- Email: [email protected]
Microstructure evolution during semi-solid powder rolling and post-treatment of 7050 aluminum alloy strips
Journal of Materials Processing Technology 214 (2014) 165–174
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Microstructure evolution during semi-solid powder rolling and post-treatment of 7050 aluminum alloy strips Yunzhong Liu, Xia Luo ∗ , Zhilong Li National Engineering Research Center of Near-Net-Shape Forming for Metallic Materials, South China University of Technology, Guangzhou 510640, PR China
a r t i c l e
i n f o
Article history: Received 16 April 2013 Received in revised form 30 August 2013 Accepted 31 August 2013 Available online 12 September 2013 Keywords: Semi-solid powder rolling Aluminum alloy strip Microstructure evolution Post-treatment
a b s t r a c t Semi-solid powder rolling (SSPR) combines semi-solid rolling with powder rolling to prepare highperformance metallic strips. Semi-solid powders were prepared under an inert atmosphere and subsequently rolled by a powder rolling machine. Conductive cooling between the pre-heated rollers and semi-solid powders results in a rapid solidification effect that is able to process alloys with a broad freezing range. The liquid in the semi-solid powders plays an important role in the microstructure evolution, which can improve the strength of strips. The 7050 aluminum alloy strips were obtained and used to evaluate the processing parameters and strip qualities for strips up to 100 mm wide and 1.5–2 mm thick. The process of semi-solid powder rolling was described and microstructure evolution during rolling and post-treatment was analyzed. The combination mechanism of semi-solid powders during rolling was also discussed. The results show that the best liquid fraction to prepare a strip ranges from 45 to 65%. Flowing and filling of liquid (>10%), densification by rolling and recrystallization (<10%) are the three combination mechanisms of the semi-solid powders during rolling. In addition, semi-solid powder rolled strips can be processed subsequently by hot rolling with the improved micro-hardness and relative density. © 2013 Elsevier B.V. All rights reserved.
1. Introduction High strength aluminum alloys such as AA7050 were used extensively for aerospace applications stated in the study by Lang et al. (2011). Flat or strip products are usually manufactured by conventional ingot metallurgy (I/M) process. Subsequently, the materials are further processed (e.g. heat treatment, hot rolling, cold rolling, etc.) to meet the required properties as shown in the study by Deshpande et al. (1998). So, the production run is long and energy wasting. In the 1970s, semi-solid forming proposed by Flemings (1991) can obtain a homogeneous defects-free globular microstructure based on the special properties of semisolid slurry. Kang et al. (1999) stated that semi-solid forming was mainly used to produce the low melting point alloys such as aluminum alloys. Later, Kiuchi and Kopp (2002) demonstrated that semi-solid rolling consisting of thixo-rolling and rheo-rolling was used to prepare metallic plates and strips at a laboratory scale. At present, rheo-rolling is not applied to industrial production because of the difficulty in slurry preparation on the production line, liquidsegregation and surface cracking as discussed by Govender and Moller (2008). Kang et al. (1997) stated that rheo-die castings were
∗ Corresponding author. Tel.: +86 20 87110099; fax: +86 20 87112111. E-mail address: [email protected] (X. Luo). 0924-0136/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2013.08.018
more into having surface crackings and Masuku et al. (2010) said that liquid segregation is easily formed during forming. These flaws have a negative effect on the mechanical properties as shown in Kim et al. (2007). Schaffer et al. (2001) stated that powder rolling can be used to make aluminum alloy strips. However, most green strips prepared by powder rolling have high porosity and need sintering or further processing to get dense materials. In addition, lubricant and binder are necessary for preparing the compacts. Then Zu and Luo (2001) studied semi-solid powder forming (SPF) and Wu et al. (2010a) discussed various processing routes. In general, here needs pre-compaction before heating in SPF as stated in Wu et al. (2010b). And it is mainly applied in the extrusion and compaction. In 2003, McHugh et al. (2004) proposed spray rolling which appeared to include the advantages of energy saving and high production rate while improving quality with a uniform fine-grain equiaxed microstructure. But this new strip/sheet manufacturing process needs dedicated and expensive apparatus, which leads to high costs. Therefore, a novel strip manufacturing process, termed “semisolid powder rolling”, is proposed by the authors. The general concept of semi-solid powder rolling is from the possible combination of semi-solid forming and powder rolling. Semi-solid powder rolling (SSPR) consists of semi-solid powder preparation under an inert atmosphere, semi-solid powder rolling at a relatively stable temperature, and post-treatment procedures if necessary. This new process is mainly used to prepare strips with high quality,
166
Y. Liu et al. / Journal of Materials Processing Technology 214 (2014) 165–174 658ºC
1.5
DSC Exothermic
1.0 0.5 524ºC
0.0 -0.5 -1.0
0
200
400 600 Temperature/ºC
800
1000
Fig. 2. DSC curve of 7050 aluminum alloy powders.
Fig. 1. Schematic of semi-solid powder rolling process.
especially for alloys that have a wide freezing range and low melting point such as aluminum alloy (AA7050, 2124, etc.). While still in the early stage of development, semi-solid powder rolling shows a promise for reducing strip manufacturing costs and saving energy while improving the qualities. The inherent rapid solidification, easily controllable process conditions and solid solubility extension may provide an interesting avenue for the development of alloys. The objective of this study is to analyze the best parameters to prepare strips and provide an insight into the mechanisms that govern the microstructure evolution during semi-solid powder rolling. The combination mechanism of semi-solid powders and the effect of liquid fraction on the microstructure evolution are emphasized. 2. Experimental procedures A schematic of the approach is shown in Fig. 1. The powders used in this work are the gas atomized 7050 aluminum alloy powders with a nearly spherical morphology (the mean particle size is about 75 m). The chemical composition of raw materials is summarized in Table 1. The experiments were carried out on a powder rolling machine with rollers pre-heated to 300 ◦ C so as to maintain the rolling temperature. The diameter of roller is 150 mm with a width of 100 mm, a rolling gap of 0.1 mm and a rotating speed of 0.4 rad/s. Firstly, in order to obtain the semi-solid powders, the gas atomized 7050 aluminum alloy powders were heated to the semi-solid temperature range and held for different time under an inert atmosphere (the actual freezing range of 7050 aluminum alloy powders is 524–658 ◦ C analyzed by DSC as shown in Fig. 2). Then, the semisolid powders were fed into the gap between pre-heated rollers and consolidated into dense strips. In order to analyze the effect of liquid fraction on microstructure evolution during rolling, the representative values of liquid fraction at each stage were selected as the processing parameters to prepare the strip (low liquid fraction, intermediate liquid fraction and high liquid fraction). Thus, the following parameters were selected: (1) heated at 555 ◦ C, held for 30 min, (2) heated at 585 ◦ C, held for 60 min, (3) heated at 625 ◦ C, held for 60 min, (4) heated at 640 ◦ C, held for 20 min, (5) heated at 650 ◦ C, held for 40 min and (6) heated at 650 ◦ C, held for 50 min. The strips up to 100 mm wide and 1.5–2.0 mm thick were obtained Table 1 Chemical composition of 7050 aluminum (wt%).
Fig. 3. A strip prepared by semi-solid powder rolling (heated at 625 ◦ C and held for 60 min).
with a good formability and a smooth surface as shown in Fig. 3. Subsequently, the semi-solid powder rolled strips were hot rolled at 471 ◦ C with 30% thickness reduction by using a laboratory rolling mill with a loading capacity of 240 t. All the hot rolled strips were treated with air cooling after hot rolling. Samples were discontinuously selected from the central zone along to the rolling direction. Finally, the relative density of each specimen was tested by using the Archimedes’ principle. The microstructures of the polished and etched samples were observed with an optical microscope (LcicaDMI500M). The phase identification was carried out by X-ray diffraction (XRD) analysis using D8 ADVANCE (Bruker, Germany) Cu (K␣1 + K␣2) radiation (the diffraction angle 2 is 10–90◦ , scan speed is 17.7 s/step). The micro-hardness was tested at HVs-100 Vickers, and the loading force was 1.96 N. For each sample, the micro-hardness was measured randomly at 6 different points, and then the mean value of micro-hardness as well as standard deviation were calculated and reported. The grain size was determined by using the linear intercept. Scanning electron micrograph (SEM) observation was carried out at NovaNanoSEM430 for identifying the morphology and distribution of second phase particles.
