SUN Yonggen, WANG Yanchun, SU Yanni, SONG Xujie, DU Lanjun,CHENG Yuansheng, DU Zhiming＊
(1. National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, China; 2. School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China; 3. Beijing North Vehicle Group Co. Ltd., Beijing 100072, China; 4. Beijing Aerospace Propulsion Institute, Beijing 100076, China)
Abstract: Al-5Cu-4.5Mg-2.5Zn alloy was prepared and the alloy ingots were fabricated by squeeze casting in this work. Considering these negative effects of composition segregation and coarse second phases, some heat treatments were adopted in this research. Microstructures, element distribution, phase constitutions and mechanical properties of Al-5Cu-4.5Mg-2.5Zn alloy ingots before and after heat treatments were investigated. It was discovered that these heat treatments would influence and extremely optimize the microstructures and properties of Al-5Cu-4.5Mg-2.5Zn alloy. Except some residual S (Al2CuMg) phase and a few of η phase, the precipitate free zone (PFZ) and the Guinier Preston zone (GPZ) formed in the alloy. It was also found that θ′′ (Al2Cu) and η′′ (MgZn2) phases formed and kept a consistent relationship with the aluminum matrix. As the result, these properties of ultimate tensile strength (UTS), percentage of elongation and Brinell hardness (HB) were greatly elevated. The UTS, percent of elongation and HB were 469 MPa, 8.1% and 208 N/mm2, respectively.
Key words: Al-Cu-Mg-Zn alloy; squeeze casting, heat treatments; second phases; mechanical properties
In recent years, the metal parts of aircraft, cars,ships, artillery, tanks and machinery[1-5]are required to be light and of high strength, so that these parts are replaced by lightweight alloy castings or forgings like aluminum[6,7]and magnesium[8,9]alloy products.Specifically, most aircraft structural parts, automobile hubs, tank wheels and track shoes are fabricated by aluminum alloys[10-13]. However, traditional technologies such as casting or die forging could not fully meet performance requirements[1,14]. Squeeze casting is a promising forming technology, with advantages of saving time, saving energy and efficient utilization,and the squeeze casting parts show high precision and good performances[15-18]. Squeeze casting technology is of huge significance and application prospect[19-21].However, it is difficult to avoid some defects, such as microporosity, segregation, and hot cracking, caused by uneven stress during the solidification process[22-24]. It has been widely recognized that hot cracking is one of the most serious defects in alloy squeeze casting, and one of the bottlenecks for commercial applications of squeeze castings[25,26].
Hot cracking mainly happens in the final stage of solidification that the solid fraction is from 85% to 95%, and some continuous reticulated microstructures form and influence the mechanical performances[27,28].Recently, many efforts had been put into the research of preventing the hot cracking. The factors dominating the generation and susceptibility of hot cracks include alloying elements, freezing range, amount of eutectic phases and solidification rate[29,30]. The researches mainly concentrated on alloying elements. It is found that the hot cracking sensitivity is largely influenced by alloying elements[31-34].
In the previous work, our team designed Al-5Cu-4.5Mg-2.5Zn alloy with low hot cracking sensitivity based on commercial 2024 alloy.[35]The as-cast Al-5Cu-4.5Mg-2.5Zn alloy was fabricated by adding industrial pure zinc, industrial pure magnesium and Al-Cu (50%) intermediate alloy into the 2024 aluminum alloy. The hot cracking behaviors of as-cast Al-5Cu-4.5Mg-2.5Zn alloy and 2024 aluminum alloy were also assessed by the restraint rod method[36]. HCS index of 2024 aluminum alloy was 68, while the index of Al-5Cu-4.5Mg-2.5Zn alloy was only 40, which was decreased by 41.18%.
