Semi-Solid Processing of Magnesium Alloys

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Semi-Solid Processing of Magnesium Alloys

Shehata, M. T.; Essadiqi, E.; Loong, C. A.

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Semi-Solid Processing of Magnesium Alloys

M.T. Shehata and E. Essadiqi

Materials Technology Laboratory (MTL)/ CANMET, 568 Booth Street

Ottawa, Ontario, Canada K1A 0G1 mshehata@nrcan.gc.ca

C.A. Loong

Industrial Materials Institute(IMI), NationalResearch Council Canada, 75 de Montagne, Boucherville, Quebec, Canada J4B 6Y4

ABSTRACT

Semi-solid Processing is a relatively new process for producing components with high integrity where the material is processed at a temperature between solidus and liquidus and then formed into components. The paper covers semi-solid processing of Mg alloys using, as an example, the Thixoforming of magnesium AZ91D. The paper describes the experimental procedure where slugs of feedstock material having a rosette structure are heated to a semi-solid state to obtain a globular structure before the material is formed into components in a die casting machine or forging press. The experimental thixotropic feedstock of magnesium alloy AZ91 was prepared using CANMET’s casting simulator with electromagnetic stirring. Forging trials were conducted at MTL while die casting of a number of components was carried out at IMI. The paper presents results of the optimized heating of 3-inch diameter and 6 inch long slugs of AZ91D alloy prepared under laboratory conditions. The optimized induction heating procedure was established to achieve a globular structure with rapid and homogeneous heating of slugs which are then subjected to high pressure die casting or forging. The optimal casting and forging parameters (slurry temperature, die temperature, ram speed and metal pressure) are given. Results on morphological changes in the microstructure of the feedstock material during heating are presented. It is shown that the material heated to 580 º C should be held at this temperature for 30-90 seconds to achieve globularisation of the primary solid phase without excessive grain growth. The microstructure at various locations in the final product was analyzed. At the optimum casting and forging conditions no significant liquid segregation is observed in the final part.

Key Word: SSM, Semi-Solid Casting, Semi-Solid Forging, Thixoforming, High Frequency Induction Heating, Magnesium Alloy, AZ91D, Pressure Die Casting

INTRODUCTION

Semi-solid metal processing is a relatively new forming process for producing components with high integrity where specially prepared slurries or slugs of the alloy at a temperature between solidus and liquidus are formed into components. Semi-solid processing can be classified into two main processing routes: Rheo and Thixo. In Rheo-processing, shown schematically in Figure 1, the process starts from a melt that is cooled to a semi-solid temperature and then formed into components. In Thixo-processing, shown schematically in Figure 2, the process starts from a solid billet that is typically cut from a rheocast or grain-refined feedstock, heated to a semi-solid temperature and then formed into components. In all cases the resulting microstructure must produce a thixotropic material having a viscosity low enough under shear to readily fill the die cavity under pressure in a die casting machine or a forging press. This technology is based on research originally conducted at MIT in the 1970s on rheological properties of semi-solid metals subjected to mechanical stirring (1). It is worthwhile to mention that the main advantages of Rheo-processing are the energy savings (since no re-melting is required) and no need to pay a premium for specially prepared feedstock, as well as recycling which can be done in-house, however, a large capital investment will be required.

Melt

Thixotropic

Microstructure

Rheoforming

s

s

s

s

s

s

Figure 1 Schematic representation of the Rheo-Processing.

Practically all high-volume components produced by semi-solid processing today start with commercial billets in aluminum alloys A356 and A357 as feedstock materials (2-4). On the other hand, for magnesium components, the proprietary Thixomolding process (which involves preparation of the thixotropic slurry from magnesium alloy chips in an injection molding machine, shown schematically in Figure 3,) is the most common production method used (5). Because fine-grained or rheocast magnesium alloy bars in different sizes are not available commercially, high-volume production of components from billets has not been possible to date.