3. Results and discussion 3.1. Effect of liquid fraction on microstructure during semi-solid powder rolling Fig. 4 shows the microstructure of semi-solid powders prepared under different conditions and the corresponding liquid fraction is summarized in Table 2.
Table 2 liquid fraction at different temperatures.
Zn
Mg
Cu
Zr
Ti
Fe
Si
Al
6.43
2.26
2.02
0.13
0.03
0.11
0.07
Bal
Temperature(◦ C) Liquid fraction (%)
555 1.38
585 13.8
625 44.2
640 65.09
650 83.2
Y. Liu et al. / Journal of Materials Processing Technology 214 (2014) 165–174
167
Fig. 4. Microstructures of semi-solid powders prepared under different conditions: heated at 555 ◦ C for holding 30 min (a), heated at 585 ◦ C for holding 60 min (b), heated at 625 ◦ C for holding 60 min (c), heated at 640 ◦ C for holding 20 min (d), heated at 650 ◦ C for holding 40 min (e), heated at 650 ◦ C for holding 50 min (f).
For alloys, Mi and Grant (2008) adopted the Scheil equation to relate the alloy liquid fraction to temperature during solidification.
T − Ts (1/(1−k)) Tl − Ts
(1)
where Ts and Tl are the solidus and liquidus temperature of the alloy, respectively, and k is a partition coefficient. Fig. 5 shows the liquid fraction of 7050 aluminum alloy as a function of temperature. Actually it is a fit to the data calculated by MATLAB using Eq. (1) with k = 0.66. An isolated liquid phase forms within the particles firstly and then extends to the particle boundary as the liquid increases (Fig. 4a). When liquid fraction increases to 13.8%, some of the liquid phase within the particles may form a network (Fig. 4b). The grains in semi-solid powders show a dendrite structure. When the liquid fraction increases to 45%, the dendrite arms begin to break up (Fig. 4c and d), which is helpful for the formation of a good strip because Lashkari and Ghomshchi (2007) found that the flow resistance of a dendrite structure is much greater than that of a non-dendrite structure. So, the deformation resistance of materials with low liquid fraction is obviously greater than
100 90 80
liquid fraction (%)
fl =
that of materials with high liquid fraction under the same rolling force. With a higher liquid fraction, the liquid phase forms a connected network and begins to agglomerate (Fig. 4e and f). Fig. 6 illustrates representative microstructures of the strips prepared
70 60 50 40 30 20 10 0
520
540
560 580 600 temperature (ºC)
620
640
660
Fig. 5. Effect of temperature on liquid fraction of 7050 aluminum alloy powders.
168
Y. Liu et al. / Journal of Materials Processing Technology 214 (2014) 165–174
Fig. 6. Microstructures of strips prepared under different conditions: heated at 555 ◦ C for holding 30 min (a), heated at 585 ◦ C for holding 60 min (b), heated at 625 ◦ C for holding 60 min (c), heated at 640 ◦ C for holding 20 min (d), heated at 650 ◦ C for holding 40 min (e), heated at 650 ◦ C for holding 50 min (f).
under different conditions. Microstructure of the strip prepared by heating at 555 ◦ C and holding for 30 min is characterized by a randomly distributed rosette structure with isolated pores and some deformed or broken powder particles (Fig. 6a). As shown in Fig. 7, its relative density and micro-hardness are low (the microhardness is 77.32 HV, and the relative density is 87.1%). So, the strip prepared under a low liquid cannot gain a satisfactory quality. When the strip was prepared at 585 ◦ C for 30 min, the original particle boundaries and most pores disappear (Fig. 6b). However, its micro-hardness and relative density just increase slightly. Those microstructures of strips prepared with liquid fraction ranging from 45 to 65% are characterized by a few rosettes, some dendrite grains and fine equiaxed grains (Fig. 6c and d). There are no original powder boundaries in the microstructures. The micro-hardness dramatically increases to 161.6 HV when the liquid fraction reaches 45%, and then gets to the peak when the liquid fraction is 65%. When the liquid fraction increases to 85%, the strips consist of flaws and coarse rosette-structure grains (Fig. 6e and f). The micro-hardness and relative density decrease fractionally as well. Therefore, the strip prepared with very low liquid fraction (<20%) has many original powder boundaries and pores and cannot become a dense
strip. Very high liquid fraction (>65%) in the semi-solid powders results in the coarse grains and poor quality such as flaws and defects. The best liquid fraction to prepare good strips for SSPR is 45–65%. microhardness HV relative density %
200 180 160 140 120 100 80 60 555-30
585-60
625-60
640-20
650-40
650-50
Fig. 7. Relative density and micro-hardness of strips prepared under different conditions.
Y. Liu et al. / Journal of Materials Processing Technology 214 (2014) 165–174
169
Fig. 8. SEM photograph of strip prepared by heating at 640 ◦ C and holding for 20 min (a), the original 7050 aluminum alloy powders (b). Tables summarize EDS composition analysis of phases.
Wu and Kim (2011) discussed the effect of different amount of liquid on the powder compaction in a semi-solid state. In SSPR, it can be divided into five ranges to discuss its effect on the microstructure of strips. In the initial stage, the liquid fraction is lower than 10%. The liquid phase is isolated within the particle during rolling. There is no liquid to fill in the interparticle gaps. In the second range, the liquid fraction is between 10 and 20%. On the one hand, for aluminum alloys, Sercombe (2003) stated that gaining a high density is not possible despite being sintered at a temperature that results in ∼20% liquid phase. On the other hand, Atkinson and Liu (2008) demonstrated that too little liquid in powders and the presence of some particles inhibiting the migration of liquid film along the grain boundary are associated with the low coarsening rates. Consequently, some pores and coarse dendrites still exist. The third range corresponds to the transition period that changes a strip from low density to nearly full density. In this stage there is about 20–45% liquid fraction in powders (the corresponding temperature is ∼625 ◦ C). The metallurgical bonding occurs accompanying with disappearance of powder boundaries during rolling. In the fourth range, liquid fraction ranges from 45 to 65%. Appropriate liquid not only fills in the gap but also reduces the deformation stress, which leads to easy deformation. Wu and Kim (2011) said that the interlocking of irregular of solid phase arms may have result in strengthening of the materials. In addition, as liquid fraction increases, the deformation temperature is also high. So there is enough time for liquid phase to flow among the powder particles, which results in full consolidation. In the final range, the liquid fraction
increases over 65%. Atkinson and Liu (2010) found that too much liquid results in high coarsening rates during cooling and the loss of some liquid materials during rolling. Fig. 7 shows that the errors of micro-hardness obtained in the final range are extremely large. So a very high liquid fraction results in an uneven distribution of microhardness. It may be due to the coarse precipitates and particles, defects and flaws in the microstructures. McHugh et al. (2004) found that in spray rolling, there is 15–30% liquid fraction in as-deposited materials and Liang and Lavernia (1993) found that there is 20–50% liquid fraction in spray deposition. However, the best liquid fraction to prepare the strip in SSPR ranges from 45 to 65%. It is due to that the measured porosity of cold rolled green strip is in the range 30–50% and there is about 10–20% liquid still isolated within the particles during rolling. If all the interparticle gaps are filled with the liquid, there needs about 40–70% liquid under an ideal condition. However, the conductive heat transfer between semi-solid powders and rollers results in solidification. Then some new posterior particle boundaries will be created, which generates new solid/solid interfaces during rolling. In addition, some reactive elements such as aluminum react with the adsorbed oxygen and form highly stable oxides (Al2 O3 ) at the powder particle surface as shown in Fig. 8. The EDX measurement in the rectangular zone shows that the oxide film formed around the powder particles as shown in Fig. 8b and the corresponding table. The oxide layer on the powder surface has a significant effect on the microstructure evolution during semi-solid powder rolling. It may impede or prevent the diffusion and combination between the powders. Therefore, more liquid should be required to fill in the gap.
