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Microstructural evolution and mechanical properties of welding seams in aluminum alloy profiles extruded by a porthole die under different billet heating temperatures and extrusion speeds
Accepted Manuscript Title: Microstructural evolution and mechanical properties of welding seams in aluminum alloy profiles extruded by a porthole die under different billet heating temperatures and extrusion speeds Authors: Junquan Yu, Guoqun Zhao, Weichao Cui, Cunsheng Zhang, Liang Chen PII: DOI: Reference:
S0924-0136(17)30165-6 http://dx.doi.org/doi:10.1016/j.jmatprotec.2017.04.030 PROTEC 15208
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
12-11-2016 1-4-2017 30-4-2017
Please cite this article as: Yu, Junquan, Zhao, Guoqun, Cui, Weichao, Zhang, Cunsheng, Chen, Liang, Microstructural evolution and mechanical properties of welding seams in aluminum alloy profiles extruded by a porthole die under different billet heating temperatures and extrusion speeds.Journal of Materials Processing Technology http://dx.doi.org/10.1016/j.jmatprotec.2017.04.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microstructural evolution and mechanical properties of welding seams in aluminum alloy profiles extruded by a porthole die under different billet heating temperatures and extrusion speeds Junquan Yua, Guoqun Zhao,a, Weichao Cuib, Cunsheng Zhanga , Liang Chena
a
Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education),
Shandong University, Jinan, Shandong 250061, PR China
b
Shandong Yancon Light Alloy Co., Ltd., Zoucheng, Shandong 273500, PR China
Abstract
Porthole die extrusion process of aluminum alloy profiles is a hot deformation process involving solid state welding. Microstructural evolution of welding seams is the key factor to determine mechanical properties of extruded profiles. In this work, the grain structure, bonding interface structure and precipitates of welding seams in the profiles extruded under different billet heating temperatures and extrusion speeds were characterized, and the hardness, strength and ductility of welding seams were analyzed. The influence of billet heating temperature and extrusion speed on the microstructure and mechanical properties of welding seams was studied. It was found that, in the porthole die extrusion process of aluminum alloy profiles, fine or coarse grains and micro-voids can be formed in welding seams. Although the new grains through the bonding interface have been formed, there are still many micro-voids in these new grains. Increasing billet heating temperature and extrusion speed not only contributes to the formation of the new grains through the bonding interface, but also promotes the closure of
Corresponding author at: Key Laboratory for Liquid–Solid Structural Evolution & Processing of Materials (Ministry of Education),
Shandong University, Jinan, Shandong 250061, PR China. Tel.: +86(0)53188393238; fax: +86(0)53188392811. E-mail address: [email protected] (Guoqun Zhao). 1
the micro-voids on the bonding interface, and thereby improves the atomic bonding degree of the material on both sides of the bonding interface. The hardness, strength and ductility of the extruded profiles can be improved by increasing billet heating temperature and extrusion speed.
Keywords: Aluminum alloy profile; Porthole die extrusion; Process parameters; Welding seam; Bonding interface structure
1. Introduction
As a kind of lightweight structural parts with an excellent comprehensive performance, the aluminum alloy profiles with hollow sections have been widely used in many fields. As reported by Chen et al. ( 2011), most of hollow section profiles, especially with large scale, thin-wall, multi-cavity and complex cross-section, are produced by using porthole die direct extrusion process. The hollow section profiles usually have many longitudinal welding seams. As reported by den Bakker et al. (2014), the longitudinal welding seams easily become the weakest part of profiles. Therefore, the control of microstructure of welding seams is very important for improving mechanical properties of the hollow section profiles.
In porthole die extrusion process, how to improve welding seams quality has attracted extensive attention. Optimizing extrusion die structure and process parameters is the main measure to improve welding quality. Currently, the effect of die structure on welding quality has been revealed deeply. For example, as reported by Valberg et al. (1995), the increase of the depth or the volume of welding chambers and the design of the pointed bridge all contribute to the improvement of the welding quality of extruded profiles. However, there are still some disputes on the effect of extrusion speed and billet heating temperature on welding quality. On the one hand, Valberg et al. (1995) found that in the porthole die extrusion of AA6082 and AA7108 aluminum alloys profiles, a
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high extrusion speed can result in a tearing of the metal in the welding zone. Based on the experimental work of Valberg et al. (1995), Donati and Tomesani (2004) concluded that increasing extrusion speed will cause the deterioration of welding quality. On the another hand, Liu et al. (2008) found that the increase of extrusion speed can improve the strength and ductility of a magnesium alloy square tube. This means that the welding quality can be improved by increasing extrusion speed. In addition, Jo et al. (2003) showed that the increase of billet heating temperature contributes to the improvement of the expanding ratio of Al7003 hollow section profiles. While Bingöl and Keskin (2007) showed that the increase of billet heating temperature results in the appearance of coarse grains in welding zone of AA6063 aluminum alloy profiles. Based on the above-mentioned reasons, it is imperative to further study the influence of billet heating temperature and extrusion speed on the welding quality of extruded profiles.
In hot extrusion process, the microstructural evolution and mechanical properties of aluminum alloy profiles were investigated by some researchers. Güzel et al. (2012) proposed a method for determining dynamic grain structure evolution during hot aluminum extrusion, and found that the grain structure in dead metal zone, shear intensive zone, inflowing material zone and exiting profile zone of the extrusion butt is significantly different. Fan et al. (2016) investigated the grain structure and texture evolution in the porthole die extrusion process of a multiport flat tube, and found that there are rolling-type and cube textures at the solid state welding region. Zhao et al. (2016) investigated the microstructure and mechanical properties of extrusion welds in continuous extrusion of AA6063 aluminum alloy with double billets, and found that the extrusion welds markedly influence the elongation of an extrudate. Güley et al. (2013) studied the evolution of microstructure during extrusion of machining chips, and found that the mechanical properties of the extruded profiles can be improved by optimizing extrusion die design. Furthermore, they embedded a criterion for the breaking of the oxide layers and an index for the welding quality into the finite element software to evaluate the welding quality of the extruded profiles. 3
Bingöl and Keskin (2007) studied the effect of different extrusion temperatures and speeds on extrusion welds, and found that the distinction of the welding zones is decreased with the increase of billet temperature and increased with the increase of ram speed according to the macrostructure of the etched profiles. Gagliardi et al. (2014) studied the influence of the profile thickness and ram velocity on the grain sizes and tensile properties of AA6060 aluminum alloy, and found that the profile thickness and ram velocity jointly affect the quality of the welding seams. Zhao et al. (2013) investigated the effect of deformation speed on the microstructure and mechanical properties of AA6063 during continuous extrusion, and showed that there is an optimum extrusion wheel velocity to obtain the products with good mechanical properties. Yu et al. (2016b) investigated the influence of the shape of bridges and the depth of welding chambers on the microstructure and mechanical properties of welding seams, and found that the ductility of extrude profiles is increased with the increase of the depth of welding chamber. Although the above mentioned works have been conducted to study the microstructural evolution and mechanical properties of aluminum alloy profiles, it should be noticed that, the porthole die extrusion process of aluminum alloy profiles is a hot deformation process involving solid state welding. The microstructural evolution of welding seams is the key factor to determine the mechanical properties of extruded profiles. Unfortunately, there is still lack of a systematic investigation on the microstructural evolution in welding zone of aluminum alloy profiles extruded under different billet heating temperatures and extrusion speeds by now. Especially, there almost has not been any investigation on the effect of billet heating temperature and extrusion speed on the bonding interface structure of the welding seams in the profiles extruded by using porthole dies.
