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Experimental investigation on hot forming–quenching integrated process of 6A02 aluminum alloy sheet
Materials Science & Engineering A 573 (2013) 154–160
Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Experimental investigation on hot forming–quenching integrated process of 6A02 aluminum alloy sheet Xiaobo Fan, Zhubin He n, Shijian Yuan, Kailun Zheng National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2013 Received in revised form 24 February 2013 Accepted 27 February 2013 Available online 7 March 2013
As important light-weight structure material, aluminum alloy has been widely used in automotive and aerospace industries, in which the manufacturing of parts with high strength and good dimensional accuracy has been the main task. In this paper, special device was developed to investigate the hot forming–quenching integrated process of cold-rolled 6A02 aluminum alloy sheet. Strengthening effect was reflected by Vickers hardness measuring and uniaxial tensile test. Microstructure examination was conducted to clarify the strengthening mechanism by scanning electron microscope (SEM), transmission electron microscopy(TEM) and electron backscattering diffraction (EBSD). Results show that Vickers hardness increases with solution time (o 50 min) increase, and improves significantly after artificial aging. The faster the cooling rate, the greater the strengthening effect. The Vickers hardness of formed part can increase to 106.5 HV from 73 HV in cold-rolled condition or 40 HV in heated condition in hot forming–quenching integrated process (solution treatment at 520 1C/50 min, cooling by 50 1C/s, being aged at 160 1C/10 h). The corresponding tensile strength and yield strength are 315.6 MPa and 253.6 MPa, respectively. The strengthening phase is underaged dispersal GP zone with about 5 nm in size. The heat-treatable aluminum alloy sheet in rolled condition can be used directly in hot forming– quenching integrated process without any prophase heat treatment. & 2013 Elsevier B.V. All rights reserved.
Keywords: Heat-treatable aluminum alloy sheet Hot forming–quenching integrated process Die quenching Strengthening Microstructure
1. Introduction To meet the challenge of energy crisis in automotive and aerospace industries, advanced manufacturing technology and weight reduction have become the urgent matter in material processing, which can be realized by optimized structural design and usage of light materials, such as magnesium, aluminum and titanium alloys [1–4]. There is an increasing demand for usage of aluminum alloy to meet a growing need for lightweight structural metals in automotive and aerospace industries [5,6], considering its high strength to weight ratio and good structural performance. Unfortunately, forming of complex shaped components at room temperature is extremely difficult due to its poor formability and high springback, which limits its wide application. The tricky problem can be solved by forming at elevated temperature. Deformation resistance decreases and formability increases with forming temperature increasing [7,8]. Such deformation behavior of aluminum alloy sheet has been conducted and reported by many researchers [9–12]. Meanwhile, springback reduces significantly while forming at high temperature [13,14].
n
Corresponding author. Tel./fax: þ 86 451 86414751. E-mail address: [email protected] (Z. He).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.02.058
Although hot forming can increase formability and reduce springback, it may lead to softening (recovery or recrystallization) and destroy the desirable microstructure. The following heattreatment is required to restore it and improve its strength, which often results in thermal distortion of the formed parts during quenching. To solve the above problems, a hot forming– quenching integrated process called solution Heat treatment, Forming and cold-die Quenching (HFQ), for producing high strength and high precision Al-alloy sheet parts, has been developed by Lin [15]. Heated sheet after solution heat treatment was formed within cold-dies, whose keys are combining the heat treatment and hot forming in one operation and using a watercooled die to do the quenching. In this process, solution heat treatment is the precondition and following aging can confirm final strength. Cooling within cold-dies after hot forming can reduce the effect of thermal stress on shape of formed parts, especially those with complex and non-closed sections. They have conducted a series of investigations on the subject like 2xxx and 6xxx series aluminum alloy sheets [16–18], especially those formability in HFQ process. 6A02 Aluminum alloy included in Al Mg Si alloys contain magnesium and silicon as major addition elements, which are used for demanding structural applications. Their age-hardening response can be very significant, leading to remarkable improvement of strength after an appropriate heat treatment [19–21].
X. Fan et al. / Materials Science & Engineering A 573 (2013) 154–160
Microstructure freezing in supersaturated solid solution is prerequisite to obtain the maximum improvement of strength, which mainly depends on quenching after full solution heat treatment. Quenching sensitivity exists universally in quenching process, and rapid cooling in critical quenching sensitivity temperature range [22,23] is required to confirm the formation of supersaturated solid solution. Enough cooling ability of cold-dies in hot forming–quenching integrated process is crucial to determine final strengthening. To date, there are few investigations on hot forming–quenching integrated process of heat-treatable aluminum alloy like 6A02 aluminum alloy sheet, especially on the effect of cooling rate of cold-dies on strengthening. In this paper, special device will be developed to experimentally investigate the hot forming–quenching integrated process of 6A02 aluminum alloy sheet. The effect of solution, quenching and aging will be analyzed, and the strengthening mechanism will be clarified by observing the microstructure with scanning electron microscope (SEM), transmission electron microscopy (TEM) and electron backscattering diffraction (EBSD) methods.