170
Y. Liu et al. / Journal of Materials Processing Technology 214 (2014) 165–174
Fig. 9. Scanning electron micrographs of semi-solid powder rolled strips (a and b) and hot rolled strips(c and d) prepared under different conditions: heated at 625 ◦ C for holding 60 min (a) and (c), heated at 640 ◦ C for holding 20 min (b) and (d).
3.2. Microstructure evolution during semi-solid powder rolling Fig. 9 shows the scanning electron micrographs (SEM) of the semi-solid powder rolled strips and hot rolled strips. X-ray radiation diffraction (XRD) was used to identify the phases. Many gray or white phase particles can be observed around the grain boundaries (Fig. 9a) and within some grains (Fig. 9b). EDX analysis of the constituents (light phase A) and surrounding matrix (dark phase B) is summarized in the table accompanying the photomicrograph (Fig. 8a). It shows that these particles are rich in Zn, Cu and Mg. Robson (2004) said they may correspond to Mphase Mg(Zn2 , AlCu) or -phase (MgZn2 ), S-phase (Al2 CuMg) or the T-phase Al32 (Mg,Zn)49 . Based on the X-ray diffraction analysis (lines a and b in Fig. 10), they are the -phase (MgZn2 ) and S-phase Al2 CuMg. Additionally, some oxide particles may exist in the microstructures according to the EDX analysis results although no corresponding peak is found in the XRD analysis, and these oxide particles may come from the original powders or from the reaction during rolling. Zabihi et al. (2013) have studied the aluminum matrix composites (AMC) with different percentage of Al2 O3 particles, and the results show that these alumina particles can improve the mechanical properties of AMC. And the existence, origin and effect of these oxide particles are discussed in the authors’ another article. In particular, the constituents of light phases in the semi-solid powder rolled strips are enriched more in Cu and Zn and with no Fe compared with those in the sprayrolled materials and the commercial materials as shown in McHugh et al. (2008). When the liquid fraction is between 45 and 65%, phases precipitate more spherical and smaller as the liquid fraction increases.
To analyze the microstructure evolution of semi-solid powder rolling process, the rollers were stopped simultaneously during rolling. A cross-section of the resulting wedge reveals four sections as shown in Fig. 1. Section 1: materials supplying region where the semi-solid powders are fed into the gap and begin to be dragged in. Section 2: drag-in region (from the drag-in plane to the neutral plane) where the semi-solid powders are dragged in the rollers and initially compacted.
Fig. 10. X-ray diffraction analysis of semi-solid powder rolled strips (a and b) and hot rolled strips (c and d) prepared under different conditions: heated at 625 ◦ C for holding 60 min (a) and (c), heated at 640 ◦ C for holding 20 min (b).
Y. Liu et al. / Journal of Materials Processing Technology 214 (2014) 165–174
171
Fig. 11. Longitudinal section micrographs illustrating microstructure evolution during semi-solid powder rolling: heated at 585 ◦ C for holding 60 min (a), heated at 625 ◦ C for holding 60 min (b).
Section 3: densification region (from the neutral plane to the rollaxis plane) where the semi-solid powders are deformed and fully consolidated under the action of rollers. Section 4: product strip.
Fig. 11 illustrates representative microstructures in each of these sections. Semi-solid powders in section 1 are accumulated randomly with a slight deformation. The original powder boundaries can be observed clearly. Basically, the semi-solid powders
Fig. 12. Microstructures of strips prepared by heating at 555 ◦ C and holding for 30 min (a), heating at 585 ◦ C and holding for 60 min by the edge (b).
172
Y. Liu et al. / Journal of Materials Processing Technology 214 (2014) 165–174
Fig. 13. Microstructures of strips after single-pass hot rolling with 30% thickness reduction: heated at 555 ◦ C for holding 30 min (a), heated at 585 ◦ C for holding 60 min (b), heated at 625 ◦ C for holding 60 min (c), heated at 640 ◦ C for holding 20 min (d), heated at 650 ◦ C for holding 40 min (e), heated at 650 ◦ C for holding 50 min (f).
remain apart in this zone. In section 2, deformation and combination among some powders occur. But many isolated pores can still be observed. Thus, the powders are just dragged in and initially compacted in this zone. Densification takes place mainly in section 3 where the metallurgical bonding occurs among powders with the disappearance of pores and powder boundaries. And then dense strips are obtained as shown in section 4. As a result, deformation, solidification and densification occur simultaneously and collaboratively during rolling. In SSPR, the powder particles jointed via three approaches: (1) Densification by rolling. It mainly occurs in the rolling region and is similar to the mechanism of hot densification rolling discussed by Vajpai et al. (2011). At last, strong metallurgical bonding between powder particles takes place with large deformation and higher temperature. (2) Recrystallization. It is well known that recrystallization proceeds by the nucleation and the migration of the new grain boundaries into the deformed matrix leaving behind the ‘strain-free grains’. Doherty et al. (1997) sated that typical
nucleation sites all have high local misorientations. For gas atomized AA7050 powders, their grain orientations are free and they have large-angle boundaries with the activation energies of 70.5 kJ/mol, which is indirectly calculated. At first, semi-solid powders with a low liquid fraction have a high deformation stress during deformation, and store much deformation energy. And then the deformation degree among the contact surfaces is larger and the stored energy of deformation is higher than other parts. Due to the deformation accompanying with high temperature, nuclei formed firstly in the inter-particle contact areas. Subsequently, nuclei boundaries moved into the contiguous particles resulting in the disappearance of particle boundaries. Barrett (1940) suggested that the mobility of nuclei boundaries depend on orientation relationship. So some nuclei boundaries satisfying the orientation relationship can grow into the contiguous particles as shown in A and B of Fig. 12a. This kind of combination mechanism just occurs when liquid fraction in powders is low (<10%). (3) Flowing and filling of liquid, it mainly happens when liquid fraction is higher
Y. Liu et al. / Journal of Materials Processing Technology 214 (2014) 165–174
microhardness HV relative density %
260 250 240 230 220 210 200 190 180 170 160 150 140 130 120 110 100 555-30
585-60
625-60
640-20
650-40
650-50
Fig. 14. Relative density and microhardness of strips after single-pass hot rolling with 30% thickness reduction.