The initial feedstock material must be cast in a way that avoids the formation of columnar dendrites or alternatively, by shearing or breaking down the dendrites to small rosettes during solidification. The latter may be achieved through stirring the solidifying melt mechanically or by electromagnetic means, whereas the former can be achieved by casting at a low super-heat, or by the use of grain refiners. Much work has been done on thixotropic aluminium alloys, particularly A356 and A357; in comparison, very little work has been done on magnesium alloys. Thixotropic feedstock billets are available commercially for both A356 and A357, but none is commercially available for magnesium alloys. The feedstock material should then be re-heated to the desired temperature (corresponding to the desired solid fraction) uniformly and rapidly and be held for sometime at this temperature to complete the globularisation of the primary phase (6). There is an optimum temperature profile to achieve the best thixotropic behaviour (7) depending on the structure and the size of the feedstock.

Optimize Reheating Optimize Microstructure Optimize Forming Thixocasting EMS Water cooled Copper Mold (Globular) Thixoforging

Figure 2 Schematic representation of Thixo-procesing

This work presents the results for the casting of laboratory thixotropic feedstock material of AZ91D using static mold casting with electromagnetic stirring techniques. It also presents the results for the optimised heating of the 76mm diameter and 152mm long (3” diameter by 6” long) AZ91D magnesium alloy slugs to the semi-solid forming temperature. In addition, microstructural changes in the as-cast thixotropic material as a result of heating to and holding at the desired semi-solid temperature are presented.

It is intended to address the three key aspects related to the semi-solid processing of magnesium alloy AZ91D components from billets: (a) production of the non-dendritic billet feedstock with equiaxed microstructure by electromagnetic stirring, (b) reheating of the billets, and (c) semi-solid forming by die casting and forging. Also, this paper will

examine the microstructure of the alloy throughout processing from feedstock to the final die cast or forged part in relation to the globularity of the primary alpha phase and liquid segregation, if any, in the final part.

Figure 3 Schematic representation of Thixomolding (5).

EXPERIMENTAL PROCEDURE

AZ91D billets of thixotropic feedstock were produced at CANMET/Materials Technology Laboratory (Natural Resources Canada) using a heat resistance furnace to melt 40kg of alloy AZ91D. A coated, water cooled, copper mould and electromagnetic stirring (EMS) was used to produce the 300 mm high and 76 mm diameter billets with a rosette or globular structure. The electromagnetic stirring unit is a JME-Inverpower Canada Inc. (now ABB) built around the water-cooled copper mold into which the molten metal is poured. The casting is performed at a low super-heat (~20 º C) where the melt pouring temperature is 615 ºC. The details on the casting simulator can be found in a separate publication (8). The chemical composition of the laboratory prepared material is a typical AZ91D alloy (8.7% Al, 0.89% Zn and 0.18% Mn).

Heating of the laboratory prepared material to the semi-solid temperature was conducted using a single induction coil operating at 10 kHz and a power of 13.5 kW. The induction power unit was supplied by Norax Canada, a manufacturer of customized induction systems based on power transistors operating at frequencies up to 200 kHz (9). The billet of 76 mm (3 in) diameter and 152 mm (6 in) in length was contained in a ceramic crucible insulated at the top. It was introduced into the coil by means of a piston

that could be raised and lowered pneumatically, as shown in Figure 4. The coil is fabricated from a 6.5 mm (0.25 in) diameter copper tubing and has a diameter of 127 mm (5 in) and a height of 203 mm (8 in). Power input into the coil is controlled by a programmable controller through a thermocouple inserted into a 37.6 mm (1.5 in) deep drilled-hole at the center of the top surface. The other three thermocouples were inserted from the side at the top, middle and bottom surfaces of the billet at a depth of 6 mm (0.24 in) so that temperature drop between the surface and core could be monitored.

In order to avoid over-heating of the surface in the workpiece, power in the coil was programmed to vary over the heating period of approximately 8 minutes. Full power was turned on for the first 3 minutes until a temperature of 460 º C (just below its solidus temperature) was attained, at which point it was reduced to 60% of its initial value. Additional reductions over the next two subsequent steps in the remaining 5 minutes brought the power input down even further. Towards the end of the heating cycle, the power required to achieve a final temperature of 575 - 580 º C was 15% of the initial value.