For a long time, predicting the welding seams quality of extruded profiles has been a challenge. Some welding criteria were proposed to predict welding seams quality. Based on the pressure in the welding plane, Akeret (1972) proposed the maximum pressure criterion, Plata and Piwnik (2000) proposed the pressure-time criterion 4
(Q criterion), and Donati and Tomesani (2004) proposed the pressure-time-flow criterion (K criterion). Recently, based on plastic deformation and diffusion mechanisms, Yu et al. (2016a) proposed the J criterion from the viewpoint of the closure behaviors of the micro-voids on bonding interfaces. According to the solid state welding mechanism proposed by Yu et al. (2016a), the closure of the micro-voids on the bonding interface is the precondition to form high quality welding seams. Therefore, in order to evaluate the welding quality, the closure degree of the micro-voids on the bonding interface should be firstly determined. However, the micro-voids on the bonding interface of welding seams in extruded profiles have not been observed by experiments so far, and the research on the influence of extrusion parameters such as the billet heating temperature and extrusion speed on the evolution of micro-voids is still a blank.
In conclusion, although some studies have been done to investigate the welding seams in extruded profiles, most of the previous works mainly focused on the mechanical properties and the grain structure of welding seams. There is still lack of a deep and careful characterization of the bonding interface structure from nanometer scale. The bonding interface structure of the welding seams determines the atomic bonding degree of the material on both sides of the bonding interface. Unfortunately, the influence of billet heating temperature and extrusion speed on the atomic bonding degree is still not well known. In addition, most of the previous works mainly investigated the effect of the grains and second-phase particles on the mechanical properties of welding seams. In fact, the bonding interface structure also influences the mechanical properties of welding seams. Thus, the effect of the bonding interface structure on the mechanical properties of welding seams still needs to be studied.
In this work, the extrusion experiments for a plate-shaped profile under different billet heating temperatures and extrusion speeds were performed. The microstructure of welding seams in extruded profiles were characterized by means of optical microscopy (OM), electron backscattered diffraction (EBSD) and transmission 5
electron microscopy (TEM), and the mechanical properties of welding seams were analyzed through microhardness and tensile tests. The influence of billet heating temperature and extrusion speed on the microstructure and mechanical properties of welding seams was studied.
2. Experimental methods and procedures 2.1. Materials preparation and extrusion experiments
The material for extrusion experiments is the AA6063 aluminum alloy billets homogenized at 530 ℃for 14 h with the dimensions of Ø152 mm × 700 mm. Its chemical compositions are shown in table 1. The extrusion die for experiments consists of an upper die, an arched bridge and a lower die, as shown in Fig. 1. The height h of the welding chamber was designed as 15.0 mm. More detailed information about the extrusion die was given in the research of Yu et al. (2016a). The extrusion ratio of the profile is 20.9. The temperatures of container and extrusion die were set as 440 ℃and 450 ℃, respectively. A 16 MN extrusion press was used for extrusion experiments. The billet heating temperature and the extrusion speed for each extrusion experiment were shown in table 2. For each extrusion experiment, two billets were extruded. In addition, an infrared temperature detector was used to monitor the temperature of extruded profiles. When the extrusion process reached the steady state, the measured exit temperatures of the profiles extruded by experiments E1-E4 are about 460 ℃, 480 ℃, 495 ℃ and 535 ℃. After extrusion, the extruded profiles were rapidly cooled with water to the room temperature, and then annealed at 175 ℃for 8 h.
2.2. Sampling, microstructure characterization and mechanical properties tests
After extrusion experiments, the plate-shaped profiles with the thickness of 12 mm and the width of 80 mm were obtained. The longitudinal welding seam is located in the mid-position of the profile’s cross-section, as
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shown in Fig. 2(a). For easy description, in this work, the width, length and thickness directions of extruded profiles were respectively defined as WD, LD and TD directions. The samples for microstructural observation were taken from the areas around the welding seam on the cross-section of extruded profiles. The samples for tensile tests were cut at 90°, 45° and 0° to the welding seam, as shown in Fig. 2(b). The dimensions of the tensile samples are given in Fig. 2(c).
For OM observations, the surfaces of samples to be observed were ground and polished into mirror-like ones, and then etched in the solution with 1.0 ml HF, 1.5 ml HCl, 2.5 ml HNO3 and 95.0 ml H2O. For EBSD measurements, firstly, the welding seams of samples were determined and marked according to OM observation results. Next, the surfaces of the samples were re-ground with emery papers up to 2000 grits and re-polished into a mirror-like ones, and then electro-polished in a solution containing 20 vol% perchloric acid and 80 vol% methanol at -20 ℃ and 15 V for 20 s to eliminate the plastically deformed region. EBSD measurements were carried out by using SEM (CARL ZEISS EVO MA 10) equipped with a device of Oxford EBSD HKL Channel 5 at an acceleration voltage of 20 kV. For TEM observations, firstly, the welding seams of samples were also marked according to the OM observations. Then, the samples were ground to a thickness about 30 μm, and then cut into disks with 3.0 mm diameter and further thinned with ion beams. TEM observations were performed in a JEM-2100F TEM at an operating voltage of 200 kV.