(>10%). With a higher liquid, the liquid phase forms a connected network and flows into the gap between the particles during rolling and then solidifies, which results in the disappearance of particle boundaries (Fig. 12b with the arrow). Yu et al. (2001) discussed the whole procedure of liquid phase sintering and supersolidus liquid phase sintering and Showaiter and Youseffi (2008) studied the effect of liquid on the sintering during densification. Seen from this perspective, the flowing and filling of liquid is similar to the liquid phase sintering. Therefore, recrystallization and densification by rolling are the two combination mechanisms among powders when the liquid fraction is low (<10%), while the flowing and filling of liquid is the main combination mechanism when liquid fraction is higher (>10%). 3.3. Microstructure evolution during post-treatment Generally, the semi-solid powder rolled strips may have a few pores. In order to improve the density and mechanical properties deeply, post-treatment would be necessary. Therefore, hot rolling is used in this study. Fig. 13 shows the microstructures of strips after single-pass hot rolling with 30% thickness reduction. The microstructures consist of fine grains (∼43 m). It can be seen that there is no evidence of apparent deformation or grain elongation in the direction of rolling in Fig. 13a and b, which is different from the rest of the pictures. From Fig. 13c–f, dynamic recrystallization is not complete in many regions. In some areas recrystallization occurs partially at grain boundaries (Fig. 13c with the arrow). Fig. 13e and f illustrates that most of grains prefer to be elongated in the direction of rolling. It means that the recrystallized region is small in strips prepared with a high liquid fraction and large in strips produced with a low liquid fraction. Fig. 14 shows the relative density and micro-hardness after hot rolling. The relative density (all of specimens over 99%) and microhardness are improved dramatically. In short, the following hot rolling is helpful for improving the mechanical properties of the semi-solid powder rolled strips. Fig. 9c and d shows that there are no secondary particles in SEM. And the diffraction peaks of secondary particles in hot-rolled strips disappear based on the lines c and d in Fig. 10. It was demonstrated that nearly complete densification during hot rolling occurs through various stages. With a low liquid fraction (<10%), the main mechanism of hot densification can be divided into three stages proposed by Vajpai et al. (2011). With a relatively high liquid fraction (>45%), the mechanism of hot densification is similar to that of the nearly dense strips discussed by Zhan et al. (2008). Plastic deformation during hot rolling results in dynamically recrystallized grains and elongated grains. But the final stage
173
of densification mechanism with low liquid fractions is the same as that with high liquid fractions. Consequently, semi-solid powder rolling has many different features compared with powder rolling and semi-solid rolling: 1. The semi-solid powder rolled strips consist of rapid solidification microstructure and features of semi-solid microstructure. There is a temperature gradient between semi-solid powders and rollers. Powders with amounts of liquid (from 5 to 85%) are fed into the gap between the pre-heated rollers, where it solidifies and forms a strip up to about 2 mm thick. Based on the results of microstructure evolution, it infers that solidification, deformation and densification occur simultaneously and collaboratively during semi-solid powder rolling. 2. The semi-solid powder rolled strips have a high density compared with those by powder rolling, because the powders for semi-solid powder rolling consist of amounts of liquid, which can serve as a “binder” like in powder rolling. The materials introduced to the rollers in powder rolling are still in a solid state. But powders used in semi-solid powder rolling have a “slushy” character. In addition, some powders will break up under the action of rolling force. Then, the liquid can flow among powders, which attributes to the consolidation. 3. It can gain fine-grain equiaxed microstructure and high solute content because of the high cooling rate, which can improve the mechanical properties significantly. Especially, strips of aluminum alloys with broad freezing range, such as AA7050, can be prepared successfully by semi-solid powder rolling. 4. Conclusions (1) SSPR is a new strip manufacturing technology that shows a promise for processing a wide variety of aluminum alloys, especially for wide freezing range alloys, such as AA7050. By combining semi-solid rolling with powder rolling, aluminum alloy strips can be prepared successfully with a nearly full density and high micro-hardness. The processing parameters can be controlled easily and simply. (2) The best liquid fraction to prepare a good strip is 45–65%. The microstructure of strip is sensitive to the liquid fraction. Too low or too high liquid fraction results in a poor microstructure with isolated pores, cracking or flaws, which affects the mechanical properties of strips. There are three combination mechanisms in semi-solid powder rolling: flowing and filling of liquid (>10%), densification by rolling and recrystallization (<10%). Hot rolling is an effective post-treatment to improve the mechanical properties of strips. Acknowledgement The authors gratefully acknowledge the financial support of Fundamental Research Funds for the Central Universities (Grant No. 2011ZZ0010). References Atkinson, H.V., Liu, D., 2008. Microstructural coarsening of semi-solid aluminium alloys. Mater. Sci. Eng. A 496, 439–446. Atkinson, H.V., Liu, D., 2010. Coarsening rate of microstructure in semi-solid aluminium alloys. Trans. Nonferrous. Metall. Soc. China 20, 1672–1676. Barrett, C.S., 1940. Trans. AIME 137, 128. Deshpande, N.U., Gokhale, A.M., Denzer, D.K., Lium, J., 1998. Relationship between fracture toughness, fracture path, and microstructure of 7050 aluminum alloy: part I. Quantitative characterization. Metal. Mater. Trans. A 29, 1191–1201. Doherty, R.D., Hughes, D.A., Humphreys, F.J., et al., 1997. Current issue in recrystallization: a review. Mater. Sci. Eng. A 238, 219–274. Flemings, M.C., 1991. Behavior of metal alloys in the semi-solid state. Metal. Trans. 22, 957–998.
174
Y. Liu et al. / Journal of Materials Processing Technology 214 (2014) 165–174
Govender, G., Moller, H., 2008. Evaluation of surface chemical segregation of semisolid metal cast aluminium alloy A356. Solid State Phenom. 141–143, 433–438. Kiuchi, M., Kopp, R., 2002. Mushy/semi-solid metal forming technology—present and future. Manufact. Technol. 51, 653–670. Kang, C.G., Yoon, J.H., Seo, Y.H., 1997. The upsetting behavior of semi-solid aluminum material fabricated by a mechanical stirring process. J. Mater. Proc. Technol. 66 (12), 30–38. Kang, C.G., Choi, J.S., Kim, K.H., 1999. The effect of strain rate on macroscopic behavior in the compression forming of semi-solid aluminum alloy. J. Mater. Process. Technol. 88, 159–168. Kim, W.Y., Kang, C.G., Kim, B.M., 2007. Deformation behavior of wrought aluminum alloys in incremental compression experiment with a closed die. J. Mater. Process. Technol. 191, 372–376. Lang, Y.J., Cai, Y.H., Cui, H., Zhang, J.S., 2011. Effect of strain-induced precipitation on the low angle grain boundary in AA7050 aluminum alloy. Mater. Des. 32, 4241–4246. Lashkari, O., Ghomshchi, R., 2007. The implication of rheology in semi-solid metal processes: an overview. J. Mater. Process. Technol. 182, 229–240. Liang, X., Lavernia, E.J., 1993. Evolution of interaction domain microstructure during spray deposition. Metall. Mater. Trans. A 25, 1994–2341. Masuku, E.P., Moller, H., Curle, U.A., 2010. Influence of surface liquid segregation on corrosion behavior of semi-solid metal high pressure die cast aluminium alloys. Trans. Nonferrous Met. Soc. China 20, s837–s841. McHugh, K.M., Delplanque, J-P., Johnson, S.B., Lavernia, E.J., Zhou, Y., Lin, Y., 2004. Spray rolling aluminum alloy strip. Mater. Sci. Eng. A 383, 96–106. Mi, J., Grant, P.S., 2008. Modelling the shape and thermal dynamics of Ni superalloy rings during spray forming. Part 2. Thermal modelling-Heat flow and solidification. Acta Metall. 56, 1597–1608. McHugh, K.M., Zhou, Y., Johnson, S.B., et al., 2008. Microstructure evolution during spray rolling and heat treatment of 2124 Al. Mater. Sci. Eng. A 447, 26–34.
Robson, J.D., 2004. Microstructural evolution in aluminium alloy 7050 during processing. Mater. Sci. Eng. A 382, 112–121. Schaffer, G.B., Sercombe, T.B., Lumley, R.N., 2001. Liquid phase sintering of aluminium alloys. Mater. Chem. Phys. 67, 85–91. Sercombe, T.B., 2003. On the sintering of uncompacted, pre-alloyed Al powder alloys. Mater. Sci. Eng. A 341, 163–168. Showaiter, N., Youseffi, M., 2008. Compaction, sintering and mechanical properties of elemental 6061 Al powder with and without sintering aids. Mater. Des. 29, 752–762. Vajpai, S.K., Dube, R.K., Sangal, S., 2011. Microstructure and properties of Cu–Al–Ni shape memory alloy strips prepared via hot densification rolling of argon atomized powder preforms. Mater. Sci. Eng. A 529, 378–387. Wu, Y.F., Kim, G.Y., Anderson, I.E., Lograsso, T.A., 2010a. Fabrication of Al6061 composite with high SiC particle loading by semi-solid powder processing. Acta Metall. 58, 4398–4405. Wu, Y.F., Kim, G.Y., Anderson, I.E., Lograsso, T.A., 2010b. Experimental study on viscosity and phase segregation of Al–Si powders in microsemisolid powder forming. J. Manufact. Sci. Eng. Trans. ASME 132, 0110031–0110037. Wu, Y.F., Kim, G.Y., 2011. Compaction behavior of Al6061 powder in the semi-solid state. Powder Technol. 214, 252–258. Yu, F.X., Cui, J.Z., Rangathan, S., 2001. Fundamental differences between spray forming and other semisolid processes. Mater. Sci. Eng. A 304–306, 621–626. Zu, L.J., Luo, S.J., 2001. Study on the powder mixing and semi-solid extrusion forming process of SiCp/2024Al composites. J. Mater. Process. Technol. 114, 189–193. Zabihi, M., Toroghinejad, M.R., Shafyei, A., 2013. Application of powder metallurgy and hot rolling processes for manufacturing aluminum/alumina composite strips. Mater. Sci. Eng. A 560, 567–574. Zhan, M.Y., Chen, Z.H., Yan, H.G., 2008. Two novel rolling processes of porous metal preforms prepared by spray deposition technique using conventional rolling mill. J. Mater. Process. Technol. 202, 269–274.