The experimental dies used for the semi-solid casting and forging trials are shown in Figure 5. The casting die was heated by hot circulating oil while the forging die was kept at the desired temperature by electrical band heaters. In die casting trials, the semi-solid slug is injected into the die cavity through a gate at the center, thereby leaving any oxide skin that could have formed during induction heating in the biscuit. The casting die produces a box-like component in a Bühler 600 T capacity SC N/53 die casting machine. The casting has a shot weight of 0.95 kg and a height of 60 mm. The box-like component has a wall thickness that varies from 9 mm near the gate to 5 mm at the bottom edge. The forging die produces a disk-like component weighing about one kilogram with an 18 mm thinner outer section of 170 mm diameter and a 58 mm thicker inner section of 90 mm diameter.

Figure 4. Schematic of set-up for reheating of billets; thermocouples were embedded at points 1, 2, 3 and 4 as shown on the right.

(a) Three-part casting die (b) Forging die Figure 5. Schematics of the casting and forging dies

The morphological changes of the primary solid phase were studied by heating smaller samples (25x25x20 mm) and quenching them in mineral oil from a temperature of 580 ºC, corresponding to about 40% solid. Metallographic samples were cut from the quenched samples and subjected to conventional metallographic preparation (mounting, grinding, polishing and etching) and etched with acetic-picral for 7-10 sec.

RESULTS AND DISCUSSIONS Feedstock Material

Microstructures of AZ91 at mid-radius of the billet with and without electromagnetic stirring are given in Figure 6. The structure is more equiaxed and rosette-like structure in the case of stirred ingot compared to the non-stirred, which consists of columnar dendritic structure. In other words, it can be seen that the globularity of the alpha phase is favoured by electromagnetic stirring.

Figure 6. Microstructures of laboratory as-cast thixotropic feedstock AZ91D, (a) without electro-magnetic stirring and, (b) with electro-magnetic stirring.

DRAWING NO REV SHTOF CUSTOMER SHRINKAGE MATERIAL PART NO REF DRAWING NO SCALE DIMENSIONS TOLERANCES 2 PL DEC 3 PL DEC 4 PL DEC ANGLE +-DRAWN CHECKED VERIFIED DATE FULL R.LALONDE 3 220 INCHES ALLOY CNRC IMI / SSM-BOX 97004 CAD FILE .OO0/IN/IN 20-MAR-97 C:\CADKEY7\PRT\CNRC DISK97004 XXXXX .01.001 .0002 0 30' MOLD ASSEMBLY FRONT SECTION OPEN 75 DE MORTAGNE, BOUCHERVILLE, QUEBEC, CANADA, J4B-6Y4 R.LALONDE ---R.LALONDE

---NATIONAL RESEARCH COUNCIL TEL 514 641-5094FAX 514 641-5104 DATE DATE ----(b) (a)

Reheating of Billets by Induction

In induction heating, heat is generated in the billet by the thermal effect of a varying electromagnetic field when an a-c current is flowing in the coil. Electrical power is transformed into heat by induction in accordance with the well-known equation derived from Ohm's Law:

P=⎪I⎮2

R /2where P is the power, ⎪I⎮ is the average value of sinusoidal current in the solenoid heating coil and R is the effective resistance of the workpiece. A detailed treatment of the theories of induction heating is beyond the scope of this paper, however there are a few important factors that need to be considered. First, commercial induction systems operate at frequencies that seldom exceed 1 kHz. The reason is that high frequency systems concentrate their energy on the surface and edges, thereby causing them to overheat. This results in a high surface temperature and an unacceptable amount of liquid metal draining from the surface. By operating the coil at a low frequency, the eddy current effect on the surface is reduced while the effective depth of penetration of the power applied is increased. This facilitates achieving a uniform temperature distribution between the core and the surface within a reasonable cycle time.

The effective resistance of a coil containing a billet is proportional to the square root of frequency for a good electrical conductor. Thus, the current required at a higher operating frequency is less than that at a lower operating frequency under the same heating power condition (9). Consequently, there is a clear advantage in using a high frequency system since the efficiency for such a system would be higher. Typical temperature-time curves registered by the four thermocouples are shown in Figure 7. For over half the reheating cycle, there is a significant temperature difference of up to 80 º C at the beginning between the hottest and coldest points. This difference becomes much narrower as the power is reduced such that towards the end of the reheating cycle, a difference in temperature of less than 6 º C is realized.

Figure 7. Temperature vs. time curves for points 1 (control), 2 (top surface), 3 (middle surface) and 4 (bottom surface) at different coil current.