Vickers hardness of the material around the welding seams was measured by using a load of 100 g and dwell time of 10 s with the 100 μm distance between successive indentations. Before tensile testing, two lines were drawn on each tensile sample to indicate the original gauge length L0 (25.00 mm). The tensile samples were tested on an electronic tensile testing machine, and the tensile speed was set as 0.01 mm/s, i.e. the strain rate is 0.0004/s. For each extrusion condition, the tensile tests were repeated three times to ensure the accuracy and 7
repeatability of the tests. After tensile tests, a vernier caliper was used to measure the fractured specimens’ gauge lengths Lf, and the elongation percentages were determined by calculating the values of (Lf-L0)/L0×100%. In addition, the fracture surfaces of tensile specimens were observed by means of SEM.
3. Results
3.1. Microstructure
3.1.1. Optical microstructure
The welding seams on the cross-sections of profiles extruded under different billet heating temperatures and extrusion speeds are almost invisible to the naked eye, as shown in Fig 3(a). However, under OM, the welding seams can be seen clearly and their microstructure is variable along the TD direction. Fig. 3 gives the optical microstructure of the zones A, B and C on the cross-sections of the profiles extruded by experiments E1 and E4. For the profile extruded by E1, there is a welding zone in the central zone A, as shown in Fig. 3(b). Along the TD direction, this welding zone gradually narrows down and finally disappears at the transition position, as shown in Fig.3 (c) and (d). For the profile extruded by E4, there is only a welding line in the central zone A, as shown in Fig. 3(e). This welding line disappears at the transition position in the zone B, as shown in Fig. 3(f). Optical observations show that wedling seams have the most obvious feature in the central zone A on the cross-sections of the profiles extruded by the porthole die designed in this study. Therefore, the following study will focus on the central zone A of the profiles extruded under different billet heating temperatures and extrusion speeds.
Fig. 4 shows the optical microstructure of the zone A on the cross-sections of the profiles extruded under different extrusion process parameters. The experiment E1 was carried out at the extrusion speed of 1.0 mm/s and the billet heating temperature of 460 ℃. The profile extruded by this experiment has a distinct welding zone, 8
as shown in Fig. 4(a). The grains in and around the welding zone are very fine. These fine grains will be further analyzed later by means of EBSD. In the experiment E2, the extrusion speed was kept as 1.0 mm/s, and the billet heating temperature was increased to 490 ℃. In comparison with the profile extruded by E1, the welding zone of the profile extruded by E2 narrows down, as shown in Fig. 4(b). The grains in this welding zone are also fine. In the experiment E3, the extrusion speed was also kept as 1.0 mm/s, and the billet heating temperature was further increased to 520 ℃. The welding zone of the profile extruded by E3 becomes a welding line, as shown in Fig. 4(c). The boundaries of grains on both sides of the welding line match very well. New grains through the welding line seem to be formed. In addition, there are some coarse grains around the welding line. In the experiment E4, the billet heating temperature was decreased to 490 ℃, while the extrusion speed was increased to 4.0 mm/s. A welding line similar to that in the profile extruded by E3 appears on the cross-section of the profile extruded by E4, as shown in Fig. 4(d). There are also some relatively large grains around the welding line. Compared to the grains shown in Fig. 4(c), the grains shown in Fig. 4(d) are smaller and more uniform. According to the optical microstructure observed above, it can be seen that fine or coarse grains can be formed in welding seams during porthole die extrusion of aluminum alloy profiles. The formation of the fine grains may be attributed to recrystallization, while the formation of the coarse grains may be due to grains growth.
3.1.2. EBSD results
Fig. 5 compares the morphology, misorientation and orientation of the grains on the cross-sections of the profiles extruded by E1 and E3. The grains morphology, welding zone and welding line observed by means of OM are shown in Fig. 5(a) and (e). The grains morphology and orientation observed by means of EBSD are shown in Fig. 5(b) and (f). The thin-grey and thick-black lines respectively indicate the low (the misorientation angle of 2° ≤
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θ < 15°) and high (the misorientation angle of θ ≥ 15°) angles grain boundaries. The misorientation angles along the lines A and B are respectively shown in Fig. 5(c) and (g). The {100} pole figures are shown in Fig. 5(d) and (h).
For the welding seam extruded by E1, the grains morphology shown in Fig. 5(b) further demonstrates that there are many fine grains in and around the welding zone. The two black arrows in Fig. 5(b) show the position of the bonding interface. It can be seen that many grains with large misorientation angles (θ ≥ 15°) are separated by the bonding interface. Furthermore, the {100} pole figure shown in Fig. 5(d) demonstrates that the main orientation of grain in this welding seam is [001]//LD.
For the welding seam extruded by E3, it can be seen from Fig. 5(e) and (f) that the grains boundaries obtained by OM and EBSD are almost consistent, but the weldling line is not detected by EBSD. A further analysis of the grain G1 shown in Fig. 5(f) indicates that the arrangement of atoms in the grain G1 has a very small misorientation angle, as shown in Fig. 5(g). Therefore, it can be concluded that the two grains with the very well matched boundaries on both sides of the welding line are a newly formed grain. It should be noticed that although the new grains are formed, a visible welding line still exists. In addition, it can be seen from Fig. 5(h) that the main orientation of the grains around the welding line is [001]//LD.
3.1.3. TEM results
TEM results shows that there are many micro-voids on the bonding interfaces of the welding seams extruded by E1-E4. The typical morphological features of these micro-voids were shown in Fig. 6. It can be seen that the shape of the micro-voids is irregular. Overall, with the incease of billet heating temperature and extrusion speed, the length and width of the micro-voids on the bonding interface decrease, more and more material on both sides of the bonding interface contact to each other, and the atomic bonding degree of the interface increases. In
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addition, compared to the micro-voids on the bonding interface of the welding seam extruded by E3, the microvoids on the bonding interface of the welding seam extruded by E4 have a very small size. The maximum size of the micro-voids on the bonding interface of the welding seam extruded by E4 is only several nanometers in width and dozens of nanometers in length, as shown in Fig. 6(d). Therefore, the bonding interface of the profile extruded by E4 has a higher atomic bonding degree than that extruded by E3. It should be pointed out that although the micro-voids on the bonding interface of the welding seam extruded by E4 are very small, the welding seam is still visible under OM. Therefore, the wedling quality of the welding seam extruded by E4 is still imperfect.