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Microstructure evolution during semi-solid powder rolling and post-treatment of 7050 aluminum alloy strips Yunzhong Liu, Xia Luo ∗ , Zhilong Li National Engineering Research Center of Near-Net-Shape Forming for Metallic Materials, South China University of Technology, Guangzhou 510640, PR China
a r t i c l e
i n f o
Article history: Received 16 April 2013 Received in revised form 30 August 2013 Accepted 31 August 2013 Available online 12 September 2013 Keywords: Semi-solid powder rolling Aluminum alloy strip Microstructure evolution Post-treatment
a b s t r a c t Semi-solid powder rolling (SSPR) combines semi-solid rolling with powder rolling to prepare highperformance metallic strips. Semi-solid powders were prepared under an inert atmosphere and subsequently rolled by a powder rolling machine. Conductive cooling between the pre-heated rollers and semi-solid powders results in a rapid solidification effect that is able to process alloys with a broad freezing range. The liquid in the semi-solid powders plays an important role in the microstructure evolution, which can improve the strength of strips. The 7050 aluminum alloy strips were obtained and used to evaluate the processing parameters and strip qualities for strips up to 100 mm wide and 1.5–2 mm thick. The process of semi-solid powder rolling was described and microstructure evolution during rolling and post-treatment was analyzed. The combination mechanism of semi-solid powders during rolling was also discussed. The results show that the best liquid fraction to prepare a strip ranges from 45 to 65%. Flowing and filling of liquid (>10%), densification by rolling and recrystallization (<10%) are the three combination mechanisms of the semi-solid powders during rolling. In addition, semi-solid powder rolled strips can be processed subsequently by hot rolling with the improved micro-hardness and relative density. © 2013 Elsevier B.V. All rights reserved.
1. Introduction High strength aluminum alloys such as AA7050 were used extensively for aerospace applications stated in the study by Lang et al. (2011). Flat or strip products are usually manufactured by conventional ingot metallurgy (I/M) process. Subsequently, the materials are further processed (e.g. heat treatment, hot rolling, cold rolling, etc.) to meet the required properties as shown in the study by Deshpande et al. (1998). So, the production run is long and energy wasting. In the 1970s, semi-solid forming proposed by Flemings (1991) can obtain a homogeneous defects-free globular microstructure based on the special properties of semisolid slurry. Kang et al. (1999) stated that semi-solid forming was mainly used to produce the low melting point alloys such as aluminum alloys. Later, Kiuchi and Kopp (2002) demonstrated that semi-solid rolling consisting of thixo-rolling and rheo-rolling was used to prepare metallic plates and strips at a laboratory scale. At present, rheo-rolling is not applied to industrial production because of the difficulty in slurry preparation on the production line, liquidsegregation and surface cracking as discussed by Govender and Moller (2008). Kang et al. (1997) stated that rheo-die castings were
∗ Corresponding author. Tel.: +86 20 87110099; fax: +86 20 87112111. E-mail address: [email protected] (X. Luo). 0924-0136/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmatprotec.2013.08.018
more into having surface crackings and Masuku et al. (2010) said that liquid segregation is easily formed during forming. These flaws have a negative effect on the mechanical properties as shown in Kim et al. (2007). Schaffer et al. (2001) stated that powder rolling can be used to make aluminum alloy strips. However, most green strips prepared by powder rolling have high porosity and need sintering or further processing to get dense materials. In addition, lubricant and binder are necessary for preparing the compacts. Then Zu and Luo (2001) studied semi-solid powder forming (SPF) and Wu et al. (2010a) discussed various processing routes. In general, here needs pre-compaction before heating in SPF as stated in Wu et al. (2010b). And it is mainly applied in the extrusion and compaction. In 2003, McHugh et al. (2004) proposed spray rolling which appeared to include the advantages of energy saving and high production rate while improving quality with a uniform fine-grain equiaxed microstructure. But this new strip/sheet manufacturing process needs dedicated and expensive apparatus, which leads to high costs. Therefore, a novel strip manufacturing process, termed “semisolid powder rolling”, is proposed by the authors. The general concept of semi-solid powder rolling is from the possible combination of semi-solid forming and powder rolling. Semi-solid powder rolling (SSPR) consists of semi-solid powder preparation under an inert atmosphere, semi-solid powder rolling at a relatively stable temperature, and post-treatment procedures if necessary. This new process is mainly used to prepare strips with high quality,
166
Y. Liu et al. / Journal of Materials Processing Technology 214 (2014) 165–174 658ºC
1.5
DSC Exothermic
1.0 0.5 524ºC
0.0 -0.5 -1.0
0
200
400 600 Temperature/ºC
800
1000
Fig. 2. DSC curve of 7050 aluminum alloy powders.
Fig. 1. Schematic of semi-solid powder rolling process.
especially for alloys that have a wide freezing range and low melting point such as aluminum alloy (AA7050, 2124, etc.). While still in the early stage of development, semi-solid powder rolling shows a promise for reducing strip manufacturing costs and saving energy while improving the qualities. The inherent rapid solidification, easily controllable process conditions and solid solubility extension may provide an interesting avenue for the development of alloys. The objective of this study is to analyze the best parameters to prepare strips and provide an insight into the mechanisms that govern the microstructure evolution during semi-solid powder rolling. The combination mechanism of semi-solid powders and the effect of liquid fraction on the microstructure evolution are emphasized. 2. Experimental procedures A schematic of the approach is shown in Fig. 1. The powders used in this work are the gas atomized 7050 aluminum alloy powders with a nearly spherical morphology (the mean particle size is about 75 m). The chemical composition of raw materials is summarized in Table 1. The experiments were carried out on a powder rolling machine with rollers pre-heated to 300 ◦ C so as to maintain the rolling temperature. The diameter of roller is 150 mm with a width of 100 mm, a rolling gap of 0.1 mm and a rotating speed of 0.4 rad/s. Firstly, in order to obtain the semi-solid powders, the gas atomized 7050 aluminum alloy powders were heated to the semi-solid temperature range and held for different time under an inert atmosphere (the actual freezing range of 7050 aluminum alloy powders is 524–658 ◦ C analyzed by DSC as shown in Fig. 2). Then, the semisolid powders were fed into the gap between pre-heated rollers and consolidated into dense strips. In order to analyze the effect of liquid fraction on microstructure evolution during rolling, the representative values of liquid fraction at each stage were selected as the processing parameters to prepare the strip (low liquid fraction, intermediate liquid fraction and high liquid fraction). Thus, the following parameters were selected: (1) heated at 555 ◦ C, held for 30 min, (2) heated at 585 ◦ C, held for 60 min, (3) heated at 625 ◦ C, held for 60 min, (4) heated at 640 ◦ C, held for 20 min, (5) heated at 650 ◦ C, held for 40 min and (6) heated at 650 ◦ C, held for 50 min. The strips up to 100 mm wide and 1.5–2.0 mm thick were obtained Table 1 Chemical composition of 7050 aluminum (wt%).
Fig. 3. A strip prepared by semi-solid powder rolling (heated at 625 ◦ C and held for 60 min).
with a good formability and a smooth surface as shown in Fig. 3. Subsequently, the semi-solid powder rolled strips were hot rolled at 471 ◦ C with 30% thickness reduction by using a laboratory rolling mill with a loading capacity of 240 t. All the hot rolled strips were treated with air cooling after hot rolling. Samples were discontinuously selected from the central zone along to the rolling direction. Finally, the relative density of each specimen was tested by using the Archimedes’ principle. The microstructures of the polished and etched samples were observed with an optical microscope (LcicaDMI500M). The phase identification was carried out by X-ray diffraction (XRD) analysis using D8 ADVANCE (Bruker, Germany) Cu (K␣1 + K␣2) radiation (the diffraction angle 2 is 10–90◦ , scan speed is 17.7 s/step). The micro-hardness was tested at HVs-100 Vickers, and the loading force was 1.96 N. For each sample, the micro-hardness was measured randomly at 6 different points, and then the mean value of micro-hardness as well as standard deviation were calculated and reported. The grain size was determined by using the linear intercept. Scanning electron micrograph (SEM) observation was carried out at NovaNanoSEM430 for identifying the morphology and distribution of second phase particles.
