Effects of quench aging treatment on microstructure and tensile properties of thixoformed ZA27 alloy

Transcription

1 Effects of quench aging treatment on microstructure and tensile properties of thixoformed ZA27 alloy T.-J. Chen*, Y. Hao and Y.-D. Li The effects of quench aging heat treatment on microstructure and tensile properties of thixoformed ZA27 alloy were investigated. The results indicated that the microstructure of the alloy became into polygonal b phase particles after solid solution treatment at 350uC for 48 h. The b particles then decomposed into a and g phases which would coarsen during the subsequent aging at 150uC. It was observed that the Zn concentration in the b phase near the polygonal boundaries was higher than that within the b particles. As a consequence, both the decomposition speed of the former b phase and the subsequent coarsening speed were faster than those of the latter b phase. Thus, a and g phases near the boundaries were always coarser than those within the particles during aging. Owing to the coarsening, the ultimate tensile strength continuously decreased with increasing aging time. The percentage elongation increased up to 10 h, but decreased with aging time owing to bad deformation accommodation and low bonding strength between particles. Cracks initiated from some defects (e.g. inclusions and porosities) during tensile test, and the path for the cracks to propagation changed with the aging time. Keywords: Thixoforming, ZA27 alloy, Quench aging treatment, Microstructure, Tensile properties Introduction Thixoforming is a novel metal processing technology which combines the elements of both casting and forging. It offers significant advantages, such as reducing macrosegregation and porosity and lowering forming efforts. 1 Therefore, in order to improve the mechanical properties of ZA27 alloy, a zinc based alloy with excellent tribological properties, this technology is applied because it suffers from easy formation of porosity owing to wide solidification interval. 2 It is well known that the performance of a final component is determined by its microstructure, and that the microstructure is determined by processing technology and subsequent heat treatment. So far, the present authors have studied microstructural evolution processes of ZA27 alloys with different initial microstructures during partial remelting and subsequent isothermal holding, 3 7 and effects of thixoforming parameters on the microstructure and mechanical properties of this alloy. 8,9 However, research on the effects of heat treatment on the microstructure and mechanical properties of this alloy is still scarce. In addition, according to the survey of the whole thixoforming field, most of the investigations have also been focused on production of non-dendritic semisolid slurry and forming methods. Key Laboratory of Gansu New Nonferrous Materials, Lanzhou University of Technology, Lanzhou , China *Corresponding author, chentj@lut.cn Only a few published papers involved the heat treatment of thixoformed aluminium or magnesium alloys The results indicated that the microstructural evolution during heat treatment and the resulting mechanical properties were obviously different from those of the corresponding alloys produced by traditional casting approaches owing to large difference in their as cast microstructures. Therefore, in order to verify the relationship of heat treatment, microstructure and mechanical properties of the thixoformed ZA27 alloy, the effects of quench aging treatment on the microstructure of this alloy and the resulting tensile properties were investigated in the present work. Experimental The nominal compositions of ZA27 alloy used for the present work were (26 28)Al ( )Cu 0. 2Zr ( )Mg (in wt-%), with a balance of Zn. The role of Zr was to achieve an as cast microstructure with fine dendrites through modification and then a non-dendritic semisolid microstructure being available for thixoforming could be obtained after partial remelting. 3 A quantity of the alloy was melted and then degassed by using C 2 Cl 6. Pouring followed at 550uC into permanent mould with ambient temperature to form rods of 190 mm long and 45 mm diameter. Chemical analysis shows that the actual compositions of the rod were Al 1. 86Cu 0. 17Zr 0. 02Mg (wt-%), with a balance of Zn. ß 2007 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 6 September 2006; accepted 21 September 2006 DOI / X Materials Science and Technology 2007 VOL 23 NO 5 535