Fig. 7 shows the TEM images of the precipitates in the welding seams extruded by experiments E2, E3 and E4. It can be seen that with the increase of billet heating temperature and extrusion speed, the density of dotlike and needle-shaped precipitates increases, as indicated by the arrows in Fig. 7(a)-(c). Fig. 7(d) shows the highresolution electron micrograph of the dotlike and needle-shaped precipitates imaged along <100> zone axis of the matrix Al. According to the studies of Sato et al. (1999) and Edwards et al. (1998), it can be concluded that the dotlike and needle-shaped precipitates in this study are the β’’ phase.
3.2. Mechanical properties 3.2.1. Micro-hardness
Micro-hardness tests were conducted at the positions with the distances of 100 μm, 200 μm, 300 μm, 400 μm and 500 μm from the welding seam. At the same distance from the welding seam, three points were chosen to test micro-hardness, and the average value and standard deviation of the hardness were calculated. The hardness distributions on the cross-sections of the different profiles extruded by experiments E1-E4 were given in Fig. 8.
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From Fig. 8, it can be seen that, no matter which extrusion process parameters were chosen for experiment, on the cross-sections of the extruded profiles, the values of hardness of the material on the welding seams are always less than those of the material near the welding seam. In addition, under the same extrusion speed, the hardness of the extruded profile increases with the increase of billet heating temperature, as shown as the curves E1, E2 and E3 in Fig.8. Under the same billet heating temperature, the hardness of the extruded profile also increases with the increase of extrusion speed, as shown as the curves E2 and E4 in Fig. 8. In conclusion, the hardness of extruded profiles increases with the increase of billet heating temperature and extrusion speed.
3.2.2. Tensile properties
Fig. 9 gives the ultimate tensile strengths and elongation percentages in the directions of 90°, 45° and 0° of the profiles extruded by E1-E4. For each extrusion experiment, the extruded profile’s ultimate tensile strengths in the directions of 90°, 45° and 0° are almost identical, but the elongation percentages have a relatively obvious difference. Overall, the elongation percentages are minimal in the direction of 90°, and maximal in the direction of 0°. The elongation percentages in the direction of 45° are close to those in the direction of 90°.
With the increase of billet heating temperature and extrusion speed, both the ultimate tensile strengths and elongation percentages in the directions of 90°, 45° and 0° increase. According to Fig. 9(b), it can be calculated that, for each extruded profile, the difference of the elongations in the directions of 0°and 90° is 7.5%, 5.6%, 4.7% and 4.0%, respectively. It can be seen that, with the increase of billet heating temperature and extrusion speed, the difference of the elongations in the directions of 0°and 90° decreases.
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3.2.3. Fracture morphologies
Fig. 10 shows the fracture positions, fracture morphologies and gauge lengths of the 90° tensile specimens after tensile tests. It can be seen that all the tensile specimens were fractured on the welding seams, but the fracture morphologies are different. For the tensile specimens cut from the profiles extruded by E1 and E2, there are some dimples on the fracture surfaces, and some areas of these fracture surfaces are smooth, as shown in Fig. 10(a) and (b). For the tensile specimen cut from the profile extruded by E3, there are many dimples and some smooth facets on the fracture surface, as shown in Fig. 10(c). A closer examination of the smooth facets at a high magnitude shows that there are some very shallow grooves. For the tensile specimen cut from the profile extruded by E4, there are many dimples and some intergranular cracking surfaces on the fracture surface, as shown in Fig. 10(d). A further observation of the intergranular cracking surfaces at a high magnitude indicates that there are some very shallow and small dimples.
4. Discussion
According to the above-mentioned characterization of the microstructure of welding seams by means of OM, EBSD and TEM, it can be seen that fine or coarse grains and micro-voids can be formed in welding seams in the porthole die extrusion process of aluminum alloy profiles. Although the new grains through the bonding interface have been formed, there are still many micro-voids in these new grains. Therefore, the formation of the new grains through the bonding interfaces does not mean that the welding seam is sound. Increasing billet heating temperature and extrusion speed not only contributes to the formation of the new grains through the bonding interface, but also promotes the closure of the micro-voids on the bonding interface, and thereby improves the atomic bonding degree of the material on both sides of the bonding interface.
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As reported by Jo et al. (2002), the increase of billet heating temperature results in the increase of the ratio of the welding pressure to material’s flow stress in welding chamber during porthole die extrusion. This increase contributes to the closure of the micro-voids by means of plastic deformation. In addition, as reported by Chen et al. (2007), the increase of temperature leads to a great improvement of the diffusion speed of the atoms on the bonding interface. Therefore, in this study, the increase of the closure degree of the micro-voids at the elevated billet heating temperature may be attributed to the increases of plastic deformation and atomic diffusion.
As reported by Zhang et al. (2012), in porthole die extrusion process, the increase of extrusion speed can obviously increase the temperature of the deforming material. As stated previously, increasing temperature can improve the diffusion speed of the atoms on the bonding interface. However, the increase of extrusion speed means the decrease of welding time. Therefore, the influence of extrusion speed on the atomic diffusion seems to be difficult to determine. In addition, the study of Donati and Tomesani (2004) has shown that the increase of extrusion speed will decrease the ratio of the welding pressure to material’s flow stress in welding chamber. This decrease is detrimental to the closure of the micro-voids under the action of plastic deformation. However, according to the experimental results in this work, increasing extrusion speed indeed promotes the closure of the micro-voids. Therefore, it is incomplete to evaluate the closure degree of the micro-voids only according to the ratio of the welding pressure to material’s flow stress in welding chamber. According to the welding criterion J proposed by Yu et al. (2016a), the closure process of the micro-voids under the action of plastic deformation is not only related to the ratio of the welding pressure to material’s flow stress in welding chamber, but also related to the effective strain surrounding the micro-voids. Increasing effective strain contributes to the closure of the micro-voids under the action of plastic deformation, as reported by Zhang et al. (2009). Therefore, for the extrusion process investigated in this work, the reason why the increase of extrusion speed can promote the
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closure of the micro-voids may be that the increase of extrusion speed causes the increase of the effective strain of the material around the bonding interface.
In order to confirm the influence of extrusion process parameters on the effective strain of the material around the bonding interface, finite element analysis was carried out by using Deform-3D software. The detailed method of establishing the finite element model can be seen in the previous research of Yu et al. (2016a). Fig. 11 shows the distribution of effective strain field on the cross-section of the profiles extruded by E1-E4. It can be seen that, with the increase of billet heating temperature and extrusion speed, the effective strain of the material around the bonding interface indeed increases, though the increase of the effective strain is not significant.