However, composition segregation and coarse second phases limited the performances of Al-5Cu-4.5Mg-2.5Zn ingots. Therefore, some heat treatments were adopted to improve the microstructure and the mechanical properties in this work. Al-5Cu-4.5Mg-2.5Zn alloy and 2024 alloy ingots were fabricated by squeeze casting, and their microstructures and mechanical properties at room temperature were studied. Then some heat treatments were performed on fresh Al-5Cu-4.5Mg-2.5Zn ingot. The performances of Al-5Cu-4.5Mg-2.5Zn ingot after heat treatments were tested and compared. On the basis of these results, the mechanism of squeeze casting and heat treatments improving the mechanical properties was discussed.
The nominal composition of Al-5Cu-4.5Mg-2.5Zn alloy is listed in Table 1. The starting materials were commercial 2024 aluminum alloy (the composition is shown in Table 1), zinc (99.99 wt%), magnesium (99.99 wt%) and Al-50Cu medium alloy. Al-5Cu-4.5Mg-2.5Zn alloy was prepared in a resistance melting furnace as the previous studies[36]. The composition of the alloys were detected by X-ray fluorescence spectrometer (XRF, Panalytical-PW4400, Netherlands). The melting process was as follows: firstly, 2024 aluminum alloy and Al-Cu intermediate alloy were put in it when the melting furnace was heated to 350 ℃; then the temperature was increased to 750 ℃ until the two alloys were melted completely; subsequently, pure magnesium and zinc were put into the furnace in turn after the melt was cooled to 700 ℃; finally, 1.0 wt% K2TiF6powder was put into the melt for refining. Consequently, the melt was prepared for the next squeeze casting forming.
Squeeze casting dies were designed and assembled as Fig.1, and the working surfaces of dies were sprayed with graphite. Firstly, the squeeze casting dies were preheated to 300 ℃. Then the Al-5Cu-4.5Mg-2.5Zn alloy melt was poured into a cylindrical mold,where the pouring temperature of the alloy melt was chosen at 730 ℃. A pressure of 50 MPa was immediately applied on the melt by a forging hydraulic machine for 30 seconds. The melt alloy solidified and deformed in the mold, and the parts which were 65 mm in diameter and 60 mm in height were obtained finally.2024 aluminum alloy ingots were also fabricated as comparative experiment.
Fig.1 The schematic of squeeze casting dies: 1-upper template;2-upper backing plate; 3-upper die set; 4-upper die; 5-blank holder; 6-lower die; 7-lower die set; 8-lower template;9-upper bolts; 10-lower bolts; 11-heating rods; 12-the alloy melt; 13-lower die core; 14-ejection rod
Table 1 Nominal chemical composition of 2024 aluminum alloy and Al-5Cu-4.5Mg-2.5Zn alloy
Because of high contents of magnesium and zinc elements in Al-5Cu-4.5Mg-2.5Zn alloy, partial elements accumulated along the grain boundaries, and some coarse reticulated eutectic microstructures tended to form[37]. By some heat treatments, phase compositions and microstructures could change and optimize and the mechanical properties were improved[38]. A reasonable heat treatment process was very important, and the optimal process in this work was as follows: firstly,the samples were heated to 470 ℃ for 24 hours and cooled in the furnace to room temperature; Subsequently these samples were heated to 495 ℃ for 4.5 hours,then quickly cooled in water; finally, they were heated to 200 ℃ for 18 hours and cooled in air.
Microstructure observations were performed through optical microscopy (OM, OLYMPUS-PEM-3,Japan), scanning electron microscope (SEM, SUPRA-55, Germany) with EDS and transmission electron microscopy (TEM, Talos-F200x, Czech Republic).The X-ray diffraction (XRD, D/MAX-RB, Japan) tests were carried out to phases analysis of the alloys. Tensile strength and elongation were conducted using the electronic universal testing machine (INSTRON-5569,USA) at room temperature, where the tensile rate was 1.0 mm/min. Three specimens were prepared for each test, and the average was taken. The hardness was tested and characterized by Brinell hardness tester (HB-3000B, China), where the load was 2 452 N and the holding time was the 30 seconds. The cross section of the specimen was taken at three different places and averaged.
