Application of equal channel angular extrusion to semi-solid processing of magnesium alloy

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1 Materials Characterization 58 (2007) Application of equal channel angular extrusion to semi-solid processing of magnesium alloy Jufu Jiang a,b,, Ying Wang a, Shoujing Luo a a School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, PO Box 435, , PR China b Postdoctoral Mobile Station of Mechanics, Harbin Institute of Technology, Harbin, , PR China Received 7 March 2006; received in revised form 21 April 2006; accepted 22 April 2006 Abstract A new method called new strain induced and melt activated (new SIMA) is introduced firstly through using equal channel angular extrusion (ECAE) as strain induced step in SIMA and completing melt activated step by semi-solid isothermal treatment, by which semi-solid billet with fine spheroidal grains with average grain size of 5 μm can be prepared. Comparing with common SIMA, semi-solid processed satellite angle frame components using semi-solid billet prepared by new SIMA have higher room temperature and 393 K high temperature mechanical properties Elsevier Inc. All rights reserved. Keywords: Mechanical properties; Microstructure; Equal channel angular extrusion (ECAE); Semi-solid processing; Magnesium alloy 1. Introduction Semi-solid processing is one of the best methods for forming magnesium alloy components because of its low resistance of deformation comparing to solid alloy forging and high mechanical properties of formed components comparing to liquid alloy die casting [1 6]. Semi-solid processing is defined as a forming process in which semi-solid billets with a spheroidal rather than a dendritic microstructure are formed to near net shaped products by dies at the semi-solid temperature, that is, the temperature interval between liquidus and solidus. For semi-solid processing of magnesium alloy, the most important is to prepare semi-solid billets of magnesium alloy, that is, to obtain alloy with non-dendritic, or Corresponding author. Tel.: address: jiangjufu@hit.edu.cn (J. Jiang). spheroidal microstructure. Comparing with other methods for preparing semi-solid billets, strain induced and melt activated (SIMA) is a good method for preparing magnesium alloy semi-solid billets with high solid fraction. In general, there are two important steps in SIMA, strain induced step and melt activated step. In common SIMA, the first step is a die upset of the cast billet and the second step is isothermal treatment of upsetted billet in semi-solid temperature in order to obtain semi-solid billets with spheroidal solid grains enclosed by liquid phase. However, for upsetting process of magnesium alloy, plastic deformation degree is limited due to its limited slip system. For example, for AZ91D magnesium alloy, a typical Mg Al Zn alloy, the ultimate plastic deformation degree only reaches to 30% corresponding to the equivalent strain of 0.3. Otherwise, if the deformation degree of AZ91D magnesium alloy is above 30%, crack will occur in the upsetted material [7] /$ - see front matter 2006 Elsevier Inc. All rights reserved. doi: /j.matchar

2 J. Jiang et al. / Materials Characterization 58 (2007) In order to overcome the difficulties of plastic deformation in die upset step, a new method for obtaining severe plastic deformation should be needed, in which magnesium alloy billet can obtain severe plastic deformation degree. Equal channel angular extrusion (ECAE) is a promising technique for obtaining ultra-grained bulk materials with increased strength and ductility through severe plastic deformation [8,9]. During ECAE, a billet is pressed through a die that consists of two channels with equal cross-section, intersecting at an angle ö. Since the cross-sectional shape of billet remains nearly the same, ECAE processing can be repeated for many passes to control the microstructure and properties of the material [10 13]. Studies on commercial and engineering application of ECAE are little reported, comparing with microstructure, mechanical properties and texture of material processed by ECAE [14]. The current study represents an attempt to prepare semi-solid billets of AZ91D magnesium alloy through semi-solid isothermal treatment of processed cast materials by ECAE. Based on the results, a new SIMA method, that is, semi-solid isothermal treatment of processed material by ECAE, will be introduced firstly, which will promote the commercial and engineering application of ECAE to semi-solid processing of magnesium alloy. 2. Experimental A typically commercial cast Mg Al Zn alloy, AZ91D magnesium alloy, was used for this study. Cast bars of AZ91D magnesium alloy were machined into cylindrical specimens having a diameter of 58 mm and height of 120 mm for ECAE processing. An ECAE die used for the present study has two equal cross-section channels with the diameter of 58.2 mm [7]. The intersecting angle between two channels is 90 and the angle of the outer arc of the intersection is 90 (Fig. 1). Samples obtained from cast materials were processed by ECAE for 4 passes at 498 K with the punch speed of 30 mm/min., in which equivalent strain of 3.63 of ECAE processed material can be obtained [15]. The billets were lubricated by graphite, inserted into the die, and then held until they reached the processing temperature of 498 K. The experimental cylindrical specimens were processed by ECAE under so-called processing route B C [16], referring to rotating the processed billet by 90 around extrusion direction between each pass. The samples were quenched quickly after ECAE to retain the processed microstructure. Fig. 1. Schematic plan of ECAE die. 1 is Mg Al Zn alloy cast ingots, 2 is half ECAE die. Some samples were used as microstructure observation and tensile test. The other were used in semi-solid isothermal treatment (SSIT) process, in which the processed materials by ECAE were machined into the specimens with the diameter of 55 mm and the height of 35 mm, and then were semi-solid isothermal treated for 25 min at 818 K. After these processed samples by ECAE were semi-solid isothermal treated, some were quenched in water quickly to observe their microstructures and the other were directly carried into the cavity of a forming die with preheating temperature of 623 K and used as semi-solid processing of a component called satellite angle frame. At the same time, cast ingots machined were upsetted by the deformation degree of 30% at 498 K, and then semi-solid isothermal treated holding for 25 min at 818 K. Furthermore, these semi-solid billets of AZ91D magnesium alloy by so-called common SIMA, referring to SSIT of upsetted cast ingots, were as semi-solid processing of a component called satellite angle frame in the same technological condition to semi-solid processing of semi-solid billet prepared by new SIMA. Tensile tests were carried out to characterize the room temperature and high temperature of 393 K mechanical properties by INSTRON 5582 universal testing machine. Tensile specimen of ECAEed and upsetted cast ingots had cylindrical geometry with the diameter of 5 mm, which were cut along the direction of ECAE processed or upsetted billets. Tensile specimen of semi-solid processed finished satellite components had plate geometry with the width of 3 mm and the thickness of 2 mm, which were cut from flank position of satellite components. Four samples cut from upsetted billets, ECAEed billets and finished satellite components in the same technological condition were measured by NSTRON 5582 universal testing machine. To ensure the accuracy of