3. Results and discussion 3.1. Effect of liquid fraction on microstructure during semi-solid powder rolling Fig. 4 shows the microstructure of semi-solid powders prepared under different conditions and the corresponding liquid fraction is summarized in Table 2.
Table 2 liquid fraction at different temperatures.
Zn
Mg
Cu
Zr
Ti
Fe
Si
Al
6.43
2.26
2.02
0.13
0.03
0.11
0.07
Bal
Temperature(◦ C) Liquid fraction (%)
555 1.38
585 13.8
625 44.2
640 65.09
650 83.2
Y. Liu et al. / Journal of Materials Processing Technology 214 (2014) 165–174
167
Fig. 4. Microstructures of semi-solid powders prepared under different conditions: heated at 555 ◦ C for holding 30 min (a), heated at 585 ◦ C for holding 60 min (b), heated at 625 ◦ C for holding 60 min (c), heated at 640 ◦ C for holding 20 min (d), heated at 650 ◦ C for holding 40 min (e), heated at 650 ◦ C for holding 50 min (f).
For alloys, Mi and Grant (2008) adopted the Scheil equation to relate the alloy liquid fraction to temperature during solidification.
T − Ts (1/(1−k)) Tl − Ts
(1)
where Ts and Tl are the solidus and liquidus temperature of the alloy, respectively, and k is a partition coefficient. Fig. 5 shows the liquid fraction of 7050 aluminum alloy as a function of temperature. Actually it is a fit to the data calculated by MATLAB using Eq. (1) with k = 0.66. An isolated liquid phase forms within the particles firstly and then extends to the particle boundary as the liquid increases (Fig. 4a). When liquid fraction increases to 13.8%, some of the liquid phase within the particles may form a network (Fig. 4b). The grains in semi-solid powders show a dendrite structure. When the liquid fraction increases to 45%, the dendrite arms begin to break up (Fig. 4c and d), which is helpful for the formation of a good strip because Lashkari and Ghomshchi (2007) found that the flow resistance of a dendrite structure is much greater than that of a non-dendrite structure. So, the deformation resistance of materials with low liquid fraction is obviously greater than
100 90 80
liquid fraction (%)
fl =
that of materials with high liquid fraction under the same rolling force. With a higher liquid fraction, the liquid phase forms a connected network and begins to agglomerate (Fig. 4e and f). Fig. 6 illustrates representative microstructures of the strips prepared
70 60 50 40 30 20 10 0
520
540
560 580 600 temperature (ºC)
620
640
660
Fig. 5. Effect of temperature on liquid fraction of 7050 aluminum alloy powders.
168
Y. Liu et al. / Journal of Materials Processing Technology 214 (2014) 165–174
Fig. 6. Microstructures of strips prepared under different conditions: heated at 555 ◦ C for holding 30 min (a), heated at 585 ◦ C for holding 60 min (b), heated at 625 ◦ C for holding 60 min (c), heated at 640 ◦ C for holding 20 min (d), heated at 650 ◦ C for holding 40 min (e), heated at 650 ◦ C for holding 50 min (f).
under different conditions. Microstructure of the strip prepared by heating at 555 ◦ C and holding for 30 min is characterized by a randomly distributed rosette structure with isolated pores and some deformed or broken powder particles (Fig. 6a). As shown in Fig. 7, its relative density and micro-hardness are low (the microhardness is 77.32 HV, and the relative density is 87.1%). So, the strip prepared under a low liquid cannot gain a satisfactory quality. When the strip was prepared at 585 ◦ C for 30 min, the original particle boundaries and most pores disappear (Fig. 6b). However, its micro-hardness and relative density just increase slightly. Those microstructures of strips prepared with liquid fraction ranging from 45 to 65% are characterized by a few rosettes, some dendrite grains and fine equiaxed grains (Fig. 6c and d). There are no original powder boundaries in the microstructures. The micro-hardness dramatically increases to 161.6 HV when the liquid fraction reaches 45%, and then gets to the peak when the liquid fraction is 65%. When the liquid fraction increases to 85%, the strips consist of flaws and coarse rosette-structure grains (Fig. 6e and f). The micro-hardness and relative density decrease fractionally as well. Therefore, the strip prepared with very low liquid fraction (<20%) has many original powder boundaries and pores and cannot become a dense
strip. Very high liquid fraction (>65%) in the semi-solid powders results in the coarse grains and poor quality such as flaws and defects. The best liquid fraction to prepare good strips for SSPR is 45–65%. microhardness HV relative density %
200 180 160 140 120 100 80 60 555-30
585-60
625-60
640-20
650-40
650-50
Fig. 7. Relative density and micro-hardness of strips prepared under different conditions.
Y. Liu et al. / Journal of Materials Processing Technology 214 (2014) 165–174
169
Fig. 8. SEM photograph of strip prepared by heating at 640 ◦ C and holding for 20 min (a), the original 7050 aluminum alloy powders (b). Tables summarize EDS composition analysis of phases.
Wu and Kim (2011) discussed the effect of different amount of liquid on the powder compaction in a semi-solid state. In SSPR, it can be divided into five ranges to discuss its effect on the microstructure of strips. In the initial stage, the liquid fraction is lower than 10%. The liquid phase is isolated within the particle during rolling. There is no liquid to fill in the interparticle gaps. In the second range, the liquid fraction is between 10 and 20%. On the one hand, for aluminum alloys, Sercombe (2003) stated that gaining a high density is not possible despite being sintered at a temperature that results in ∼20% liquid phase. On the other hand, Atkinson and Liu (2008) demonstrated that too little liquid in powders and the presence of some particles inhibiting the migration of liquid film along the grain boundary are associated with the low coarsening rates. Consequently, some pores and coarse dendrites still exist. The third range corresponds to the transition period that changes a strip from low density to nearly full density. In this stage there is about 20–45% liquid fraction in powders (the corresponding temperature is ∼625 ◦ C). The metallurgical bonding occurs accompanying with disappearance of powder boundaries during rolling. In the fourth range, liquid fraction ranges from 45 to 65%. Appropriate liquid not only fills in the gap but also reduces the deformation stress, which leads to easy deformation. Wu and Kim (2011) said that the interlocking of irregular of solid phase arms may have result in strengthening of the materials. In addition, as liquid fraction increases, the deformation temperature is also high. So there is enough time for liquid phase to flow among the powder particles, which results in full consolidation. In the final range, the liquid fraction
increases over 65%. Atkinson and Liu (2010) found that too much liquid results in high coarsening rates during cooling and the loss of some liquid materials during rolling. Fig. 7 shows that the errors of micro-hardness obtained in the final range are extremely large. So a very high liquid fraction results in an uneven distribution of microhardness. It may be due to the coarse precipitates and particles, defects and flaws in the microstructures. McHugh et al. (2004) found that in spray rolling, there is 15–30% liquid fraction in as-deposited materials and Liang and Lavernia (1993) found that there is 20–50% liquid fraction in spray deposition. However, the best liquid fraction to prepare the strip in SSPR ranges from 45 to 65%. It is due to that the measured porosity of cold rolled green strip is in the range 30–50% and there is about 10–20% liquid still isolated within the particles during rolling. If all the interparticle gaps are filled with the liquid, there needs about 40–70% liquid under an ideal condition. However, the conductive heat transfer between semi-solid powders and rollers results in solidification. Then some new posterior particle boundaries will be created, which generates new solid/solid interfaces during rolling. In addition, some reactive elements such as aluminum react with the adsorbed oxygen and form highly stable oxides (Al2 O3 ) at the powder particle surface as shown in Fig. 8. The EDX measurement in the rectangular zone shows that the oxide film formed around the powder particles as shown in Fig. 8b and the corresponding table. The oxide layer on the powder surface has a significant effect on the microstructure evolution during semi-solid powder rolling. It may impede or prevent the diffusion and combination between the powders. Therefore, more liquid should be required to fill in the gap.
170
Y. Liu et al. / Journal of Materials Processing Technology 214 (2014) 165–174
Fig. 9. Scanning electron micrographs of semi-solid powder rolled strips (a and b) and hot rolled strips(c and d) prepared under different conditions: heated at 625 ◦ C for holding 60 min (a) and (c), heated at 640 ◦ C for holding 20 min (b) and (d).