According to the aforementioned test results of mechanical properties, it can be seen that the increases of billet heating temperature and extrusion speed can improve the hardness, ultimate tensile strength and elongation percentage of the extruded profiles. On the one hand, the increases of billet heating temperature and extrusion speed can increase the temperature of the deforming material, and therefore promote second-phase particles to dissolve into the aluminum matrix, as reported by Ma et al. (2015). During subsequent aging, more fine precipitates can be formed, as reported by Fan et al. (2015). As a result, the hardness and strength of profiles were improved. On the other hand, the increase of billet heating temperature and extrusion speed will enhance the material’s plastic deformation and atomic diffusion, and therefore promote the closure of the micro-voids on the bonding interface to improve welding quality. Furthermore, with the increase of billet heating temperature and extrusion speed, the grains of welding seams gradually become uniform. As a result, the elongation percentage of the extruded profiles increases in the directions of 90°, 45° and 0°, and the difference of the elongations in the directions of 0°and 90° decreases. It should be noticed that, for the welding seam extruded by E4, although the micro-voids on the bonding interface are very small, the welding seam is still visible under OM. 15
Therefore, the elongation in the direction 90° is still lower than that in the direction of 0°. In addition, it was found that the hardness of the material on the bonding interface is always lower than that of the material outside the bonding interface, regardless of whether the grains around the bonding interface are fine or coarse. This phenomenon may be due to the existence of micro-voids on the bonding interface.
The observations of the fracture morphologies in this work have shown that some areas of the fracture surfaces of the tensile specimens cut from the profiles extruded by experiments E1 and E2 are smooth, as shown in Fig. 10(a) and (b). This is because that there are many micro-voids on the bonding interfaces of the profiles and most of the grains in the welding zones are separated by the bonding interfaces. With the increase of billet heating temperature, the new grains are formed, but some micro-voids shown in Fig. 6(c) still exist in the newly formed grains. In the process of tensile test, some cracks originated at the micro-voids and propagated along the bonding interface. As a result, there are some smooth facets on the fracture surface of the tensile specimen cut from the profile extruded by experiments E3, as shown in Fig. 10(c). With the increase of extrusion speed, the temperature of the deforming material will increase obviously, and the grains boundaries may weaken under the combined action of second-phase particles’ solution and recrystallization, as reported by Zhou et al. (2014). As a result, there are some intergranular cracking surfaces on the fracture surface of the tensile specimen cut from the profile extruded by experiments E4, as shown in Fig. 10(d). 5. Conclusions
In this work, the extrusion experiments for a plate-shaped profile under different billet heating temperatures and extrusion speeds were performed. The microstructure of welding seams in extruded profiles were characterized by means of optical microscopy (OM), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM), and the mechanical properties of welding seams were analyzed through microhardness and tensile tests. The influence of billet heating temperature and extrusion speed on the microstructure and mechanical properties of welding seams was studied. The following conclusions were drawn: (1) In the porthole die extrusion processes of aluminum alloy profiles, fine or coarse grains and micro-voids can be formed in welding seams and have a significant influence on the mechanical properties of the extruded profiles. (2) Although the new grains through the bonding interface have been formed, there are still many micro-voids in these 16
new grains. Therefore, the formation of the new grains through the bonding interface does not mean that the welding seam is sound. (3) Increasing billet heating temperature and extrusion speed not only contributes to the formation of the new grains through the bonding interface, but also promotes the closure of the micro-voids on the bonding interface, and thereby improves the atomic bonding degree of the material on both sides of the bonding interface. (4) The hardness, strength and ductility of the extruded profiles can be improved by increasing billet heating temperature and extrusion speed. Acknowledgements
The authors would like to acknowledge the financial support from National Natural Science Foundation of China (51375270, 51575315 and 51405268).
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Fan, X.B., He, Z.B., Zheng, K.L., Yuan, S.J., 2015. Strengthening behavior of Al–Cu–Mg alloy sheet in hot forming– quenching integrated process with cold–hot dies. Materials & Design 83, 557-565.
Fan, X.H., Tang, D., Fang, W.L., Li, D.Y., Peng, Y.H., 2016. Microstructure development and texture evolution of aluminum multi-port extrusion tube during the porthole die extrusion. Materials Characterization 118, 468-480.
Güley, V., Güzel, A., Jäger, A., Ben Khalifa, N., Tekkaya, A.E., Misiolek, W.Z., 2013. Effect of die design on the welding quality during solid state recycling of AA6060 chips by hot extrusion. Materials Science and Engineering: A 574, 163175.
Güzel, A., Jäger, A., Parvizian, F., Lambers, H.G., Tekkaya, A.E., Svendsen, B., Maier, H.J., 2012. A new method for determining dynamic grain structure evolution during hot aluminum extrusion. Journal of Materials Processing Technology 212, 323-330.
Gagliardi, F., Citrea, T., Ambrogio, G., Filice, L., 2014. Influence of the process setup on the microstructure and mechanical properties evolution in porthole die extrusion. Materials & Design 60, 274-281. 18
Jo, H.H., Jeong, C.S., Lee, S.K., Kim, B.M., 2003. Determination of welding pressure in the non-steady-state porthole die extrusion of improved Al7003 hollow section tubes. Journal of Materials Processing Technology 139, 428-433.
Jo, H.H., Lee, S.K., Lee, S.B., Kim, B.M., 2002. Prediction of welding pressure in the non-steady state porthole die extrusion of Al7003 tubes. International Journal of Machine Tools and Manufacture 42, 753-759.
Liu, G., Zhou, J., Duszczyk, J., 2008. FE analysis of metal flow and weld seam formation in a porthole die during the extrusion of a magnesium alloy into a square tube and the effect of ram speed on weld strength. Journal of materials processing technology 200, 185-198.
Ma, W.Y., Wang, B.Y., Yang, L., Tang, X.F., Xiao, W.C., Zhou, J., 2015. Influence of solution heat treatment on mechanical response and fracture behaviour of aluminium alloy sheets: An experimental study. Materials & Design 88, 1119-1126.
Plata, M., Piwnik, J., 2000. Theoretical and experimental analysis of seam weld formation in hot extrusion of aluminum alloys. Proceedings of the 7th International Aluminum Extrusion Technology Seminar, Chicago, USA, 2000, pp. 205212.