2. Experimental 2.1. Materials The material used in this study is aluminum alloy 6A02 sheet with 0.8 mm thickness in rolled condition, provided by Northeast Light Alloy Co., Ltd. The main chemical composition is shown in Table 1. The original Vickers hardness is 73 HV, but it reduces to 40 HV after being heated at 350 1C for 2 min. Table 1 Chemical composition of the 6A02 sheet (wt%). Element
Si
Fe
Cu
Mn
Mg
Ni
Zn
Ti
Al
Composition
0.91
0.24
0.26
0.26
0.68
o0.05
o 0.15
0.025
Rem
155
2.2. Hot forming–quenching integrated process testing Fig. 1 shows the temperature change in hot forming–quenching integrated process. Heated sheet after solution heat treatment (which enables the precipitates to be dissolved within Al matrix) was transferred and located to the cold-dies quickly, hereafter formed into required shape, then held for 5 s within cold-dies to reduce the temperature rapidly to a lower temperature, aiming at freezing the microstructure as a supersaturated solid solution (without precipitating) and avoiding thermal distortion of the formed part during quenching. Finally, aging is often followed to improve and obtain the full strength. In order to investigate the strengthening behavior during the integrated process, experiments were carried out with the process parameters shown in Table 2. Constant solution temperature and artificial aging were formulated generally. The residence time was 2 h between hot forming and artificial aging. Different solution time within 50 min was used to realize various supersaturated degree. Cooling rate depends on the flow rate of cooling water at room temperature (20 1C). The forming test was carried out in the platform developed by engineering research center of hydroforming in Harbin Institute of Technology (ERCH/HIT), as shown in Fig. 2. Blank holder force was provided by 8 die-springs. The sheet specimen was heated by resistance furnace (close to the die) with temperature control accuracy of 71.0 1C. Cooling channels constituted by two half molds were manufactured within die, punch and blank-holder around forming surface by distance of about 8 mm. The highest cooling rate of formed part within cold-dies is about 50 1C/s, whose temperature diversification was measured by infrared camera (FLIR SC325). The cooling rate is about 20 1C/s without cooling water flowing. The specimen was cut with 240 mm in length and 120 mm in width. Down displacement of punch was 19 mm. 2.3. Strength measurement Vickers hardness (HV0.1) at different positions was measured to reflect strengthening effect, whose dwell time is 15 s. The measuring positions were selected according to the arrangement of cooling channels as shown in Fig. 3(b). At the same time, mechanical properties of formed parts were measured to reflect directly the strength by uniaxial tensile test with gauge section 15 mm in parallel length and 5 mm in width. 2.4. Microstructure examination
Fig. 1. Schematic diagram of temperature diversification in hot forming–quenching integrated process.
The distribution of precipitates was observed by scanning electron microscope (SEM), and grain structure was analyzed by electron backscattering diffraction (EBSD). The specimen was prepared by electropolishing working at 20 1C within 15 s, whose electrolytic solution contains 70% CH3OH and 30% HNO3. Meanwhile, transmission electron microscopy (TEM) inspections were carried out to identify the strengthening precipitates, whose specimen was prepared by using MTP-1A two Jet thinning.
Table 2 Process parameters in hot forming–quenching integrated process. Subject
Solution Quenching Aging
Experimental procedure Solution temperature (1C)
Solution time
Cooling rate
Aging
520 520 520
5 min, 25 min, 50 min 50 min 50 min
50 1C/s 20 1C/s, 50 1C/s 50 1C/s
160 1C 10 h 160 1C 10 h None15 1C 120 h160 1C 10 h
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X. Fan et al. / Materials Science & Engineering A 573 (2013) 154–160
Fig. 2. Forming device for hot forming–quenching integrated process.
Fig. 3. Formed parts by: (a) traditional forming and (b) hot forming–quenching integrated process.
Fig.4. Microstructure of the as-received sheet: (a) SEM image, (b) and (c) grain structure and pole figure after being heated at 350 1C/2 min, respectively.
3. Results and discussion
3.1. Microstructure of as-received sheet
Fig. 3 shows the formed parts by two different forming processes. Part of Fig. 3(a) was formed before the heat treatment, serious shape distortion occurred during quenching process. However, no obvious shape distortion appeared (Fig. 3(b)) when the part was formed in hot forming–quenching integrated process. Effect of thermal stress on the shape of formed parts reduces obviously, dimensional shaped accuracy can be guaranteed. Strengthening effect and the related microstructure evolution will be discussed in detail as following.
Fig. 4 shows the microstructure of as-received sheet. From Fig. 4(a), it is clear that the second phase particles (shown in light) are dispersed in Al matrix (shown in dark), which is identified as Mg2Si with EDS method showing the composition of 1.54 (wt%) Mg and 2.90 (wt%) Si [27]. The grain orientation could not be achieved by EBSD because big deformation degree in the rolling process leads to serious lattice distortion taking place in microstructure. Kikuchi pattern can be analyzed clearly after the sheet was heated at 350 1C for 2 min. Original grain structure can be
X. Fan et al. / Materials Science & Engineering A 573 (2013) 154–160
observed again with uneven grain sizes (Fig. 4(b)), whose average value is 116.6 mm, where white contours represent low angle grain boundaries(11–151) with the percentage of 35.8%. Low angle grain exists almost inside large angle grain, which illustrates that a large number of defects begin to disappear. Recovery takes place in this condition, lattice distortion falls off with relative low defects density. No strong recrystallization texture was observed (Fig. 4(c)) with maximum volume fraction only 10.458%, therefore, recovery is the reason why Vickers hardness decreases obviously for heated sheet.