2 Some small ingots, 50 mm long, were cut from these rods as the starting ingots for thixoforming. These small ingots were heated in an electric resistance furnace at semisolid temperature of 470uC for 90 min, and then transported to a die to be pressed using a 40 ton pressure. The die temperature of 300uC was employed. The thixoformed products are some slender rods of 165 mm long and 15 mm diameter. These slender rods were solution treated for 48 h at 360uC and then water quenched. Subsequently, they were aged for different durations ranging from 5 to 2880 min (48 h) at 150uC and then also water quenched. Finally, these rods were machined into tensile bars with a gauge of 40 mm and a diameter of 8 mm. For comparison, some traditional cast ingots and as thixoformed rods were directly machined into tensile bars. The tensile test was carried out on a universal testing machine at a nominal strain rate of s 21. The final ultimate tensile strength (UTS) and percentage elongation was the average of three bars. The fracture and side surfaces of the tested tensile specimens were examined by secondary and backscattered electron imaging on a scanning electron microscope (SEM) respectively. In order to verify the microstructural evolution during quench aging, the microstructures of the water quenched specimens were mainly observed by backscattered electron imaging on the SEM. In addition, in order to confirm if all of the phases in the as thixoformed alloy transformed into single b phase after solution treatment, one as thixoformed rod and one solution treated rod were examined by an X-ray diffractometer (XRD) with Nickel filtered Cu K a radiation and a scanning speed of 0. 02u (2h) per second. Results and discussion Effect of quench aging treatment on microstructure Figure 1 presents the microstructures of the traditional cast ZA27 alloy, the as thixoformed ZA27 alloy and the thixoformed ZA27 alloy after being solution treated at 360uC for 48 h. It can be seen that the microstructure of the traditional cast alloy consisted of primary dendrites (in black) and interdendritic eutectics (in grey and white) (Fig. 1a). But the microstructure of the thixoformed alloy consisted of spherical primary particles (in black) and interparticle secondarily solidified structures (solidified from the liquid phase after the semisolid slurry filling mould, also in grey and white) (Fig. 1b). This indicated that the thixoforming process had a significant effect on the microstructure of the alloy. The small white particles (surrounded by grey structure) within the primary particles, similar to the interparticle secondarily solidified structures, were also from solidification of the liquid phase in semisolid state. 8,9 The microstructure is characterised by liquid pools entrapped within the primary particles. After solution treatment, its microstructure evolved into a uniform grey structure without obvious boundaries (Fig. 1c). A previous investigation indicated when the thixoformed ZA27 alloy had just solidified, its microstructure was composed of primary a phase, peritectic b phase and eutectic g and b phases. 8 All of the a and b phases were unstable and would decompose into equilibrium phases of a, g and a little e phase during subsequent 1 Images (SEM) of a as cast ZA27 alloy, b as thixoformed ZA27and c thixoformed ZA27 alloy after being solution treated cooling and natural aging. 15 It is well known that there is a single b phase interval in the Zn Al binary equilibrium diagram and a single b phase can be obtained after solution treatment at this interval. The temperature of 360uC is just within this interval. 16 Therefore, in the present work this temperature was chosen as the solution treatment temperature so that a single b phase could be obtained after solution 536 Materials Science and Technology 2007 VOL 23 NO 5

3 2 X-ray diffractometer results of as thixoformed and solution treated thixoformed ZA27 alloys treatment. Figure 2 shows the results from XRD of the as thixoformed and solution treated thixoformed ZA27 alloys. It shows that the as thixoformed alloy consisted of a, g and e phases while the solution treated alloy had a single b phase, which was consistent with the above discussion. This implied that the uniform grey structure shown in Fig. 1b was the b phase. It must be noted that for the ZA27 alloy, a and e phases are in black, g phase is in white and b phase is in grey in the backscattered electron image. During the solution treatment at 360uC, the microstructure of the as thixoformed alloy would gradually transform into b phase in exhaustion of a, g and e phases by a reaction of a zg ze Rb. 15 The primary particles are the Al rich a phase and the b is a Zn rich phase, so their transformation needs a large amount of Zn. 4,7 However, the plentiful Zn rich phase existed in the form of interparticle eutectic g