3.2. Microstructure evolution during semi-solid powder rolling Fig. 9 shows the scanning electron micrographs (SEM) of the semi-solid powder rolled strips and hot rolled strips. X-ray radiation diffraction (XRD) was used to identify the phases. Many gray or white phase particles can be observed around the grain boundaries (Fig. 9a) and within some grains (Fig. 9b). EDX analysis of the constituents (light phase A) and surrounding matrix (dark phase B) is summarized in the table accompanying the photomicrograph (Fig. 8a). It shows that these particles are rich in Zn, Cu and Mg. Robson (2004) said they may correspond to Mphase Mg(Zn2 , AlCu) or -phase (MgZn2 ), S-phase (Al2 CuMg) or the T-phase Al32 (Mg,Zn)49 . Based on the X-ray diffraction analysis (lines a and b in Fig. 10), they are the -phase (MgZn2 ) and S-phase Al2 CuMg. Additionally, some oxide particles may exist in the microstructures according to the EDX analysis results although no corresponding peak is found in the XRD analysis, and these oxide particles may come from the original powders or from the reaction during rolling. Zabihi et al. (2013) have studied the aluminum matrix composites (AMC) with different percentage of Al2 O3 particles, and the results show that these alumina particles can improve the mechanical properties of AMC. And the existence, origin and effect of these oxide particles are discussed in the authors’ another article. In particular, the constituents of light phases in the semi-solid powder rolled strips are enriched more in Cu and Zn and with no Fe compared with those in the sprayrolled materials and the commercial materials as shown in McHugh et al. (2008). When the liquid fraction is between 45 and 65%, phases precipitate more spherical and smaller as the liquid fraction increases.
To analyze the microstructure evolution of semi-solid powder rolling process, the rollers were stopped simultaneously during rolling. A cross-section of the resulting wedge reveals four sections as shown in Fig. 1. Section 1: materials supplying region where the semi-solid powders are fed into the gap and begin to be dragged in. Section 2: drag-in region (from the drag-in plane to the neutral plane) where the semi-solid powders are dragged in the rollers and initially compacted.
Fig. 10. X-ray diffraction analysis of semi-solid powder rolled strips (a and b) and hot rolled strips (c and d) prepared under different conditions: heated at 625 ◦ C for holding 60 min (a) and (c), heated at 640 ◦ C for holding 20 min (b).
Y. Liu et al. / Journal of Materials Processing Technology 214 (2014) 165–174
171
Fig. 11. Longitudinal section micrographs illustrating microstructure evolution during semi-solid powder rolling: heated at 585 ◦ C for holding 60 min (a), heated at 625 ◦ C for holding 60 min (b).
Section 3: densification region (from the neutral plane to the rollaxis plane) where the semi-solid powders are deformed and fully consolidated under the action of rollers. Section 4: product strip.
Fig. 11 illustrates representative microstructures in each of these sections. Semi-solid powders in section 1 are accumulated randomly with a slight deformation. The original powder boundaries can be observed clearly. Basically, the semi-solid powders
Fig. 12. Microstructures of strips prepared by heating at 555 ◦ C and holding for 30 min (a), heating at 585 ◦ C and holding for 60 min by the edge (b).
172
Y. Liu et al. / Journal of Materials Processing Technology 214 (2014) 165–174
Fig. 13. Microstructures of strips after single-pass hot rolling with 30% thickness reduction: heated at 555 ◦ C for holding 30 min (a), heated at 585 ◦ C for holding 60 min (b), heated at 625 ◦ C for holding 60 min (c), heated at 640 ◦ C for holding 20 min (d), heated at 650 ◦ C for holding 40 min (e), heated at 650 ◦ C for holding 50 min (f).
remain apart in this zone. In section 2, deformation and combination among some powders occur. But many isolated pores can still be observed. Thus, the powders are just dragged in and initially compacted in this zone. Densification takes place mainly in section 3 where the metallurgical bonding occurs among powders with the disappearance of pores and powder boundaries. And then dense strips are obtained as shown in section 4. As a result, deformation, solidification and densification occur simultaneously and collaboratively during rolling. In SSPR, the powder particles jointed via three approaches: (1) Densification by rolling. It mainly occurs in the rolling region and is similar to the mechanism of hot densification rolling discussed by Vajpai et al. (2011). At last, strong metallurgical bonding between powder particles takes place with large deformation and higher temperature. (2) Recrystallization. It is well known that recrystallization proceeds by the nucleation and the migration of the new grain boundaries into the deformed matrix leaving behind the ‘strain-free grains’. Doherty et al. (1997) sated that typical
nucleation sites all have high local misorientations. For gas atomized AA7050 powders, their grain orientations are free and they have large-angle boundaries with the activation energies of 70.5 kJ/mol, which is indirectly calculated. At first, semi-solid powders with a low liquid fraction have a high deformation stress during deformation, and store much deformation energy. And then the deformation degree among the contact surfaces is larger and the stored energy of deformation is higher than other parts. Due to the deformation accompanying with high temperature, nuclei formed firstly in the inter-particle contact areas. Subsequently, nuclei boundaries moved into the contiguous particles resulting in the disappearance of particle boundaries. Barrett (1940) suggested that the mobility of nuclei boundaries depend on orientation relationship. So some nuclei boundaries satisfying the orientation relationship can grow into the contiguous particles as shown in A and B of Fig. 12a. This kind of combination mechanism just occurs when liquid fraction in powders is low (<10%). (3) Flowing and filling of liquid, it mainly happens when liquid fraction is higher
Y. Liu et al. / Journal of Materials Processing Technology 214 (2014) 165–174
microhardness HV relative density %
260 250 240 230 220 210 200 190 180 170 160 150 140 130 120 110 100 555-30
585-60
625-60
640-20
650-40
650-50
Fig. 14. Relative density and microhardness of strips after single-pass hot rolling with 30% thickness reduction.