3.2. Effect of solution on strengthening Fig. 5 shows the dependence of Vickers hardness on solution time at the general solution temperature of 520 1C and cooling rate of 50 1C/s. It is obvious that Vickers hardness increases with solution time increase in quenched and aged condition within 50 min. Vickers hardness in quenched condition is lower than original hardness in rolled condition as shown in Fig. 5(a), whose maximum hardness just adds to 70 HV with 50 min in solution time. The Vickers hardness of formed parts in aged condition reduces obviously to 40–50 HV when the solution time is 5 min. The hardness increases to about 70 HV when the solution time is 25 min, which reaches the level of original hardness in rolled condition. In pace with the further increase of solution time, the maximum hardness in aged condition could increase to 106.5 HV while the solution time is 50 min as shown in Fig. 5(b). Fig. 6 shows the mechanical properties of formed parts (at measuring point 1) in aged condition with different solution time at solution temperature of 520 1C and cooling rate of 50 1C/s. Those mechanical properties including tensile strength (sb), yield strength (s0.2) and material constant (K) increase with solution time increase within 50 min, but work-hardening exponent (n value) decreases significantly. As observed, a significant strengthening appears after enough solution treatment, being in agreement with the hardness results plotted in Fig. 5(b). It illustrates that hardness measuring can be used to reflect strengthening effect from another point of view. The tensile strength and yield strength are 315.6 MPa and 253.6 MPa when solution time is 50 min, respectively, being 134.3% and 298.7% higher than the strength of formed parts when solution time is 5 min, which can be considered as forming under hot forming condition without heat treatment.
157
The hardness in quenched condition drops and work-hardening exponent decreases due to softening at high temperature, which leads to work hardening eliminated gradually. Although work hardening is neutralized, the hardness of formed parts in quenched condition was improved again to the level ranging from 61 HV to 69.7 HV, which can be explained by solid solution strengthening. It is an interaction between the mobile dislocations and the solute atoms. The most relevant mechanism is the elastic interactions due to the size misfit and modulus misfit between solute atoms and Al matrixs atoms, a strain field around the atom and hard or soft ‘‘spot’’in the matrix were created respectively [24]. Presence of solute atoms increases the flow stress, so does strength, correlation expressed as:
s ¼ spure þ Hcn
ð1Þ
where spure is a flow stress of a pure metal, H and n are constants, c is the solid solubility. Dissolution of second phase particles can be certificated by SEM images from Fig. 7 although those are in aged condition. It is seen clearly that second phase particles just dissolved sparsely within Al matrix when solution time is 5 min with grain boundary becoming visible. Most of the irregularly shaped second phase particles distributed inhomogeneously and chaotically in Al matrix (Fig. 7(a)),
Fig. 6. Mechanical properties of formed parts with different solution times.
Fig. 5. Dependence of hardness on solution time: (a) in quenched condition and (b) in aged condition.
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X. Fan et al. / Materials Science & Engineering A 573 (2013) 154–160
Fig. 7. SEM images with different solution times: (a) 5 min, (b) 25 min, (c) 50 min.
Fig. 8. Dependence of hardness on cooling rate.
Fig. 9. Dependence of hardness on aging.
which are the same particles generated during sheet manufacturing. More and more original second phase particles have been dissolved within Al matrix (Fig. 7(b) and (c)), which reflects solid solubility increases with solution time increasing within 50 min in quenched condition.
together inside supersaturated solid solution has no time to move out, especially in the quenching sensitivity temperature range. Higher supersaturated solid solubility and vacancy density have much greater potentiality to promote exsolution transformation and lattice precipitation taking place in artificial aging, which leads to final strength improvement. That is why hardness increases with the increase of cooling rate.
3.3. Effect of quenching on strengthening Fig. 8 shows dependence of Vickers hardness on cooling rate by the way of cooling within cold-dies when solution heat treatment at 520 1C for 50 min and artificial aging at 160 1C for 10 h. It is clear that the faster the cooling rate, the greater the strengthening effect. When the cooling rate is about 50 1C/s, the hardness can rise in an average value of 103 HV, and maximum hardness can arrive in 106.5 HV, which is lower than the hardness of as-received sheet (115 HV) by water quenching in the other same heat treatment condition. While there is no cooling water to pass into (cooling rate of 20 1C/s), the average value drops to 97.8 HV, which is just equivalent to the level of air quenching (98 HV). Quenching is used to freeze metastable supersaturated solid solution by the way of rapid cooling so that a higher strength can be received in the subsequent aging. Precipitation occurs during quenching regardless of cooling rate, which mainly consists of coarse precipitates formed and substantially smaller intergranular precipitates. The volume fraction of quench-induced precipitation depends strongly on the cooling rate [25]. When the cooling rate is higher, less precipitation occurs, which has less negative effect on disperse nucleation during following exsolution transformation. Meanwhile, solute atoms and vacancy bundled
3.4. Effect of aging on strengthening Fig. 9 shows the effect of three different aging processes on strengthening with solution treatment condition of 520 1C 50 min and cooling rate of 50 1C/s. It is clear that the hardness of formed parts improves significantly after artificial aging, especially the raise of Vickers hardness in point 1 changing from 63.0 HV to 106.5 HV. Average value of 40 HV was increased at different points. Of course, natural aging also can make the hardness improve sometimes. Unluckily, the maximum increase rate is just about 13% in point 2. Hence, artificial aging is necessary to improve the final strength in hot forming–quenching integrated process for 6A02 aluminum alloy sheet. However, there is no much improvement in hardness while solution time is 5 min from Fig. 5. When the solution time is relatively longer, obvious aging hardening effect existed. Therefore, obtaining of supersaturated solid solution is the fundamental factor to affect following strengthening effect, which determines whether it could strengthen in hot forming–quenching integrated process or not. Fig. 10 shows the microstructure of formed part in hot forming–quenching integrated process (solution treatment at 520 1C/50 min, cold-dies forming by cooling rate of 50 1C/s, aged at 160 1C/10 h). As shown in this figure, there is no obvious
X. Fan et al. / Materials Science & Engineering A 573 (2013) 154–160
159
Fig. 10. Microstructure of final formed parts by EBSD: (a) grain structure and (b) pole figure.