phase. Thus, Zn atoms in the eutectic g phase diffused towards the primary particles and the primary particles gradually became into b phase. Simultaneously, the decomposed b phase also transformed back into b phase. This resulted in the expansion of the primary particles till the neighbouring particles came into contact with each other. The contacting particles could not completely merge owing to the difference in their crystal orientations and a thin linear boundary always formed between them (Fig. 3a). Therefore, it can be expected that the solution treated microstructure was composed of polygonal particles. During the subsequent aging at 150uC, the b phase would decompose into a and g phases. In fact, the b phase had already decomposed because of its instability during the period from the quenching of the specimens to the SEM observations. From Fig. 3a some fine white particles could be obviously observed, which indicated that g phase had formed. During aging at 150uC, the decomposed products gradually coarsened as the aging time prolonged (Fig. 3b). Simultaneously, the linear boundaries became wide and obvious. The uniform grey structure then evolved into independent particles with obvious polygonalal boundaries (Fig. 3d and e). In addition, it was observed that the decomposed products from the former secondarily solidified structures were always coarser than those from the primary particles. This difference became more and more marked with increasing aging time. When the aging time was.16 h, the microstructure became similar to that of the as thixoformed alloy. The morphologies of the structures at the sites of the former secondarily solidified structures were distinctly differentiated from those of the primary particles (Fig. 3e and f). This can be more obviously seen by comparing the low magnification micrographs of these two kinds of alloys shown in Fig. 1b and Fig. 4. Previous investigations showed that the decomposition rate of b phase in Zn Al Cu alloys during aging depended on the concentration of Zn in the b phase and increased as the concentration increased. 17,18 Because the Zn rich eutectic g phase existed in the secondarily solidified structures and the atom diffusion in solid state was very limited, the composition distribution was not completely uniform although the alloy had been solution treated for 48 h at 360uC. The Zn concentration in the b phase from the secondarily solidified structures was still higher than that from the primary particles. This can be demonstrated by the result from EDXA on the SEM, which showed that the Zn concentration close to the linear boundaries was y while that apart from them was So the decomposition rate of b phase from the secondarily solidified structures and the subsequent coarsening speed of the decomposed products were higher than those within the primary particles. The resulting size of the former decomposed products was significantly coarser than that of the latter products. Effect of quench aging treatment on tensile properties The present results showed that the UTSs and percentage elongations of the as cast and as thixoformed alloys were MPa, 1. 54% and MPa, 2. 38% respectively. This implied that the thixoforming process could obviously improve the tensile properties of the alloy, especially the UTS. This was attributed that the thixoforming significantly decreased porosities. 9 Table 1 presents the results of the tensile properties of the thixoformed ZA27 alloy after being quench aged for different times. By comparing with the as thixoformed alloy, it was observed that the solution treatment increased the UTS and decreased the percentage elongation owing to the solution strengthening of Al and Cu elements. This was consistent with the common rule resulted from most of traditional cast materials. In order to visually seen the variations of the tensile properties with aging time, the values in Table 1 were presented as two dot curves as shown in Fig. 5. It shows that the UTS sharply decreased as the aging time increased to 3 h and then slowly decreased (Fig. 5b). However, the percentage elongation first increased and then decreased with the time (Fig. 5b). The percentage elongation was enhanced from the initial 0. 1% to a maximum value of 12. 5% after being aged for 10 h. This indicated that the main advantage of the quench aging treatment was to improve the ductility of this alloy through proper treatment. It is well known that aging treatment can strengthen a solution treated alloy by decomposition or precipitation. But in the present work, the UTS decreased continuously during aging. This indicated that the supersaturated b phase on quenching had already decomposed before tensile test. The microstructure discussed in section Effect of quench aging treatment on microstructure also showed that the b phase in the Materials Science and Technology 2007 VOL 23 NO 5 537