3 192 J. Jiang et al. / Materials Characterization 58 (2007) Fig. 2. Microstructure of cast material and four-pass processed material by ECAE at 498 K. mechanical properties, the average values of testing experiment results of four samples in the same technological condition was determined as every mechanical index of this material. The microstructure observations were performed by optical microscopy. Alcohol solution with 3% hydrogen nitrate was employed as etchant for microstructure observations. 3. Results and discussion 3.1. Microstructure and mechanical properties of ECAE processed AZ91D alloy Fig. 2 shows the microstructures of cast material, upsetted material and four-pass processed material by ECAE at 498 K. As indicated in Fig. 2, the microstructure of cast material consists of coarse dendrites with the average grain size above 200 μm. Comparing with cast material, the microstructure of upsetted material consists of relatively fine grains with the average grain size of 120 μm due to limited plastic deformation obtained in upsetting process. In contrast, the microstructure of four-pass processed material by ECAE is composed of fine equiaxed grains with the average grain size of 2 μm. The severe plastic deformation occurring in ECAE processing leads to the grain refinement of processed material, which will contribute to prepare semi-solid billet with fine spheroidal microstructure in the following semi-solid isothermal treatment and improve the thixotropic behaviors of semi-solid billet [7]. Fig. 3 shows the engineering stress strain curves of cast material, upsetted material and four-pass ECAE processed material at 498 K under route B c. As indicated in Fig. 3, room temperature mechanical properties of four-pass processed materials by ECAE, such as yield strength (YS), ultimate tensile strength (UTS) elongation, Fig. 3. Engineering stress strain curve of cast material and four-pass processed material by ECAE at 498 K under route B C.