Fig. 11. TEM images of precipitates: (a) 12000 and (b) 49000.
diversification in grain structure (compared to Fig. 4(b)), but low angle grain boundaries reduce obviously with the percentage of 15.1%. Defects produced in rolling process disappeared numerously. The grain grows not too much with 127.5 mm in average size. It is concluded that strengthening effect is not caused by fine grain strengthening. Likewise, the recrystallization texture is not strong as shown in Fig. 10(b), recovery was carried out more thoroughly during heat treatment. TEM images of the same part were obtained to better understand the precipitates as shown in Fig. 11. Dispersal and small precipitates with 5 nm in size, distributed homogeneously, are the strengthening phase particles, whose diffraction spots could not be analyzed clearly due to the relative tiny underaged precipitates, which can be defined GP zone [26]. Because the precipitation sequence for Al–Mg–Si alloys is generally accepted [19,27] as: Al(super saturated solid solution){atomic clusters(Mg, Si)}-{formation of GP zones}-{b0 precipitates}-{b00 precipitates}- {b Mg2Si precipitates}. Among these, the b00 precipitates are considered to give the main strengthening contribution and hence they are mostly responsible for the maximum age hardening effect. Also, it has been shown that the GP zone, b00 , b0 and b precipitates have typical morphologies/sizes of near-spherical/ ˚ ribbons/several mm long 1–2 nm, needles/up to 40 A˚ 40 A˚ 350 A, and plates or cubes/up to 10–20 mm, respectively [26]. GP zones keeping coherent relationship with matrix are the soft particles precipitating at this time [28]. Both the particles themselves and stress fields caused by lattice distortion around the particles could impede the progress of dislocation movement,
which plays a dominant role in strengthening effect. The number and space of particles determine strengthening effect simultaneously, relationship shown as follows:
Dss ¼ a
1 dT
ð2Þ
where Dss is the strengthening effect, a is the proportion constant and dT is the space between second phase particles. In addition, dislocation produced in rolling process did not disappear entirely, which can be seen clearly from Fig. 11(a). Existence of dislocation obstructs diffusion of solute atoms. Segregation area of solute atoms was formed gradually in artificial aging accompanying with its merging and growing. Spherical phase was visible on dislocations, which was identified as Al6(MnFeSi) in terms of the composition with EDS method [29,30]. Formation of spherical phase takes part of silicon away, which is used to form strengthening phase (Mg2Si). Although interaction happens between spherical phase and dislocation, it will reduce the final strengthening effect. Fig. 12 shows the SEM images that indicates the evolution of second phase particles in different aging processes. Few dispersed precipitates separate out during natural aging, which can be seen clearly from Fig. 12(a). The reason is that solute atoms cannot receive enough energy to diffuse at room temperature, high density vacancy cannot work. Though supersaturated solid solution is under an unstable state, it is hard to transfer to a stable state. Segregation enhancement area has no ability formed that
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Fig. 12. SEM images in different aged conditions: (a) natural aging and (b) artificial aging.
leads to arising in hardness slightly. Existence of vacancy helps contribute a certain amount of stored energy that accelerates solute atoms diffuse during artificial aging. Therefore, more percentage tiny and dispersed white particles can be observed not only on the grain boundaries but also within the grains as shown in Fig.12(b). That indicates more percentage GP zones were formed so that hardness increases significantly.
4. Conclusions Strengthening effect and the related microstructure evolution of 6A02 aluminum alloy sheet in hot forming–quenching integrated process have been analyzed. Conclusions can be drawn as follows:
(1) Vickers hardness of formed part increases with solution time ( o50 min) and cooling rate increase. It can increase to 106.5 HV from cold-rolled condition or heated condition in hot forming–quenching integrated process (solution treatment at 520 1C/50 min, cold-dies forming by cooling rate of 50 1C/s, aged at 160 1C/10 h). The corresponding tensile strength and yield strength are 315.6 MPa and 253.6 MPa, respectively, being 134.3% and 298.7% higher than the strength of formed parts when solution time is 5 min. (2) Recovery is the chief mechanism for Vickers hardness decrease of the heated sheet and formed parts in quenched condition. Aging hardening is the main strengthening mechanism. Artificial aging is necessary to improve the hardness of 6A02 aluminum alloy formed parts. The strengthening phase is underaged dispersal GP zone with 5 nm in size after being aged at 160 1C/10 h. (3) The heat-treatable aluminum alloy sheet in rolled condition can be used directly in hot forming–quenching integrated process without any prophase heat treatment.