4 a 0h;b 0.5 h;c 3h;d 10 h; e 16 h; f 48 h 3 Images (SEM) of thixoformed ZA27 alloy after being quench aged for different times quenched alloy surely decomposed. So the present results (Table 1 and Fig. 5) showed only the overaging behaviour of the b phase. During overaging, the rapid coarsening of the decomposed products resulted in the sudden decrease in UTS (Fig. 5a). When the aging time was.3 h, the coarsening speed of the decomposed products might decrease owing to the increase in their sizes, so the decrease in the UTS with aging time Table 1 Ultimate tensile strength and percentage elongation of solution treated thixoformed ZA27 alloy after being aged for different periods Aging time, min (h) (1) 180 (3) 360 (6) 600 (10) 960 (16) 2160(36) 2880(48) UTS, MPa Elongation, % Materials Science and Technology 2007 VOL 23 NO 5

5 4 Image (SEM) of thixoformed ZA27 alloy after being quench aged for 48 h changed from sudden to slow. It was just because of softening from the coarsening that the percentage elongation obviously increased with the increase in aging duration within 10 h (Fig. 5b). But when the aging duration exceeded 10 h, the deformation accommodation became bad owing to the coarsening, and thus the elongation decreased with further prolonging aging time. It is well know that the g phase is a hard brittle phase. 2 The dissever role of the g phase particles to the matrix was enhanced as they coarsened during aging. Thus the deformation accommodation of a and g phases would obviously worsen when the g phase coarsened to some extent. As discussed above in section Effect of quench aging treatment on microstructure, the microstructure of the thixoformed ZA27 alloy after being solution treated was composed of polygonal particles and the polygonalal boundaries was so fine that they could be observed at only relatively higher magnification. It can be expected that the strength at the boundaries was the weakest in the alloy although the boundaries were very fine. So, similar to the as thixoformed state ZA27 alloy, cracks also preferentially propagated along the boundaries during tensile test. 9 In addition, a colattice or semicolattice relationship possibly formed between some neighbouring particles with small crystal mismatch through the long time solution treatment. The strength at such boundaries was possibly higher than that within the particles, and thus cracks might propagate within the particles. Observation on the fracture surface shows that there were some dimples and planes on the surface (Fig. 6a). The dimples might form from the cracks propagation within the particles. In fact, the 3D morphologies of the 2D polygonalal particles were polyhedral particles. Thus, it can be expected that the planes might form from the cracks propagation along the boundaries. These can be further demonstrated by the side view of the fracture area shown in Fig. 7a. The linear borders just corresponded to the planes on the fracture surface and the sinuate borders corresponded to the curved surfaces around the dimples. During the subsequent aging, the polygonal particles were softened and the boundaries became wide owing to coarsening of the decomposed products, which decreased the strength of the particles and the bonding 5 Variations of a UTS and b percentage elongation of solution treated thixoformed ZA27 alloy with aging time strength between particles respectively. Image (SEM) of the fracture surface of the alloy after being aged for 1 h showed some polyhedral particles on the surface (Fig. 6b). The side view of the fractured area obviously showed some linear borders formed from debonding between the boundaries (Fig. 7b). This implied that the crack preferred to propagate along the boundaries. That is to say that the weakening contribution to the boundaries was higher than that to the particles during aging, and thus the path to crack propagation gradually changed from the mixture of within the particles and along the boundaries to mainly along the boundaries as the aging proceeded. This is because that the decomposition of b phase close to the boundaries and the subsequent coarsening were all rapider than those within the particles. As the aging further proceeded, the coarsening especially within the particles progressed, and the particles were significantly softened. Their strength gradually became equivalent to the bonding strength of the boundaries. Therefore, the path to crack propagation became back to the mixture mode of within the particles and along the boundaries. Simultaneously, the plastic deformation was obviously improved. When Materials Science and Technology 2007 VOL 23 NO 5 539