Sato, Y.S., Kokawa, H., Enomoto, M., Jogan, S., 1999. Microstructural evolution of 6063 aluminum during friction-stir welding. Metallurgical and Materials Transactions A 30, 2429-2437.
Valberg, H., Loeken, T., Hval, M., Nyhus, B., Thaulow, C., 1995. The extrusion of hollow profiles with a gas pocket behind the bridge. International Journal of Materials and Product Technology 10, 222-267.
Yu, J.Q., Zhao, G.Q., Chen, L., 2016a. Analysis of longitudinal weld seam defects and investigation of solid-state bonding criteria in porthole die extrusion process of aluminum alloy profiles. Journal of Materials Processing Technology 237, 19
31-47.
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Zhao, Y., Song, B.Y., Yan, Z.Y., Zhang, X., Pei, J.Y., 2016. Microstructure and mechanical properties of extrusion welds in continuous extrusion of AA6063 aluminium alloy with double billets. Journal of Materials Processing Technology 235, 149-157.
Zhou, M., Lin, Y.C., Deng, J., Jiang, Y.Q., 2014. Hot tensile deformation behaviors and constitutive model of an Al–Zn– Mg–Cu alloy. Materials & Design 59, 141-150.
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Fig. 1. The extrusion die for experiments.
21
Fig. 2. The positions and orientations of different samples as well as the dimensions of tensile specimens: (a) the positions of the welding seam and the observed cross-section for microstructure, (b) the orientations of tensile samples, (c) the dimensions of tensile samples.
22
Fig. 3. The optical microstructure on the cross-sections of the profiles extruded by experiments E1 and E4.
23
Fig. 4. The optical microstructure of the zone A on the cross-sections of the profiles extruded by different experiments: (a) E1, (b) E2, (c) E3, (d) E4.
24
Fig. 5. The comparison of morphology, misorientation and orientation of the grains on the cross-sections of the profiles extruded by experiments E1 and E3: (a) and (e) optical microstructure, (b) and (f) EBSD images of grains morphology, (c) and (g) misorientation angles along the lines A and B, (d) and (h) {100} pole figures.
25
Fig. 6. Bright field TEM images of the micro-voids on the bonding interfaces of the welding seams extruded by different experiments: (a) E1, (b) E2, (c) E3, (d) E4.
26
Fig. 7. The TEM images of the precipitates: (a), (b), (c) bright field TEM images of the precipitates in the welding seams extruded by experiments E2, E3, E4, (d) high-resolution electron micrograph of the dotlike and needleshaped precipitates imaged along <100> zone axis of the matrix Al.
27
Fig. 8. Hardness distributions on the cross-sections of the profiles extruded by experiments E1-E4.
28
Fig. 9. Tensile properties of the profiles extruded by E1-E4: (a) ultimate tensile strength, (b) elongation percentage.
29
Fig. 10. The fracture positions, fracture morphologies and gauge lengths of the 90° tensile specimens after tensile tests: (a) E1, (b) E2, (c) E3, (d) E4.
30
Fig. 11. Distribution of effective strain field on the cross-section of the profiles extruded by different experiments: (a) E1, (b) E2, (c) E3, (d) E4.
31
Table 1 Chemical compositions of the as-homogenized AA6063 aluminum alloy billets. Element
Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Ni
Al
(wt %)
0.3942
0.1221
0.0844
0.0063
0.5563
0.0035
0.0085
0.0103
0.0048
Bal.
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Table 2 Extrusion process parameters for experiments E1-E4. Experiment No.
E1
E2
E3
E4
Billet heating temperature (℃)
460
490
520
490
Extrusion speed (mm/s)
1.0
1.0
1.0
4.0
33
S0924-0136(17)30165-6 http://dx.doi.org/doi:10.1016/j.jmatprotec.2017.04.030 PROTEC 15208
To appear in:
Journal of Materials Processing Technology
Received date: Revised date: Accepted date:
12-11-2016 1-4-2017 30-4-2017
Please cite this article as: Yu, Junquan, Zhao, Guoqun, Cui, Weichao, Zhang, Cunsheng, Chen, Liang, Microstructural evolution and mechanical properties of welding seams in aluminum alloy profiles extruded by a porthole die under different billet heating temperatures and extrusion speeds.Journal of Materials Processing Technology http://dx.doi.org/10.1016/j.jmatprotec.2017.04.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microstructural evolution and mechanical properties of welding seams in aluminum alloy profiles extruded by a porthole die under different billet heating temperatures and extrusion speeds Junquan Yua, Guoqun Zhao,a, Weichao Cuib, Cunsheng Zhanga , Liang Chena
a
Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education),
Shandong University, Jinan, Shandong 250061, PR China
b
Shandong Yancon Light Alloy Co., Ltd., Zoucheng, Shandong 273500, PR China
Abstract
Porthole die extrusion process of aluminum alloy profiles is a hot deformation process involving solid state welding. Microstructural evolution of welding seams is the key factor to determine mechanical properties of extruded profiles. In this work, the grain structure, bonding interface structure and precipitates of welding seams in the profiles extruded under different billet heating temperatures and extrusion speeds were characterized, and the hardness, strength and ductility of welding seams were analyzed. The influence of billet heating temperature and extrusion speed on the microstructure and mechanical properties of welding seams was studied. It was found that, in the porthole die extrusion process of aluminum alloy profiles, fine or coarse grains and micro-voids can be formed in welding seams. Although the new grains through the bonding interface have been formed, there are still many micro-voids in these new grains. Increasing billet heating temperature and extrusion speed not only contributes to the formation of the new grains through the bonding interface, but also promotes the closure of
Corresponding author at: Key Laboratory for Liquid–Solid Structural Evolution & Processing of Materials (Ministry of Education),
Shandong University, Jinan, Shandong 250061, PR China. Tel.: +86(0)53188393238; fax: +86(0)53188392811. E-mail address: [email protected] (Guoqun Zhao). 1
the micro-voids on the bonding interface, and thereby improves the atomic bonding degree of the material on both sides of the bonding interface. The hardness, strength and ductility of the extruded profiles can be improved by increasing billet heating temperature and extrusion speed.