China (No. 51175111). The authors would like to take this opportunity to express their sincere appreciation to the fundings. Special thanks for Mr. Peng Lin for his assistance in the microstructure analysis.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
Acknowledgments This study was financially supported by the Program for New Century Excellent Talents in University (NCET-11-0799), the Fundamental Research Funds for the Central Universities (HIT.BRETIII.201204) and the National Natural Science Foundation of
[27] [28] [29] [30]
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Contents lists available at SciVerse ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Experimental investigation on hot forming–quenching integrated process of 6A02 aluminum alloy sheet Xiaobo Fan, Zhubin He n, Shijian Yuan, Kailun Zheng National Key Laboratory for Precision Hot Processing of Metals, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2013 Received in revised form 24 February 2013 Accepted 27 February 2013 Available online 7 March 2013
As important light-weight structure material, aluminum alloy has been widely used in automotive and aerospace industries, in which the manufacturing of parts with high strength and good dimensional accuracy has been the main task. In this paper, special device was developed to investigate the hot forming–quenching integrated process of cold-rolled 6A02 aluminum alloy sheet. Strengthening effect was reflected by Vickers hardness measuring and uniaxial tensile test. Microstructure examination was conducted to clarify the strengthening mechanism by scanning electron microscope (SEM), transmission electron microscopy(TEM) and electron backscattering diffraction (EBSD). Results show that Vickers hardness increases with solution time (o 50 min) increase, and improves significantly after artificial aging. The faster the cooling rate, the greater the strengthening effect. The Vickers hardness of formed part can increase to 106.5 HV from 73 HV in cold-rolled condition or 40 HV in heated condition in hot forming–quenching integrated process (solution treatment at 520 1C/50 min, cooling by 50 1C/s, being aged at 160 1C/10 h). The corresponding tensile strength and yield strength are 315.6 MPa and 253.6 MPa, respectively. The strengthening phase is underaged dispersal GP zone with about 5 nm in size. The heat-treatable aluminum alloy sheet in rolled condition can be used directly in hot forming– quenching integrated process without any prophase heat treatment. & 2013 Elsevier B.V. All rights reserved.
Keywords: Heat-treatable aluminum alloy sheet Hot forming–quenching integrated process Die quenching Strengthening Microstructure
1. Introduction To meet the challenge of energy crisis in automotive and aerospace industries, advanced manufacturing technology and weight reduction have become the urgent matter in material processing, which can be realized by optimized structural design and usage of light materials, such as magnesium, aluminum and titanium alloys [1–4]. There is an increasing demand for usage of aluminum alloy to meet a growing need for lightweight structural metals in automotive and aerospace industries [5,6], considering its high strength to weight ratio and good structural performance. Unfortunately, forming of complex shaped components at room temperature is extremely difficult due to its poor formability and high springback, which limits its wide application. The tricky problem can be solved by forming at elevated temperature. Deformation resistance decreases and formability increases with forming temperature increasing [7,8]. Such deformation behavior of aluminum alloy sheet has been conducted and reported by many researchers [9–12]. Meanwhile, springback reduces significantly while forming at high temperature [13,14].
n
Corresponding author. Tel./fax: þ 86 451 86414751. E-mail address: [email protected] (Z. He).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.02.058
Although hot forming can increase formability and reduce springback, it may lead to softening (recovery or recrystallization) and destroy the desirable microstructure. The following heattreatment is required to restore it and improve its strength, which often results in thermal distortion of the formed parts during quenching. To solve the above problems, a hot forming– quenching integrated process called solution Heat treatment, Forming and cold-die Quenching (HFQ), for producing high strength and high precision Al-alloy sheet parts, has been developed by Lin [15]. Heated sheet after solution heat treatment was formed within cold-dies, whose keys are combining the heat treatment and hot forming in one operation and using a watercooled die to do the quenching. In this process, solution heat treatment is the precondition and following aging can confirm final strength. Cooling within cold-dies after hot forming can reduce the effect of thermal stress on shape of formed parts, especially those with complex and non-closed sections. They have conducted a series of investigations on the subject like 2xxx and 6xxx series aluminum alloy sheets [16–18], especially those formability in HFQ process. 6A02 Aluminum alloy included in Al Mg Si alloys contain magnesium and silicon as major addition elements, which are used for demanding structural applications. Their age-hardening response can be very significant, leading to remarkable improvement of strength after an appropriate heat treatment [19–21].
X. Fan et al. / Materials Science & Engineering A 573 (2013) 154–160
Microstructure freezing in supersaturated solid solution is prerequisite to obtain the maximum improvement of strength, which mainly depends on quenching after full solution heat treatment. Quenching sensitivity exists universally in quenching process, and rapid cooling in critical quenching sensitivity temperature range [22,23] is required to confirm the formation of supersaturated solid solution. Enough cooling ability of cold-dies in hot forming–quenching integrated process is crucial to determine final strengthening. To date, there are few investigations on hot forming–quenching integrated process of heat-treatable aluminum alloy like 6A02 aluminum alloy sheet, especially on the effect of cooling rate of cold-dies on strengthening. In this paper, special device will be developed to experimentally investigate the hot forming–quenching integrated process of 6A02 aluminum alloy sheet. The effect of solution, quenching and aging will be analyzed, and the strengthening mechanism will be clarified by observing the microstructure with scanning electron microscope (SEM), transmission electron microscopy (TEM) and electron backscattering diffraction (EBSD) methods.