6 6 Images (SEM) of fracture surfaces of solution treated thixoformed ZA27 alloy after being aged for a 0, b 1, c 10 and d 48 h aged for 10 h, the fracture surface was covered by lots of dimples with different sizes and showed no plane characteristics (Fig. 6c). As presented in Table 1 or Fig. 5b, the elongation was up to the maximum value of 12. 5%. The side view of tensile fractured area revealed obvious evidence about crack propagation within a particle (Fig. 7c). It also showed that both a and g phases largely deformed and a texture like microstructure parallel to the tensile orientation formed. In addition, it can be seen that a boundary (indicated in Fig. 7c by arrow) partially debonded and became wide, which implied that cracks could also propagate along boundaries. When the aging time was.10 h, the plane characteristics appeared again on the fracture surface. This is because that the bonding strength between particles gradually became inferior to that within the particles owing to the greater coarsening of a and g phases near the boundaries than those within the particles. Thus, cracks preferentially propagated along the boundaries, which can be demonstrated by Fig. 7d. In addition, it can be expected that the time for deformation during tensile test must decrease owing to the easy debonding between particles after aged for 10 h, and thus the percentage elongation decreased (Fig. 5b). So it can be concluded that the reasons why the elongation decreased after aging for 10 h include two aspects, bad deformation accommodation and low bonding strength between particles. The present results also indicated that, similar to the as thixoformed ZA27 alloy, cracks initiated from some defects, such as porosity and inclusions. 9 But the path to preferentially propagate within the primary particles or along the boundaries was determined by the aging time and the resulting microstructures. Conclusions 1. After being solid solution treated for 48 h at 350uC, the microstructure of thixoformed ZA27 alloy became into polygonalal b phase particles, and then the b phase decomposed into small a and g phases which would coarsen during the subsequent aging at 150uC. 2. The decomposition of b phase near the polygonal boundaries and the subsequent coarsening were rapider than those within the particles because the compositions at these two sites were different. 3. The UTS of the solution treated thixoformed ZA27 alloy continuously decreased, but the percentage elongation increased up to 10 h and then decreased as the aging time increased. 4. Crack initiated from some defects (e.g. inclusions and porosities) during tensile test and the path to preferentially propagate within the polygonalal particles or along the boundaries changed with the aging time. 540 Materials Science and Technology 2007 VOL 23 NO 5

7 7 Side views (SEM) of tensile fractured areas of solution treated thixoformed ZA27 alloy after being aged for a 0, b 1, c 10 and d 48 h Acknowledgements The authors would like to thank for financial support the Development Programme for Outstanding Young Teachers in Lanzhou University of Technology and the Opening Foundation of State Key Laboratory of Advanced Non-ferrous Materials. References 1. D. H. Kirkwood: Int. Mater. Rev., 1994, 39, E. Gervias, R. J. Barnhurst and C. A. Loong: J. Met., 1985, 37, T. Chen, Y. Hao, Y. Ma, S. Lu and G. Xu: Trans. Nonferrous Met. Soc. China, 2001, 11, T.-J. Chen, Y. Hao and J. Sun: J. Wuhan Univ. Tech. Mater. Sci. Edt., 2004, 19, T.-J. Chen, Y. Hao and J. Sun: J. Mater. Sci. Technol., 2001, 18, T.-J. Chen, Y. Hao and J. Sun: Mater. Sci. Eng. A, 2002, A337, T.-J. Chen, Y. Hao and J. Sun: J. Mater. Proc. Tech., 2004, 148, T.-J. Chen, Y. Hao, J. Sun and Y.-D. Li: Mater. Sci. Eng. A, 2005, A396, T.-J. Chen, Y. Hao, J. Sun and Y.-D. Li: Mater. Sci. Eng. A, 2004, A38, P. Cavalieret, E. Cerri and P. Leo: Mater. Charact., 2006, 55, P. Cavalieret, E. Cerri and P. Leo: J. Mater. Sci., 2004, 39, J. J. Blandin, D. Giunchi, M. SueÂry and E. Evangelista: Mater. Sci. Technol., 2002, 18, E. Cerri, M. Cabibbo and E. Evangelista: Mater. Sci. Eng. A, 2002, A33, Y.-B. Yu, P.-Y. Song, S. S. Kim and J. H. Lee: Scr. Mater., 1999, 41, Z.-M. Zhang, J.-C. Wang and G.-C. Yang: J. Mater. Sci., 2000, 35, T. Savaskan and S. Murphy: Mater. Sci. Technol., 1990, 6, X.-L. Xu, Z.-W. Yu, S.-J. Ji, J.-C. Sun and Z.-K. Hei: Acta Metall. Sinica, 2001, 14, T.-J. Chen and Y. Hao: Hot Work. Technol., 2005, 10, 4 7. Materials Science and Technology 2007 VOL 23 NO 5 541