, September 27 th 29 th, 2, Editors: Chiarmetta, G. L., Rosso, M., pp. 289-294. /5/ Meßmer, G. Simulation of Thixoforging Processes with Flow-3D; In: K. Siegert (Editor): Proc. of the International Conference "New Developments in Forging Technology", May 14 th 16 th, 21; Fellbach/Stuttgart. /6/ Europäische Norm DIN EN 515; p. 5