Keywords: Aluminum alloy profile; Porthole die extrusion; Process parameters; Welding seam; Bonding interface structure
1. Introduction
As a kind of lightweight structural parts with an excellent comprehensive performance, the aluminum alloy profiles with hollow sections have been widely used in many fields. As reported by Chen et al. ( 2011), most of hollow section profiles, especially with large scale, thin-wall, multi-cavity and complex cross-section, are produced by using porthole die direct extrusion process. The hollow section profiles usually have many longitudinal welding seams. As reported by den Bakker et al. (2014), the longitudinal welding seams easily become the weakest part of profiles. Therefore, the control of microstructure of welding seams is very important for improving mechanical properties of the hollow section profiles.
In porthole die extrusion process, how to improve welding seams quality has attracted extensive attention. Optimizing extrusion die structure and process parameters is the main measure to improve welding quality. Currently, the effect of die structure on welding quality has been revealed deeply. For example, as reported by Valberg et al. (1995), the increase of the depth or the volume of welding chambers and the design of the pointed bridge all contribute to the improvement of the welding quality of extruded profiles. However, there are still some disputes on the effect of extrusion speed and billet heating temperature on welding quality. On the one hand, Valberg et al. (1995) found that in the porthole die extrusion of AA6082 and AA7108 aluminum alloys profiles, a
2
high extrusion speed can result in a tearing of the metal in the welding zone. Based on the experimental work of Valberg et al. (1995), Donati and Tomesani (2004) concluded that increasing extrusion speed will cause the deterioration of welding quality. On the another hand, Liu et al. (2008) found that the increase of extrusion speed can improve the strength and ductility of a magnesium alloy square tube. This means that the welding quality can be improved by increasing extrusion speed. In addition, Jo et al. (2003) showed that the increase of billet heating temperature contributes to the improvement of the expanding ratio of Al7003 hollow section profiles. While Bingöl and Keskin (2007) showed that the increase of billet heating temperature results in the appearance of coarse grains in welding zone of AA6063 aluminum alloy profiles. Based on the above-mentioned reasons, it is imperative to further study the influence of billet heating temperature and extrusion speed on the welding quality of extruded profiles.
In hot extrusion process, the microstructural evolution and mechanical properties of aluminum alloy profiles were investigated by some researchers. Güzel et al. (2012) proposed a method for determining dynamic grain structure evolution during hot aluminum extrusion, and found that the grain structure in dead metal zone, shear intensive zone, inflowing material zone and exiting profile zone of the extrusion butt is significantly different. Fan et al. (2016) investigated the grain structure and texture evolution in the porthole die extrusion process of a multiport flat tube, and found that there are rolling-type and cube textures at the solid state welding region. Zhao et al. (2016) investigated the microstructure and mechanical properties of extrusion welds in continuous extrusion of AA6063 aluminum alloy with double billets, and found that the extrusion welds markedly influence the elongation of an extrudate. Güley et al. (2013) studied the evolution of microstructure during extrusion of machining chips, and found that the mechanical properties of the extruded profiles can be improved by optimizing extrusion die design. Furthermore, they embedded a criterion for the breaking of the oxide layers and an index for the welding quality into the finite element software to evaluate the welding quality of the extruded profiles. 3
Bingöl and Keskin (2007) studied the effect of different extrusion temperatures and speeds on extrusion welds, and found that the distinction of the welding zones is decreased with the increase of billet temperature and increased with the increase of ram speed according to the macrostructure of the etched profiles. Gagliardi et al. (2014) studied the influence of the profile thickness and ram velocity on the grain sizes and tensile properties of AA6060 aluminum alloy, and found that the profile thickness and ram velocity jointly affect the quality of the welding seams. Zhao et al. (2013) investigated the effect of deformation speed on the microstructure and mechanical properties of AA6063 during continuous extrusion, and showed that there is an optimum extrusion wheel velocity to obtain the products with good mechanical properties. Yu et al. (2016b) investigated the influence of the shape of bridges and the depth of welding chambers on the microstructure and mechanical properties of welding seams, and found that the ductility of extrude profiles is increased with the increase of the depth of welding chamber. Although the above mentioned works have been conducted to study the microstructural evolution and mechanical properties of aluminum alloy profiles, it should be noticed that, the porthole die extrusion process of aluminum alloy profiles is a hot deformation process involving solid state welding. The microstructural evolution of welding seams is the key factor to determine the mechanical properties of extruded profiles. Unfortunately, there is still lack of a systematic investigation on the microstructural evolution in welding zone of aluminum alloy profiles extruded under different billet heating temperatures and extrusion speeds by now. Especially, there almost has not been any investigation on the effect of billet heating temperature and extrusion speed on the bonding interface structure of the welding seams in the profiles extruded by using porthole dies.
For a long time, predicting the welding seams quality of extruded profiles has been a challenge. Some welding criteria were proposed to predict welding seams quality. Based on the pressure in the welding plane, Akeret (1972) proposed the maximum pressure criterion, Plata and Piwnik (2000) proposed the pressure-time criterion 4
(Q criterion), and Donati and Tomesani (2004) proposed the pressure-time-flow criterion (K criterion). Recently, based on plastic deformation and diffusion mechanisms, Yu et al. (2016a) proposed the J criterion from the viewpoint of the closure behaviors of the micro-voids on bonding interfaces. According to the solid state welding mechanism proposed by Yu et al. (2016a), the closure of the micro-voids on the bonding interface is the precondition to form high quality welding seams. Therefore, in order to evaluate the welding quality, the closure degree of the micro-voids on the bonding interface should be firstly determined. However, the micro-voids on the bonding interface of welding seams in extruded profiles have not been observed by experiments so far, and the research on the influence of extrusion parameters such as the billet heating temperature and extrusion speed on the evolution of micro-voids is still a blank.
In conclusion, although some studies have been done to investigate the welding seams in extruded profiles, most of the previous works mainly focused on the mechanical properties and the grain structure of welding seams. There is still lack of a deep and careful characterization of the bonding interface structure from nanometer scale. The bonding interface structure of the welding seams determines the atomic bonding degree of the material on both sides of the bonding interface. Unfortunately, the influence of billet heating temperature and extrusion speed on the atomic bonding degree is still not well known. In addition, most of the previous works mainly investigated the effect of the grains and second-phase particles on the mechanical properties of welding seams. In fact, the bonding interface structure also influences the mechanical properties of welding seams. Thus, the effect of the bonding interface structure on the mechanical properties of welding seams still needs to be studied.
In this work, the extrusion experiments for a plate-shaped profile under different billet heating temperatures and extrusion speeds were performed. The microstructure of welding seams in extruded profiles were characterized by means of optical microscopy (OM), electron backscattered diffraction (EBSD) and transmission 5
electron microscopy (TEM), and the mechanical properties of welding seams were analyzed through microhardness and tensile tests. The influence of billet heating temperature and extrusion speed on the microstructure and mechanical properties of welding seams was studied.