4 J. Jiang et al. / Materials Characterization 58 (2007) are improved heavily. There is only a little plastic deformation character in the engineering stress strain curve of cast material, which implies that cast AZ91D alloy is characterized by brittleness. In contrast, there is a clear plastic deformation character in engineering stress strain curve of upsetted material and four-pass processed materials by ECAE, which implies that ductility of upsetted and four-pass processed AZ91D alloy is enhanced effectively and it has become a ductile material. The yield strength (YS), ultimate tensile strength (UTS) and elongation of four-pass processed material by ECAE increased to MPa, MPa and 13.2%, respectively, from 85.8 MPa, MPa and 0.87% of cast material. Comparing with those of cast material, room mechanical properties of upsetted material such as YS, UTS and elongation are also enhanced. However, the increasing extent of room temperature mechanical properties between cast material and upsetted material is less than that between cast material and four-pass ECAE processed material. It is evident that the increasing extent of room temperature mechanical properties of four-pass processed material by ECAE is very large. The increasing extent of room mechanical properties such as YS, UTS and elongation between cast material and four-pass processed material by ECAE are 115%, 197% and 1420%, respectively, which are more than increasing extents of 60%, 76% and 573% between cast material and upsetted material. The reason for the difference of mechanical properties of samples of cast material, upsetted material and fourpass processed material by ECAE lies in the different grain size of these materials. The grain size of cast material is very coarse, which leads to the lowest mechanical properties. Comparing with cast material, the grain size of upsetted material relatively decreases a little, which leads to some improvement of mechanical properties. Comparing with the former two materials, the grain size of four-pass ECAE processed material is very fine, leading to the great enhancement of mechanical properties Microstructure of semi-solid billet prepared by new SIMA In this study, new SIMA is defined as a method for preparing semi-solid billet, in which cast material firstly is four-pass processed by ECAE, and then semisolid isothermal treated for 25 min at 818 K, that is, heated to semi-solid temperature of 818 K and the low melting point phases partially are melt in holding time of 25 min. In contrast, common SIMA is defined as a method for preparing semi-solid billet, in which cast material firstly is upsetted by the deformation degree 30% corresponding to equivalent strain of 0.3, and then semi-solid isothermal treated for 25 min at 818 K. Fig. 4 shows the optical micrographs of semi-solid billet of AZ91D alloy prepared by common SIMA and new SIMA. As indicated in Fig. 4, the average grain size of semi-solid billet prepared by new SIMA is about 5 μm and more fine spheroidal grains occur in the microstructure of semi-solid billet. In contrast, the average grain size of semi-solid billet prepared by common SIMA is about 180 μm, which is larger than that of semi-solid billet prepared by new SIMA. Furthermore, the microstructure of semi-solid billet prepared common SIMA is very inhomogeneous and some grains does not reach spheroidal shape. The different microstructures with different grain size before semi-solid isothermal treatment result in the different average grain size of semi-solid billet. In new SIMA, due to severe plastic deformation, the microstructure of four-pass ECAE processed material is refined well and more fine equiaxed grains occur in the microstructure before semi-solid isothermal treatment. The microstructure with fine equiaxed grains with average grain size of 2 μm will accelerate the Fig. 4. Optical micrographs of semi-solid billet of AZ91D alloy prepared by common SIMA and new SIMA.

5 194 J. Jiang et al. / Materials Characterization 58 (2007) Fig. 5. Macrophotograph of satellite angle frame components formed by semi-solid processing. Note the left three components are semisolid processed using semi-solid billet prepared by common SIMA, the right three components are semi-solid processed using semi-solid billet prepared by new SIMA. formation of semi-solid billet with fine spheroidal grains. However, due to elevated temperature, the grains in semi-solid state will grow, which leads to grains' growing into 5 μm from average grain size of 2 μm of microstructure of four-pass processed material by ECAE. The semi-solid billets with different average grain size have different viscosities in semi-solid state, which leads to different resistance to deformation during semi-solid processing. Comparing with common SIMA, lower resistance to deformation is required for semi-solid billet with fine spheroidal grains prepared by new SIMA during semi-solid processing and its ability to fill the die cavity is stronger Semi-solid processing of semi-solid billet prepared by new SIMA In order to demonstrate that new SIMA is a good method for preparing semi-solid billet of AZ9D alloy, semi-solid processing experiments of satellite angle frame components using semi-solid billets prepared by common SIMA and new SIMA were done. Fig. 5 shows the macrophotograph of satellite angle frame components formed by semi-solid processing. Here, semi-solid processing technology used is called semisolid extrusion. It is defined as a forming process in which semi-solid billets with spheroidal solid grains and liquid phase are extruded into final products in a preheated die. In semi-solid extrusion process of satellite angle frame component, the technological parameters involve the pressure of 2000 KN, preheating temperature of 623 K, ECAEed billet's holding time of 25 min and ECAEed billet's heating temperature of 818 K. In Fig. 5, the left three components are semisolid processed using semi-solid billets prepared by common SIMA, the right three components are semisolid processed using semi-solid billets prepared by new SIMA. As indicated in Fig. 5, there is no evident difference between two kinds of satellite angle frame components in the macrograph. The surface quality and dimensional accuracy are all very high due to advantages of semi-solid extrusion. However, there is an evident difference between two kinds of satellite angle frame components in the micrographs (Fig. 6). As indicated in Fig. 6, comparing with common SIMA, the satellite angle frame components formed using semisolid billet prepared by new SIMA have finer spheroidal grains and more homogeneous microstructure. The average grain size of satellite angle frame components formed using semi-solid billet prepared by new SIMA is about 5 μm. In contrast, the average grain size of satellite angle frame components formed using semi-solid billet prepared by common SIMA is about 150 μm. Furthermore, the grain shape still remains close to spheroidal shape after semi-solid billets are semi-solid processed, which illustrates that during semi-solid processing plastic deformation of solid Fig. 6. Optical micrographs of satellite angle frame components using semi-solid billet prepared by (a) common SIMA and (b) new SIMA.