2. Experimental 2.1. Materials The material used in this study is aluminum alloy 6A02 sheet with 0.8 mm thickness in rolled condition, provided by Northeast Light Alloy Co., Ltd. The main chemical composition is shown in Table 1. The original Vickers hardness is 73 HV, but it reduces to 40 HV after being heated at 350 1C for 2 min. Table 1 Chemical composition of the 6A02 sheet (wt%). Element
Si
Fe
Cu
Mn
Mg
Ni
Zn
Ti
Al
Composition
0.91
0.24
0.26
0.26
0.68
o0.05
o 0.15
0.025
Rem
155
2.2. Hot forming–quenching integrated process testing Fig. 1 shows the temperature change in hot forming–quenching integrated process. Heated sheet after solution heat treatment (which enables the precipitates to be dissolved within Al matrix) was transferred and located to the cold-dies quickly, hereafter formed into required shape, then held for 5 s within cold-dies to reduce the temperature rapidly to a lower temperature, aiming at freezing the microstructure as a supersaturated solid solution (without precipitating) and avoiding thermal distortion of the formed part during quenching. Finally, aging is often followed to improve and obtain the full strength. In order to investigate the strengthening behavior during the integrated process, experiments were carried out with the process parameters shown in Table 2. Constant solution temperature and artificial aging were formulated generally. The residence time was 2 h between hot forming and artificial aging. Different solution time within 50 min was used to realize various supersaturated degree. Cooling rate depends on the flow rate of cooling water at room temperature (20 1C). The forming test was carried out in the platform developed by engineering research center of hydroforming in Harbin Institute of Technology (ERCH/HIT), as shown in Fig. 2. Blank holder force was provided by 8 die-springs. The sheet specimen was heated by resistance furnace (close to the die) with temperature control accuracy of 71.0 1C. Cooling channels constituted by two half molds were manufactured within die, punch and blank-holder around forming surface by distance of about 8 mm. The highest cooling rate of formed part within cold-dies is about 50 1C/s, whose temperature diversification was measured by infrared camera (FLIR SC325). The cooling rate is about 20 1C/s without cooling water flowing. The specimen was cut with 240 mm in length and 120 mm in width. Down displacement of punch was 19 mm. 2.3. Strength measurement Vickers hardness (HV0.1) at different positions was measured to reflect strengthening effect, whose dwell time is 15 s. The measuring positions were selected according to the arrangement of cooling channels as shown in Fig. 3(b). At the same time, mechanical properties of formed parts were measured to reflect directly the strength by uniaxial tensile test with gauge section 15 mm in parallel length and 5 mm in width. 2.4. Microstructure examination
Fig. 1. Schematic diagram of temperature diversification in hot forming–quenching integrated process.
The distribution of precipitates was observed by scanning electron microscope (SEM), and grain structure was analyzed by electron backscattering diffraction (EBSD). The specimen was prepared by electropolishing working at 20 1C within 15 s, whose electrolytic solution contains 70% CH3OH and 30% HNO3. Meanwhile, transmission electron microscopy (TEM) inspections were carried out to identify the strengthening precipitates, whose specimen was prepared by using MTP-1A two Jet thinning.
Table 2 Process parameters in hot forming–quenching integrated process. Subject
Solution Quenching Aging
Experimental procedure Solution temperature (1C)
Solution time
Cooling rate
Aging
520 520 520
5 min, 25 min, 50 min 50 min 50 min
50 1C/s 20 1C/s, 50 1C/s 50 1C/s
160 1C 10 h 160 1C 10 h None15 1C 120 h160 1C 10 h
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Fig. 2. Forming device for hot forming–quenching integrated process.
Fig. 3. Formed parts by: (a) traditional forming and (b) hot forming–quenching integrated process.
Fig.4. Microstructure of the as-received sheet: (a) SEM image, (b) and (c) grain structure and pole figure after being heated at 350 1C/2 min, respectively.
3. Results and discussion
3.1. Microstructure of as-received sheet
Fig. 3 shows the formed parts by two different forming processes. Part of Fig. 3(a) was formed before the heat treatment, serious shape distortion occurred during quenching process. However, no obvious shape distortion appeared (Fig. 3(b)) when the part was formed in hot forming–quenching integrated process. Effect of thermal stress on the shape of formed parts reduces obviously, dimensional shaped accuracy can be guaranteed. Strengthening effect and the related microstructure evolution will be discussed in detail as following.
Fig. 4 shows the microstructure of as-received sheet. From Fig. 4(a), it is clear that the second phase particles (shown in light) are dispersed in Al matrix (shown in dark), which is identified as Mg2Si with EDS method showing the composition of 1.54 (wt%) Mg and 2.90 (wt%) Si [27]. The grain orientation could not be achieved by EBSD because big deformation degree in the rolling process leads to serious lattice distortion taking place in microstructure. Kikuchi pattern can be analyzed clearly after the sheet was heated at 350 1C for 2 min. Original grain structure can be
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observed again with uneven grain sizes (Fig. 4(b)), whose average value is 116.6 mm, where white contours represent low angle grain boundaries(11–151) with the percentage of 35.8%. Low angle grain exists almost inside large angle grain, which illustrates that a large number of defects begin to disappear. Recovery takes place in this condition, lattice distortion falls off with relative low defects density. No strong recrystallization texture was observed (Fig. 4(c)) with maximum volume fraction only 10.458%, therefore, recovery is the reason why Vickers hardness decreases obviously for heated sheet.