0 100 200 300 400 500 600 0 2 4 6 8 10 12 14 16 0 100 200 300 400 500 600 Control Top Surface Middle Surface Bottom Surface Current (Amp) Cu rr e n t ( A m p ) Time (s) T e m p e ra tu re ( d eg re e C )

The microstructures of the samples were then examined by optical microscopy. Results in Figure 8 show that at 580 oC, the material becomes more globular with increasing holding time. At longer holding times grain growth of the primary phase is observed, which is undesirable as this may lead to lower strength in the final part after forming. It appears that an optimum holding time for globularisation of the primary phase without excessive grain growth is achieved at a holding time of about 90 seconds.

As Cast 0 sec

90 sec 30 sec

210 sec 600 sec

Figure 8. Microstructure evolution during the isothermal heat-treatment at various holding times at 580º C of laboratory prepared AZ91D.

Semi-Solid Forming

The optimal parameters used for successfully producing high quality castings and forgings, shown in Figure 9 may be outlined as follows:

Figure 9. Cross-sectional and external views of the box-like die cast (left) and disk-like forged (right) components

Billet Temperature - Billet temperature is critical in that fluidity of the semi-solid AZ91D alloy is dependant on viscosity and the percentage of solid. For consistency, it is vital to maintain the feedstock temperature to be as uniform as possible. It was found that a temperature range of 575 - 580 ºC was most suitable. This temperature range corresponds to a solid fraction of approximately 0.3 – 0.4 (11).

Die Temperature - Heating the dies prior to the introduction of semi-solid billet is found necessary. For semi-solid forging the die temperature was kept in the range 250- 290 ºC using electrical band heaters. The die temperature for semi-solid die-casting was kept in the range of 230 – 260 ºC by hot circulating oil.

Ram Speed - The metal injection speed has a major influence on the nature of the flow front in the die cavity. In general, a slow speed should be employed, as one of the principal reasons behind semi-solid forming is the ability to fill the die cavity in a

laminar manner. A fast speed leads to flow turbulence, which negates this advantage and could result in an unacceptably high level of porosity. However, for complex components cast at a given metal pressure, it may not be possible to fill the die completely under a low ram speed. Die casting trials conducted for the box-like components showed that best results were obtained when the ram speed was 1.0 m/s. Forging test trials conducted for the disk-like component showed that a ram speed of 10 cm/s gave good results.

Metal Pressure - In liquid pressure die casting, metal pressure often plays a very important role in expanding the castability window in the production of complex castings. Problems with castings not being able to meet quality specifications can sometimes be solved successfully by using a machine with a higher injection force. This is also the case with semi-solid die casting where the minimal pressure requirement for most parts is at least two times greater. This is because the heat loss during injection results in a material that is increasingly viscous as it fills the cavity. Tests showed that under conditions of equal billet temperature and injection speed, defects observed at a low metal pressure could often be eliminated when a much higher pressure is applied. A pressure of 1200 bars (120 MPa) or higher was found to produce castings with high integrity. For semi-solid forging the final pressure applied onto the part did not exceed 100 bars (10MPa) which was found to be sufficient to produce very sound forging.

Microstructure Characterization of the Final Part

The final part was examined to determine if there was any significant liquid segregation during the filling of the dies with the semi-solid slurry. For this purpose metallographic samples produced under optimal conditions were cut from various locations in the final die cast and forged parts. The locations represent different thickness sections and corner areas where liquid segregation may be suspected. The different areas examined in the die cast part are shown on Figure 10. Some typical micrographs taken from these areas of the die cast part are shown in Figure 11, while those from the forged component from areas in Figure 12 are shown in Figure 13. There is no significant liquid segregation near the wall in all areas except for small spots close to the surface at the end of the thinnest wall (area 5) and at the corner (area 4) in Figure 10, corresponding to micrographs in Figure 11 f and e, respectively.

Figure 10. Photo showing the various areas where metallographic samples were cut from the die cast part shown in Figure 9.

5

4

3 2

(a) (b)

(c) (d)

(e) (f)

Figure 11. Micrographs taken near surface from various areas indicated in Figure 10: (a) near surface in area 1, (b) near surface in area 2, (c) from area 3 at corner, (d) from area 4 at center, (e) from areas 4 at inner corner, and (f) from areas 5 at surface.

2

3 1 4

Figure 12. Photo showing the various areas where metallographic samples were cut from the forged part shown in Figure 9.