3.1 Observations of microstructure and texture
Fig.2 shows the corresponding microstructures of 2024 aluminum and Al-5Cu-4.5Mg-2.5Zn alloy oringnal samples. It can be seen that there are free dendrites and nondendritic structures in both alloys. The free dendrites formed from the grain growth during the solidification, while the nondendritic structures formed from the deformed or broken dendrites under the pressure.During the squeeze casting process, the solidification of the alloy tissue speeded up because of the influence of the applied load. In one aspect, some free dendrites deformed and broken under the pressure, to produce a large number of the new nucleus. These nucleus grew up quickly to form the fine equiaxed crystals. In the other aspect, some dendritic crystals deformed and produced columnar crystals or rosette grains under deformation conditions. As statistical analysis with Image-Pro Plus 6.0 software, the total number of grains in Al-5Cu-4.5Mg-2.5Zn alloy was more than that in the 2024 aluminum alloy, and the dendrite cells and nondendritic structures in the Al-5Cu-4.5Mg-2.5Zn alloy were relatively finer. The main difference between the two alloys was that the contents of magnesium and zinc in Al-5Cu-4.5Mg-2.5Zn alloy were higher than those in 2024 aluminum alloy according to Table 1. During the squeeze casting process of Al5Cu-4.5Mg-2.5Zn alloy,some second phases formed in the grain and along the grain boundary under the applied pressure. At the same time, a small amount of non-equilibrium eutectic tissues formed on the grain boundary due to partial aggregation of the alloying elements. These phases could pin the grain boundaries, stabilizing the fine grain size[39].
To explore the reason for the second phase formation, the phase constitutions and particle distribution,some further microstructure analysis of Al-5Cu4.5Mg-2.5Zn alloy were conducted. Fig.3(a) is the scanning electron microscopy of Al-5Cu-4.5Mg-2.5Zn as-cast alloy, which show that there are at least three phases.The most phases are the aluminum matrix according to Fig.3(b). Two presented eutectic structures formed along the grain boundaries, and the darker were obviously more than the lighter ones. The distribution of alloying elements in Al-5Cu-4.5Mg-2.5Zn alloy is shown in Figs.3(c)-3(e). As a result of different solubility of zinc, magnesium, and copper elements in aluminum substrate, these three main alloying elements distributed unevenly in Al-5Cu4.5Mg-2.5Zn alloy[40]. The limit solubility of copper in aluminum is relatively low, thus the copper element is hardly absorbed by aluminum in solid solution, and there is almost no copper element inside the grains, as showed in Fig.3(c). The copper element mainly segregated at the grain boundaries and formed coarse reticulated eutectics. Although the solubility of magnesium is greater than that of copper,most of the magnesium element enriches along grain boundaries (Fig.3(d)). However, it can be observed in Fig.3(e) that zinc element mostly dissolves in the aluminum substrate and distributes evenly in the alloy. The non-uniform distribution of the three main alloying elements would cause the infertile region in the alloy[41].
Fig.2 As-cast microstructures: (a) 2024 aluminum alloy; (b) Al-5Cu-4.5Mg-2.5Zn alloy
Fig.3 (a) Microstructure; Elemental mappings of (b) Al, (c) Cu, (d)Mg, (e) Zn in Al-5Cu-4.5Mg-2.5Zn alloy
Fig.4 (a) XRD analysis result; (b) SEM micrograph; Energy spectrum analysis of (c) point 1, (d) point 2 and (e) point 3
The XRD patterns of Al-5Cu-4.5Mg-2.5Zn alloy are shown in Fig.4(a). Except for the aluminum matrix,there are three second phases, Al2CuMg, Al2Cu and MgZn2, respectively. And the content of Al2CuMg is higher than that of Al2Cu and MgZn2. According to the previous studies[42], the reaction process of Al-5Cu-4.5Mg-2.5Zn alloy deduced during solidification as follows: firstly, someα(Al) phases precipitated; thereafter, eutectic structures synthesized including two reactions:L→α(Al) +θ(Al2Cu) +S(Al2CuMg) andL→α(Al) +η(MgZn2); finally, Al-5Cu-4.5Mg-2.5Zn alloy completely solidified. Figs.4(c)-4(e) are the energy spectrum analysis results of the three points in the SEM micrograph as Fig.4(b). It is seen that point 1 (black parts) isα(Al) solid solution dissolved with magnesium and zinc elements. The skeletal organizations as point 2 isS(Al2CuMg) phase consisting of aluminum,copper and magnesium elements. And the bright parts in point 3 are obviously different, which are eutectic structures doped with lessθ(Al2Cu) phases. It could be considered that Al-5Cu-4.5Mg-2.5Zn alloy formed on the basis of Al-Cu-Mg alloy dissolving with a large of zinc.