3.2. Effect of solution on strengthening Fig. 5 shows the dependence of Vickers hardness on solution time at the general solution temperature of 520 1C and cooling rate of 50 1C/s. It is obvious that Vickers hardness increases with solution time increase in quenched and aged condition within 50 min. Vickers hardness in quenched condition is lower than original hardness in rolled condition as shown in Fig. 5(a), whose maximum hardness just adds to 70 HV with 50 min in solution time. The Vickers hardness of formed parts in aged condition reduces obviously to 40–50 HV when the solution time is 5 min. The hardness increases to about 70 HV when the solution time is 25 min, which reaches the level of original hardness in rolled condition. In pace with the further increase of solution time, the maximum hardness in aged condition could increase to 106.5 HV while the solution time is 50 min as shown in Fig. 5(b). Fig. 6 shows the mechanical properties of formed parts (at measuring point 1) in aged condition with different solution time at solution temperature of 520 1C and cooling rate of 50 1C/s. Those mechanical properties including tensile strength (sb), yield strength (s0.2) and material constant (K) increase with solution time increase within 50 min, but work-hardening exponent (n value) decreases significantly. As observed, a significant strengthening appears after enough solution treatment, being in agreement with the hardness results plotted in Fig. 5(b). It illustrates that hardness measuring can be used to reflect strengthening effect from another point of view. The tensile strength and yield strength are 315.6 MPa and 253.6 MPa when solution time is 50 min, respectively, being 134.3% and 298.7% higher than the strength of formed parts when solution time is 5 min, which can be considered as forming under hot forming condition without heat treatment.
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The hardness in quenched condition drops and work-hardening exponent decreases due to softening at high temperature, which leads to work hardening eliminated gradually. Although work hardening is neutralized, the hardness of formed parts in quenched condition was improved again to the level ranging from 61 HV to 69.7 HV, which can be explained by solid solution strengthening. It is an interaction between the mobile dislocations and the solute atoms. The most relevant mechanism is the elastic interactions due to the size misfit and modulus misfit between solute atoms and Al matrixs atoms, a strain field around the atom and hard or soft ‘‘spot’’in the matrix were created respectively [24]. Presence of solute atoms increases the flow stress, so does strength, correlation expressed as:
s ¼ spure þ Hcn
ð1Þ
where spure is a flow stress of a pure metal, H and n are constants, c is the solid solubility. Dissolution of second phase particles can be certificated by SEM images from Fig. 7 although those are in aged condition. It is seen clearly that second phase particles just dissolved sparsely within Al matrix when solution time is 5 min with grain boundary becoming visible. Most of the irregularly shaped second phase particles distributed inhomogeneously and chaotically in Al matrix (Fig. 7(a)),
Fig. 6. Mechanical properties of formed parts with different solution times.
Fig. 5. Dependence of hardness on solution time: (a) in quenched condition and (b) in aged condition.
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Fig. 7. SEM images with different solution times: (a) 5 min, (b) 25 min, (c) 50 min.
Fig. 8. Dependence of hardness on cooling rate.
Fig. 9. Dependence of hardness on aging.
which are the same particles generated during sheet manufacturing. More and more original second phase particles have been dissolved within Al matrix (Fig. 7(b) and (c)), which reflects solid solubility increases with solution time increasing within 50 min in quenched condition.
together inside supersaturated solid solution has no time to move out, especially in the quenching sensitivity temperature range. Higher supersaturated solid solubility and vacancy density have much greater potentiality to promote exsolution transformation and lattice precipitation taking place in artificial aging, which leads to final strength improvement. That is why hardness increases with the increase of cooling rate.
3.3. Effect of quenching on strengthening Fig. 8 shows dependence of Vickers hardness on cooling rate by the way of cooling within cold-dies when solution heat treatment at 520 1C for 50 min and artificial aging at 160 1C for 10 h. It is clear that the faster the cooling rate, the greater the strengthening effect. When the cooling rate is about 50 1C/s, the hardness can rise in an average value of 103 HV, and maximum hardness can arrive in 106.5 HV, which is lower than the hardness of as-received sheet (115 HV) by water quenching in the other same heat treatment condition. While there is no cooling water to pass into (cooling rate of 20 1C/s), the average value drops to 97.8 HV, which is just equivalent to the level of air quenching (98 HV). Quenching is used to freeze metastable supersaturated solid solution by the way of rapid cooling so that a higher strength can be received in the subsequent aging. Precipitation occurs during quenching regardless of cooling rate, which mainly consists of coarse precipitates formed and substantially smaller intergranular precipitates. The volume fraction of quench-induced precipitation depends strongly on the cooling rate [25]. When the cooling rate is higher, less precipitation occurs, which has less negative effect on disperse nucleation during following exsolution transformation. Meanwhile, solute atoms and vacancy bundled
3.4. Effect of aging on strengthening Fig. 9 shows the effect of three different aging processes on strengthening with solution treatment condition of 520 1C 50 min and cooling rate of 50 1C/s. It is clear that the hardness of formed parts improves significantly after artificial aging, especially the raise of Vickers hardness in point 1 changing from 63.0 HV to 106.5 HV. Average value of 40 HV was increased at different points. Of course, natural aging also can make the hardness improve sometimes. Unluckily, the maximum increase rate is just about 13% in point 2. Hence, artificial aging is necessary to improve the final strength in hot forming–quenching integrated process for 6A02 aluminum alloy sheet. However, there is no much improvement in hardness while solution time is 5 min from Fig. 5. When the solution time is relatively longer, obvious aging hardening effect existed. Therefore, obtaining of supersaturated solid solution is the fundamental factor to affect following strengthening effect, which determines whether it could strengthen in hot forming–quenching integrated process or not. Fig. 10 shows the microstructure of formed part in hot forming–quenching integrated process (solution treatment at 520 1C/50 min, cold-dies forming by cooling rate of 50 1C/s, aged at 160 1C/10 h). As shown in this figure, there is no obvious
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Fig. 10. Microstructure of final formed parts by EBSD: (a) grain structure and (b) pole figure.