(a) (b)

(c) (d)

Figure 13. Micrographs taken at various areas indicated in Figure 12: (a) from area 1 near top surface, (b) from area 2 near surface at corner, (c) from area 3 at center, and (d) from area 4 near bottom surface.

SUMMARY AND CONCLUSION

With the casting simulator of MTL/CANMET we were able to prepare laboratory scale thixotropic 3-inch diameter feedstock of AZ91D alloy through a combination of super-heat reduction and electromagnetic stirring of the melt. The microstructure of feedstock consists of rosettes of primary alpha phase that become globular after reheating to 580 º C.

Reheating of AZ91D billets for semi-solid forming can be done efficiently in a single-coil high-frequency induction system with controlled power input. Sound parts can be routinely produced with a high input power to heat the billet to the solidus temperature, followed by a rapid reduction of power, down to 15 % of its original value, prior to forming.

There is a need to hold the material at the semi-solid temperature (580 oC) for an optimum time range of 30-90 seconds to ensure that the primary solid phase becomes reasonably globular. Longer holding time causes excessive grain growth.

The quality of the semi-solid formed components is influenced by factors that includes the temperature of the semi-solid slurry, the temperature of the die, the ram speed (injection speed) and the metal pressure used. The optimum conditions for semi-solid casting and semi-semi-solid forging for AZ91D are established. At these optimum casting and forging conditions, no significant liquid segregation is observed in the final part.

ACKNOWLEDGMENTS

The authors would like to acknowledge the financial support for this project from the Canadian Lightweight Materials Research Initiative (CLiMRI) of Natural Resources Canada. The authors would like to acknowledge the technical contributions of A. Rivard and R. Canaj of MTL to this work. Thanks are also extended to the many other technical staff both at MTL/CANMET and IMI/NRC who contributed in this project.

REFERENCES

1. M.C. Flemings, "Behavior of Metal Alloys in the Semi-Solid State," Metallurgical Transaction, Volume A22, 952-981, May 1991.

2. K.P. Young and R. Fitze, "Semi-Solid Metal Cast Aluminum Automotive Components, " 3rd Int’l Conf. on Semi-Solid Processing of Alloys and Composites, pp.155-177, 1994.

3. K.P. Young, “Recent Advances in Semi-Solid Metal (SSM) Cast Aluminium and Magnesium Components,” 4th International Conference on Semi-Solid Processing of Alloys and Composites, pp. 229-233, 1996.

4. K. Young and P. Eisen, " SSM (Semi-Solid Metal) Technological Alternatives For Different Applications, " 6th Int’l Conf. on Semi-Solid Processing of Alloys and Composites, pp.97-102, 2000.

5. D. Walukas, S. LeBeau, N. Prewitt and R. Decker. “Thixomolding Technology Opportunities and Practical Uses,” 6th Int’l Conf. on Semi-Solid Processing of Alloys and Composites, pp 109-120, 2000.

6. G. Hirt, R. Cremer, A. Winkelmann, T. Witulski, M. Zillgen, 3th Int. Conf. Semi-Solid Processing of Alloys and Composites, Tokyo, Japan, September (1994), pp. 107-116.

7. E. J. Zoqui, M. T. Shehata, M. Paes, V. Kao, E. Es-Sadiq, Journal of Materials Science and Engineering A, Submitted, January 2001.

8. M. T. Shehata, E. J. Zoqui, V. Kao, E. Essadiqi, “Proceedings of the International Symposium on Materials in the Automotive Industry” Conference of Metallurgists, Toronto (2001) pp.207-224.

9. H. Shahani and M. Shahani, "Resonance Switching Power Supply,"US Patent 5715155, 1998.

10. J-W. Liaw, T-F. Chen, C.A. Loong, C-Q. Zheng, P. Raducanu, K.T. Nguyen and C.K. Jen, “ Theoretical Modeling and Experimental Verification of Induction Heating of Semi-Solid Billets,” 6th Int’l Conf. on Semi-Solid Processing of Alloys and Composites, pp.571-577, 2000.

11. F.C Bennett, T.E. Leontis, and S.L. Couling, Proceeding International Magnesium Association, 34th Annual Meeting, Columbus, Ohio, May 1977, pp. 23-29.