The OM, SEM, TEM of the second phases were conducted as shown in Fig.5. There are obviously two kinds of eutectic structures distributed along grain boundaries,θ(Al2Cu) andS(Al2CuMg) phases, as showed in Fig.5(a).θ(Al2Cu) phases are distributed as a strip inside the alloy (Fig.5(b)), andS(Al2CuMg)phases are present around the grain boundary as ripple-like or needle-like eutectics. All the both second phases are distributed irregularly in Al-5Cu-4.5Mg-2.5Zn alloy.
Fig.5 (a) Light microscopy of θ and S phases; SEM micrographs of(b) θ and (c) S phases; (d) TEM image of S phase
The high-resolution image ofS(Al2CuMg) phase is shown in Fig.6(a) and the diffraction spots are displayed in Fig.6(b). A lot of alloying elements enriched and formedSphases, thus, some dilution zones of alloying elements produced inside the holes ofSphases,like point A in Fig.6(a), whose energy spectrum is shown in Fig.6(c). However, far away from eutectic structures, the content of alloying elements in aluminum matrix was high, like point B in Fig.6(a), whose energy spectrum is shown in Fig.6(d). It also could be seen that there were some dislocations away from the eutectic organizations, caused by the plastic flow behavior. It revealed that some plastic deformations occurred during the squeeze casting process. Moreover, there were a large number of dispersed stripθ(Al2Cu) phases, with two special types as Fig.6(e). One type was composed of two mutually verticalθphases distributed like the letter “L”, whose diffraction spots are shown in Fig.6(f). Some were distributed parallel to the zone axis [001] of the aluminum matrix and the others were distributed perpendicular to the zone axis.Fig.6(h) illustrates another type, single stripθphases distributed perpendicular or parallel to the matrix. In high-resolution photos ofθphase (Fig.6(g)), both of the two types kept a coherent relationship with the aluminum matrix.
Fig.6 (a) TEM image of S phase; (b) diffraction spots of S phase; energy spectrum analysis of: (c) point A and (d) point B; (e) TEM image of θ phase; (f) and (h) diffraction spots of θ phase; (g) high-resolution image of θ phase
To sum up, there were three second phases in the original Al-5Cu-4.5Mg-2.5Zn alloy,S(Al2CuMg)phases,θ(Al2Cu) phases andη(MgZn2) phases, respectively. SomeS(Al2CuMg) phases distributed as ripple-like or needle-like, mainly presented along the grain boundaries. A small number of blockyθ(Al2Cu)phases and a little stripθphases dispersed along the dislocation line, which kept a coherent relationship with the matrix. The phase composition was related to the high content and uneven distribution of elements in the original Al-5Cu-4.5Mg-2.5Zn alloy. The composition segregation and the coarse second phases at the grain boundaries seriously affected the mechanical performances of Al-5Cu-4.5Mg-2.5Zn alloy.
Fig.7 Elemental mappings of Al-5Cu-4.5Mg-2.5Zn alloy after heat treatment: (a) microstructure; elemental distributions of (b)Al, (c) Cu, (d) Mg and (e) Zn
To resolve the above problem, some heat treatments were employed in this work. The treatments included the homogenizing treatment, solution treatment,and aging treatment. The homogenizing treatment was to eliminate residual stress and promote the nonequilibrium phase dissolution diffusion. The solution and aging treatments were to make the coarse phases dissolve in the aluminum matrix as much as possible, forming the super-saturated solid solution, and then decomposing and precipitating numerous fine dispersive phases on grain boundaries[37]. The performances of the alloy after heat treatment were determined by the reinforcing phases dispersed in Al matrix, the precipitating phases and precipitate free zones around grain boundaries[43].