Fig. 11. TEM images of precipitates: (a) 12000 and (b) 49000.
diversification in grain structure (compared to Fig. 4(b)), but low angle grain boundaries reduce obviously with the percentage of 15.1%. Defects produced in rolling process disappeared numerously. The grain grows not too much with 127.5 mm in average size. It is concluded that strengthening effect is not caused by fine grain strengthening. Likewise, the recrystallization texture is not strong as shown in Fig. 10(b), recovery was carried out more thoroughly during heat treatment. TEM images of the same part were obtained to better understand the precipitates as shown in Fig. 11. Dispersal and small precipitates with 5 nm in size, distributed homogeneously, are the strengthening phase particles, whose diffraction spots could not be analyzed clearly due to the relative tiny underaged precipitates, which can be defined GP zone [26]. Because the precipitation sequence for Al–Mg–Si alloys is generally accepted [19,27] as: Al(super saturated solid solution){atomic clusters(Mg, Si)}-{formation of GP zones}-{b0 precipitates}-{b00 precipitates}- {b Mg2Si precipitates}. Among these, the b00 precipitates are considered to give the main strengthening contribution and hence they are mostly responsible for the maximum age hardening effect. Also, it has been shown that the GP zone, b00 , b0 and b precipitates have typical morphologies/sizes of near-spherical/ ˚ ribbons/several mm long 1–2 nm, needles/up to 40 A˚ 40 A˚ 350 A, and plates or cubes/up to 10–20 mm, respectively [26]. GP zones keeping coherent relationship with matrix are the soft particles precipitating at this time [28]. Both the particles themselves and stress fields caused by lattice distortion around the particles could impede the progress of dislocation movement,
which plays a dominant role in strengthening effect. The number and space of particles determine strengthening effect simultaneously, relationship shown as follows:
Dss ¼ a
1 dT
ð2Þ
where Dss is the strengthening effect, a is the proportion constant and dT is the space between second phase particles. In addition, dislocation produced in rolling process did not disappear entirely, which can be seen clearly from Fig. 11(a). Existence of dislocation obstructs diffusion of solute atoms. Segregation area of solute atoms was formed gradually in artificial aging accompanying with its merging and growing. Spherical phase was visible on dislocations, which was identified as Al6(MnFeSi) in terms of the composition with EDS method [29,30]. Formation of spherical phase takes part of silicon away, which is used to form strengthening phase (Mg2Si). Although interaction happens between spherical phase and dislocation, it will reduce the final strengthening effect. Fig. 12 shows the SEM images that indicates the evolution of second phase particles in different aging processes. Few dispersed precipitates separate out during natural aging, which can be seen clearly from Fig. 12(a). The reason is that solute atoms cannot receive enough energy to diffuse at room temperature, high density vacancy cannot work. Though supersaturated solid solution is under an unstable state, it is hard to transfer to a stable state. Segregation enhancement area has no ability formed that
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Fig. 12. SEM images in different aged conditions: (a) natural aging and (b) artificial aging.
leads to arising in hardness slightly. Existence of vacancy helps contribute a certain amount of stored energy that accelerates solute atoms diffuse during artificial aging. Therefore, more percentage tiny and dispersed white particles can be observed not only on the grain boundaries but also within the grains as shown in Fig.12(b). That indicates more percentage GP zones were formed so that hardness increases significantly.
4. Conclusions Strengthening effect and the related microstructure evolution of 6A02 aluminum alloy sheet in hot forming–quenching integrated process have been analyzed. Conclusions can be drawn as follows:
(1) Vickers hardness of formed part increases with solution time ( o50 min) and cooling rate increase. It can increase to 106.5 HV from cold-rolled condition or heated condition in hot forming–quenching integrated process (solution treatment at 520 1C/50 min, cold-dies forming by cooling rate of 50 1C/s, aged at 160 1C/10 h). The corresponding tensile strength and yield strength are 315.6 MPa and 253.6 MPa, respectively, being 134.3% and 298.7% higher than the strength of formed parts when solution time is 5 min. (2) Recovery is the chief mechanism for Vickers hardness decrease of the heated sheet and formed parts in quenched condition. Aging hardening is the main strengthening mechanism. Artificial aging is necessary to improve the hardness of 6A02 aluminum alloy formed parts. The strengthening phase is underaged dispersal GP zone with 5 nm in size after being aged at 160 1C/10 h. (3) The heat-treatable aluminum alloy sheet in rolled condition can be used directly in hot forming–quenching integrated process without any prophase heat treatment.
China (No. 51175111). The authors would like to take this opportunity to express their sincere appreciation to the fundings. Special thanks for Mr. Peng Lin for his assistance in the microstructure analysis.
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Acknowledgments This study was financially supported by the Program for New Century Excellent Talents in University (NCET-11-0799), the Fundamental Research Funds for the Central Universities (HIT.BRETIII.201204) and the National Natural Science Foundation of
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