(>10%). With a higher liquid, the liquid phase forms a connected network and flows into the gap between the particles during rolling and then solidifies, which results in the disappearance of particle boundaries (Fig. 12b with the arrow). Yu et al. (2001) discussed the whole procedure of liquid phase sintering and supersolidus liquid phase sintering and Showaiter and Youseffi (2008) studied the effect of liquid on the sintering during densification. Seen from this perspective, the flowing and filling of liquid is similar to the liquid phase sintering. Therefore, recrystallization and densification by rolling are the two combination mechanisms among powders when the liquid fraction is low (<10%), while the flowing and filling of liquid is the main combination mechanism when liquid fraction is higher (>10%). 3.3. Microstructure evolution during post-treatment Generally, the semi-solid powder rolled strips may have a few pores. In order to improve the density and mechanical properties deeply, post-treatment would be necessary. Therefore, hot rolling is used in this study. Fig. 13 shows the microstructures of strips after single-pass hot rolling with 30% thickness reduction. The microstructures consist of fine grains (∼43 m). It can be seen that there is no evidence of apparent deformation or grain elongation in the direction of rolling in Fig. 13a and b, which is different from the rest of the pictures. From Fig. 13c–f, dynamic recrystallization is not complete in many regions. In some areas recrystallization occurs partially at grain boundaries (Fig. 13c with the arrow). Fig. 13e and f illustrates that most of grains prefer to be elongated in the direction of rolling. It means that the recrystallized region is small in strips prepared with a high liquid fraction and large in strips produced with a low liquid fraction. Fig. 14 shows the relative density and micro-hardness after hot rolling. The relative density (all of specimens over 99%) and microhardness are improved dramatically. In short, the following hot rolling is helpful for improving the mechanical properties of the semi-solid powder rolled strips. Fig. 9c and d shows that there are no secondary particles in SEM. And the diffraction peaks of secondary particles in hot-rolled strips disappear based on the lines c and d in Fig. 10. It was demonstrated that nearly complete densification during hot rolling occurs through various stages. With a low liquid fraction (<10%), the main mechanism of hot densification can be divided into three stages proposed by Vajpai et al. (2011). With a relatively high liquid fraction (>45%), the mechanism of hot densification is similar to that of the nearly dense strips discussed by Zhan et al. (2008). Plastic deformation during hot rolling results in dynamically recrystallized grains and elongated grains. But the final stage
173
of densification mechanism with low liquid fractions is the same as that with high liquid fractions. Consequently, semi-solid powder rolling has many different features compared with powder rolling and semi-solid rolling: 1. The semi-solid powder rolled strips consist of rapid solidification microstructure and features of semi-solid microstructure. There is a temperature gradient between semi-solid powders and rollers. Powders with amounts of liquid (from 5 to 85%) are fed into the gap between the pre-heated rollers, where it solidifies and forms a strip up to about 2 mm thick. Based on the results of microstructure evolution, it infers that solidification, deformation and densification occur simultaneously and collaboratively during semi-solid powder rolling. 2. The semi-solid powder rolled strips have a high density compared with those by powder rolling, because the powders for semi-solid powder rolling consist of amounts of liquid, which can serve as a “binder” like in powder rolling. The materials introduced to the rollers in powder rolling are still in a solid state. But powders used in semi-solid powder rolling have a “slushy” character. In addition, some powders will break up under the action of rolling force. Then, the liquid can flow among powders, which attributes to the consolidation. 3. It can gain fine-grain equiaxed microstructure and high solute content because of the high cooling rate, which can improve the mechanical properties significantly. Especially, strips of aluminum alloys with broad freezing range, such as AA7050, can be prepared successfully by semi-solid powder rolling. 4. Conclusions (1) SSPR is a new strip manufacturing technology that shows a promise for processing a wide variety of aluminum alloys, especially for wide freezing range alloys, such as AA7050. By combining semi-solid rolling with powder rolling, aluminum alloy strips can be prepared successfully with a nearly full density and high micro-hardness. The processing parameters can be controlled easily and simply. (2) The best liquid fraction to prepare a good strip is 45–65%. The microstructure of strip is sensitive to the liquid fraction. Too low or too high liquid fraction results in a poor microstructure with isolated pores, cracking or flaws, which affects the mechanical properties of strips. There are three combination mechanisms in semi-solid powder rolling: flowing and filling of liquid (>10%), densification by rolling and recrystallization (<10%). Hot rolling is an effective post-treatment to improve the mechanical properties of strips. Acknowledgement The authors gratefully acknowledge the financial support of Fundamental Research Funds for the Central Universities (Grant No. 2011ZZ0010). References Atkinson, H.V., Liu, D., 2008. Microstructural coarsening of semi-solid aluminium alloys. Mater. Sci. Eng. A 496, 439–446. Atkinson, H.V., Liu, D., 2010. Coarsening rate of microstructure in semi-solid aluminium alloys. Trans. Nonferrous. Metall. Soc. China 20, 1672–1676. Barrett, C.S., 1940. Trans. AIME 137, 128. Deshpande, N.U., Gokhale, A.M., Denzer, D.K., Lium, J., 1998. Relationship between fracture toughness, fracture path, and microstructure of 7050 aluminum alloy: part I. Quantitative characterization. Metal. Mater. Trans. A 29, 1191–1201. Doherty, R.D., Hughes, D.A., Humphreys, F.J., et al., 1997. Current issue in recrystallization: a review. Mater. Sci. Eng. A 238, 219–274. Flemings, M.C., 1991. Behavior of metal alloys in the semi-solid state. Metal. Trans. 22, 957–998.
174
Y. Liu et al. / Journal of Materials Processing Technology 214 (2014) 165–174
Govender, G., Moller, H., 2008. Evaluation of surface chemical segregation of semisolid metal cast aluminium alloy A356. Solid State Phenom. 141–143, 433–438. Kiuchi, M., Kopp, R., 2002. Mushy/semi-solid metal forming technology—present and future. Manufact. Technol. 51, 653–670. Kang, C.G., Yoon, J.H., Seo, Y.H., 1997. The upsetting behavior of semi-solid aluminum material fabricated by a mechanical stirring process. J. Mater. Proc. Technol. 66 (12), 30–38. Kang, C.G., Choi, J.S., Kim, K.H., 1999. The effect of strain rate on macroscopic behavior in the compression forming of semi-solid aluminum alloy. J. Mater. Process. Technol. 88, 159–168. Kim, W.Y., Kang, C.G., Kim, B.M., 2007. Deformation behavior of wrought aluminum alloys in incremental compression experiment with a closed die. J. Mater. Process. Technol. 191, 372–376. Lang, Y.J., Cai, Y.H., Cui, H., Zhang, J.S., 2011. Effect of strain-induced precipitation on the low angle grain boundary in AA7050 aluminum alloy. Mater. Des. 32, 4241–4246. Lashkari, O., Ghomshchi, R., 2007. The implication of rheology in semi-solid metal processes: an overview. J. Mater. Process. Technol. 182, 229–240. Liang, X., Lavernia, E.J., 1993. Evolution of interaction domain microstructure during spray deposition. Metall. Mater. Trans. A 25, 1994–2341. Masuku, E.P., Moller, H., Curle, U.A., 2010. Influence of surface liquid segregation on corrosion behavior of semi-solid metal high pressure die cast aluminium alloys. Trans. Nonferrous Met. Soc. China 20, s837–s841. McHugh, K.M., Delplanque, J-P., Johnson, S.B., Lavernia, E.J., Zhou, Y., Lin, Y., 2004. Spray rolling aluminum alloy strip. Mater. Sci. Eng. A 383, 96–106. Mi, J., Grant, P.S., 2008. Modelling the shape and thermal dynamics of Ni superalloy rings during spray forming. Part 2. Thermal modelling-Heat flow and solidification. Acta Metall. 56, 1597–1608. McHugh, K.M., Zhou, Y., Johnson, S.B., et al., 2008. Microstructure evolution during spray rolling and heat treatment of 2124 Al. Mater. Sci. Eng. A 447, 26–34.
Robson, J.D., 2004. Microstructural evolution in aluminium alloy 7050 during processing. Mater. Sci. Eng. A 382, 112–121. Schaffer, G.B., Sercombe, T.B., Lumley, R.N., 2001. Liquid phase sintering of aluminium alloys. Mater. Chem. Phys. 67, 85–91. Sercombe, T.B., 2003. On the sintering of uncompacted, pre-alloyed Al powder alloys. Mater. Sci. Eng. A 341, 163–168. Showaiter, N., Youseffi, M., 2008. Compaction, sintering and mechanical properties of elemental 6061 Al powder with and without sintering aids. Mater. Des. 29, 752–762. Vajpai, S.K., Dube, R.K., Sangal, S., 2011. Microstructure and properties of Cu–Al–Ni shape memory alloy strips prepared via hot densification rolling of argon atomized powder preforms. Mater. Sci. Eng. A 529, 378–387. Wu, Y.F., Kim, G.Y., Anderson, I.E., Lograsso, T.A., 2010a. Fabrication of Al6061 composite with high SiC particle loading by semi-solid powder processing. Acta Metall. 58, 4398–4405. Wu, Y.F., Kim, G.Y., Anderson, I.E., Lograsso, T.A., 2010b. Experimental study on viscosity and phase segregation of Al–Si powders in microsemisolid powder forming. J. Manufact. Sci. Eng. Trans. ASME 132, 0110031–0110037. Wu, Y.F., Kim, G.Y., 2011. Compaction behavior of Al6061 powder in the semi-solid state. Powder Technol. 214, 252–258. Yu, F.X., Cui, J.Z., Rangathan, S., 2001. Fundamental differences between spray forming and other semisolid processes. Mater. Sci. Eng. A 304–306, 621–626. Zu, L.J., Luo, S.J., 2001. Study on the powder mixing and semi-solid extrusion forming process of SiCp/2024Al composites. J. Mater. Process. Technol. 114, 189–193. Zabihi, M., Toroghinejad, M.R., Shafyei, A., 2013. Application of powder metallurgy and hot rolling processes for manufacturing aluminum/alumina composite strips. Mater. Sci. Eng. A 560, 567–574. Zhan, M.Y., Chen, Z.H., Yan, H.G., 2008. Two novel rolling processes of porous metal preforms prepared by spray deposition technique using conventional rolling mill. J. Mater. Process. Technol. 202, 269–274.