Fig.9 (a) TEM micrograph after heat treatment; (b) diffraction spot of residue S phase; (c) distribution of precipitations; (d) diffraction spots of Al matrix; (e) diffraction spots of θ′′ phase; (f) diffraction spots of η′′ phase; (g) high-resolution image of η′′ phase; (h) highresolution image of η phase; (i) diffraction spots of η phase
Fig.8 Grain boundary precipitate free zone (PFZ)
Fig.7(a) shows the scanning electron microscopy of Al-5Cu-4.5Mg-2.5Zn alloy after heat treatments.There were some dispersible reinforcing phases with different shapes, some precipitating phases and precipitate free zones on the grain boundary. The coarse reticulated phases decreased and disappeared after heat treatments. The distribution of elements in Al-5Cu-4.5Mg-2.5Zn alloy after heat processing are shown in Figs.7(b)-7(e). There were still some eutectic structures with rich Cu and rich Mg along grain boundaries, but the eutectic structures reduced significantly. Zn element evenly distributed in the grains (Fig.7(e)) while a lot of tiny aging precipitated phases dispersed with rich Cu.Furthermore, the precipitate free zone (PFZ) formed in grain boundaries (Fig.8). ThePFZwas wider about 2 μm, where there were some residual eutectic structures. Besides thePFZ, some precipitated phases were close to thePFZ, which were small and dense. Some studies indicated that there was no relation between the strength and the width ofPFZ. However, the plasticity of the alloy would be reduced when the width ofPFZincreased[39].
Figs.9 are the TEM analysis results of Al-5Cu-4.5Mg-2.5Zn alloy after heat treatments, which illustrates that the organizations in the alloy are composed of the Guinier Preston zone (GPZ), thePFZand the residual eutectic structures. The residual phases were mainlySphases, but the shapes ofSphases changed and displayed like an elliptical sheet on the grain boundary (Fig.9(a)). The reason was that a lot of magnesium and copper dissolved inSphases during heat treatments. The diffraction spots of residualSphases are displayed in Fig.9(b). It also could be seen in Fig.9(c) that theGPZcontained a lot of dispersed precipitation phases, which distributed in the aluminum matrix. The diffraction spots of Al matrix are shown in Fig.9(d). Some flaky phases distributed like type “==”wereη′′ phases, whose length were about 5 nm. The long striped or disc phases distributed like the letter “L”wereθ′′ phases, whose length or diameter were about 10 nm. Theθ′′ phases distributed [001] crystal surface and kept a coherent relationship with the matrix. The diffraction spots ofη′′ andθ′′ phases are shown in Figs.9(e)-9(f), respectively. However, there were also a few ofηphases holding a non-coherent relation with the Al matrix, as showed in Fig.9(h), and the diffraction spots are seen in Fig.9(i). To summarize,θ′′ andη′′ phases formed after heat treatments, which maintained a consistent relationship with the aluminum matrix. Moreover, small amount ofη(MgZn2) phases precipitated,so that the alloy kept the strength without softening.The formation of new phases and the changes of second phases influenced the properties of Al-5Cu-4.5Mg-2.5Zn alloy.