6 J. Jiang et al. / Materials Characterization 58 (2007) Table 1 Mechanical properties at room temperature and at 393 K of components formed by semi-solid processing using semi-solid billets prepared by common SIMA and new SIMA Mechanical properties YS at 293 K phase mainly depends on rotation and sliding among solid phases. Table 1 presents the mechanical properties of satellite angle components using semi-solid billet prepared by common SIMA and new SIMA at room temperature and 393 K. Room temperature mechanical properties of satellite angle frame components formed using semisolid billet prepared by new SIMA are improved heavily, comparing with common SIMA. The yield strength (YS), ultimate tensile strength (UTS) and elongation increased to MPa, MPa and 16.1%, from MPa, MPa and 7.8%. As with room temperature mechanical properties, high temperature mechanical properties at 393 K, such as YS, UTS and elongation, also increased from 78.9 MPa, MPa and 13.2%, to MPa, MPa and 18.9%. Therefore, not only room temperature but also high temperature mechanical properties all are enhanced effectively by using semi-solid billets with fine spheroidal grains prepared by new SIMA. It is found that new SIMA is a desirable method for preparing semi-solid billet by which microstructure of semi-solid billet with fine spheroidal grains with the grain size of 5 μm and high mechanical properties of formed components formed, including at room temperature and 393 K can be obtained successfully. Furthermore, it is found that it is very good choice to apply ECAE to strain induced step in SIMA, which is a good engineering application of ECAE to semi-solid processing of magnesium alloy. 4. Conclusions UTS at 293 K Elongation at 293 K (%) YS at 393 K UTS at 393 K Common SIMA New SIMA Elongation at 393 K (%) Room temperature mechanical properties of AZ91D alloy, such as yield strength (YS), ultimate tensile strength (UTS) and elongation, are enhanced heavily by four-pass equal channel angular extrusion (ECAE) at 498 K under route B c and microstructure of AZ91D alloy is refined to the average grain size of 2 μm. Through using ECAE as strain induced step in SIMA and completing melt activated step by semisolid isothermal treatment, a new method called new SIMA is introduced firstly, by which semi-solid billet with fine spheroidal grains with the average grain size of 5 μm can be prepared successfully. The experimental results of satellite angle frame components semi-solid processed using semi-solid billet prepared by new SIMA demonstrate that high mechanical properties of formed components at room temperature and at high temperature of 393 K can be obtained successfully. Therefore, desirable microstructure with fine spheroidal grains and high mechanical properties of formed components at room temperature and high temperature of 393 K all show using ECAE as strain induced step in SIMA is a good choice and new SIMA is a good method for preparing semi-solid of AZ91D alloy. Acknowledgements Dr. Jufu. Jiang and Prof. Shoujing Luo are grateful to the Natural Science Foundation of China (NSFC) for support on those researches under Grant No and Grant No References [1] Liu D, Atkinson HV, Kapranos P, Jirattiticharoean W, Jones H. Microstructural evolution and tensile mechanical properties of thixoformed high performance aluminium alloys. Mater Sci Eng A 2003;361: [2] Tzimas E, Zavaliangos A. Evolution of near-equiaxed microstructure in the semisolid state. Mater Sci Eng A 2000;289: [3] Haga T, Kapranos P. Thixoforming of laminate made from semisolid cast strips. J Mater Process Technol 2004; : [4] Lapkowski W. Some studies regarding thixoforming of metal alloys. J Mater Process Technol 1998;80 81: [5] Haga T, Kapranos P. Billetless simple thixoforming process. J Mater Process Technol 2002; : [6] Chino Y, Kobata M, Iwasaki H, Mabuchi M. An investigation of compressive deformation behaviour for AZ91 Mg alloy containing a small volume of liquid. Acta Mater 2003;51: [7] Jiang JF. Research on preparing AZ91D magnesium alloy semi-solid billets by new SIMA method and thixoforging. PhD thesis, Harbin Institute of Technology, Harbin, PR China, 2005, pp [8] Senkonv ON, Senkova SV, Scott JM, Miracle DB. Compaction of amorphous aluminum alloy powder by direction extrusion and equal channel angular extrusion. Mater Sci Eng A 2005;393: [9] Sun PL, Kao PW, Chang CP. High angle boundary formation by grain subdivision in equal channel angular extrusion. Scr Mater 2004;51: [10] Mahesh S, Beyerlein IJ, Tome CN. Loading and substructureinduced irreversibility in texture during route C equal channel angular ectrusion. Scr Mater 2005;53:965 9.

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