3.2 Mechanical properties
Fig.10 Mechanical properties: (a) tensile stress-strain curves; (b) ultimate tensile strength; (c) percentage of elongation; (d) Brinell hardness.(A0 is 2024 alloy without HT; B0 is Al-5Cu-4.5Mg-2.5Zn alloy without HT; B1 is Al-5Cu-4.5Mg-2.5Zn alloy after HT)
Figs.10 show the properties of Al-5Cu-4.5Mg-2.5Zn alloy before and after heat treatments, while the properties of 2024 aluminum alloy without heat treatments are as the references. Compared with the properties of 2024 alloy without heat treatments, the mechanical performances of the original Al-5Cu-4.5Mg-2.5Zn alloy without heat treatments increased slightly as shown in Figs.10. In view of compositions,the additions of Mg and Zn elements enhanced the effect of solid solution strengthening[35,40]. The above microstructures and phases analysis also illustrate that solid solution strengthening and second phases dispersed on the grain boundary are the two main reasons for the better performances of Al-5Cu-4.5Mg-2.5Zn alloy. According to XRD phase analysis results (Fig.4(a)),except forα(Al) matrix phases,S(Al2CuMg) phases,θ(Al2Cu) phases and a small number ofη(MgZn2)phases formed in Al-5Cu-4.5Mg-2.5Zn alloy. The unevenly distributed second phases strengthened the alloy. From the morphology images (Fig.5 and Fig.6), theSandθphases distributed along the grain boundaries,and tinyηphases dispersed in the grains. During the squeeze casting of Al-5Cu-4.5Mg-2.5Zn alloy, some magnesium dissolved and formedα(Al) matrix phases,and the strength and hardness enhanced as the effect of solid solution strengthening[43]. Simultaneously, some eutectic structures formed with Mg and Cu elements around the grain boundaries, therefore the elongation increased gently. What’s more, the zinc element dissolved and distributed evenly, which intensified solution strengthening and improved the strength and the hardness of Al-5Cu-4.5Mg-2.5Zn alloy.
As showed in Figs. 10, the ultimate tensile strength, percentage of elongation, and hardness of Al-5Cu-4.5Mg2.5Zn alloy improved greatly after heat treatments, whereσb= 469 MPa,δ= 8.1% andHB= 208 N/mm2. Specifically, percentage of elongation raised about three times than that of original Al-5Cu-4.5Mg-2.5Zn alloy without any heat treatments. As the above analysis, the alloying element segregations effectively reduced and the eutectic structures dissolved in the matrix by heat treatments. Although there were still some eutectic structures formed with rich Cu and rich Mg on grain boundary, the number of eutectic structures decreased. All the change of eutectic structures made the elongation of the alloy get better.Moreover, the dispersed second phases distributed more evenly, and theGPZandPFZalso formed after heat treatments. Except a few ofηphases,θ′′ andη′′phases formed, which maintained a consistent relationship with the aluminum matrix. These second phases improved the alloy by the solution and dispersion strengthening, so the strength and hardness increased.Thus the microstructures and the phases both optimized in Al-5Cu-4.5Mg-2.5Zn alloy after heat treatments[44].Therefore, Al-5Cu-4.5Mg-2.5Zn alloy obtained extra high elongation and showed the perfect performances.
a) The microstructures of the alloys fabricated by squeeze casting consisted of free dendrites and nondendritic structures as the applied pressure during the process. Because of partial aggregation of the alloying elements, second phases on the grain boundary could pin the grain boundaries, and stabilize the finer grain size of Al-5Cu-4.5Mg-2.5Zn alloy.
b) The basic components of Al-5Cu-4.5Mg-2.5Zn alloy after squeeze casting mainly contained two eutectic structures. Ones wereS(Al2CuMg) phases distributed a lot as ripple-like or needle-like in the alloy. And the other ones were blocky or stripθ(Al2Cu) phases distributed around the grain boundaries. The two eutectic structures were distributed irregularly and alternately.
c) After heat treatments, there were still some eutectic structures with rich Cu and rich Mg along grain boundaries. However, Zn element became evenly distributed in the grains. Moreover,θ′′ andη′′ phases formed, and a small amount ofηphases precipitated in Al-5Cu-4.5Mg-2.5Zn alloy after heat treatments.
d) Compared with the properties of 2024 aluminum alloy without heat treatments, the mechanical properties of original Al-5Cu-4.5Mg-2.5Zn alloy improved slightly. Further, after heat treatments, the ultimate properties of Al-5Cu-4.5Mg-2.5Zn alloy became optimized, and the best mechanical performances were as follows:σb= 469 MPa,δ= 8.1%, andHB= 208 N/mm2.