2. Experimental methods and procedures 2.1. Materials preparation and extrusion experiments
The material for extrusion experiments is the AA6063 aluminum alloy billets homogenized at 530 ℃for 14 h with the dimensions of Ø152 mm × 700 mm. Its chemical compositions are shown in table 1. The extrusion die for experiments consists of an upper die, an arched bridge and a lower die, as shown in Fig. 1. The height h of the welding chamber was designed as 15.0 mm. More detailed information about the extrusion die was given in the research of Yu et al. (2016a). The extrusion ratio of the profile is 20.9. The temperatures of container and extrusion die were set as 440 ℃and 450 ℃, respectively. A 16 MN extrusion press was used for extrusion experiments. The billet heating temperature and the extrusion speed for each extrusion experiment were shown in table 2. For each extrusion experiment, two billets were extruded. In addition, an infrared temperature detector was used to monitor the temperature of extruded profiles. When the extrusion process reached the steady state, the measured exit temperatures of the profiles extruded by experiments E1-E4 are about 460 ℃, 480 ℃, 495 ℃ and 535 ℃. After extrusion, the extruded profiles were rapidly cooled with water to the room temperature, and then annealed at 175 ℃for 8 h.
2.2. Sampling, microstructure characterization and mechanical properties tests
After extrusion experiments, the plate-shaped profiles with the thickness of 12 mm and the width of 80 mm were obtained. The longitudinal welding seam is located in the mid-position of the profile’s cross-section, as
6
shown in Fig. 2(a). For easy description, in this work, the width, length and thickness directions of extruded profiles were respectively defined as WD, LD and TD directions. The samples for microstructural observation were taken from the areas around the welding seam on the cross-section of extruded profiles. The samples for tensile tests were cut at 90°, 45° and 0° to the welding seam, as shown in Fig. 2(b). The dimensions of the tensile samples are given in Fig. 2(c).
For OM observations, the surfaces of samples to be observed were ground and polished into mirror-like ones, and then etched in the solution with 1.0 ml HF, 1.5 ml HCl, 2.5 ml HNO3 and 95.0 ml H2O. For EBSD measurements, firstly, the welding seams of samples were determined and marked according to OM observation results. Next, the surfaces of the samples were re-ground with emery papers up to 2000 grits and re-polished into a mirror-like ones, and then electro-polished in a solution containing 20 vol% perchloric acid and 80 vol% methanol at -20 ℃ and 15 V for 20 s to eliminate the plastically deformed region. EBSD measurements were carried out by using SEM (CARL ZEISS EVO MA 10) equipped with a device of Oxford EBSD HKL Channel 5 at an acceleration voltage of 20 kV. For TEM observations, firstly, the welding seams of samples were also marked according to the OM observations. Then, the samples were ground to a thickness about 30 μm, and then cut into disks with 3.0 mm diameter and further thinned with ion beams. TEM observations were performed in a JEM-2100F TEM at an operating voltage of 200 kV.
Vickers hardness of the material around the welding seams was measured by using a load of 100 g and dwell time of 10 s with the 100 μm distance between successive indentations. Before tensile testing, two lines were drawn on each tensile sample to indicate the original gauge length L0 (25.00 mm). The tensile samples were tested on an electronic tensile testing machine, and the tensile speed was set as 0.01 mm/s, i.e. the strain rate is 0.0004/s. For each extrusion condition, the tensile tests were repeated three times to ensure the accuracy and 7
repeatability of the tests. After tensile tests, a vernier caliper was used to measure the fractured specimens’ gauge lengths Lf, and the elongation percentages were determined by calculating the values of (Lf-L0)/L0×100%. In addition, the fracture surfaces of tensile specimens were observed by means of SEM.
3. Results
3.1. Microstructure
3.1.1. Optical microstructure
The welding seams on the cross-sections of profiles extruded under different billet heating temperatures and extrusion speeds are almost invisible to the naked eye, as shown in Fig 3(a). However, under OM, the welding seams can be seen clearly and their microstructure is variable along the TD direction. Fig. 3 gives the optical microstructure of the zones A, B and C on the cross-sections of the profiles extruded by experiments E1 and E4. For the profile extruded by E1, there is a welding zone in the central zone A, as shown in Fig. 3(b). Along the TD direction, this welding zone gradually narrows down and finally disappears at the transition position, as shown in Fig.3 (c) and (d). For the profile extruded by E4, there is only a welding line in the central zone A, as shown in Fig. 3(e). This welding line disappears at the transition position in the zone B, as shown in Fig. 3(f). Optical observations show that wedling seams have the most obvious feature in the central zone A on the cross-sections of the profiles extruded by the porthole die designed in this study. Therefore, the following study will focus on the central zone A of the profiles extruded under different billet heating temperatures and extrusion speeds.
Fig. 4 shows the optical microstructure of the zone A on the cross-sections of the profiles extruded under different extrusion process parameters. The experiment E1 was carried out at the extrusion speed of 1.0 mm/s and the billet heating temperature of 460 ℃. The profile extruded by this experiment has a distinct welding zone, 8
as shown in Fig. 4(a). The grains in and around the welding zone are very fine. These fine grains will be further analyzed later by means of EBSD. In the experiment E2, the extrusion speed was kept as 1.0 mm/s, and the billet heating temperature was increased to 490 ℃. In comparison with the profile extruded by E1, the welding zone of the profile extruded by E2 narrows down, as shown in Fig. 4(b). The grains in this welding zone are also fine. In the experiment E3, the extrusion speed was also kept as 1.0 mm/s, and the billet heating temperature was further increased to 520 ℃. The welding zone of the profile extruded by E3 becomes a welding line, as shown in Fig. 4(c). The boundaries of grains on both sides of the welding line match very well. New grains through the welding line seem to be formed. In addition, there are some coarse grains around the welding line. In the experiment E4, the billet heating temperature was decreased to 490 ℃, while the extrusion speed was increased to 4.0 mm/s. A welding line similar to that in the profile extruded by E3 appears on the cross-section of the profile extruded by E4, as shown in Fig. 4(d). There are also some relatively large grains around the welding line. Compared to the grains shown in Fig. 4(c), the grains shown in Fig. 4(d) are smaller and more uniform. According to the optical microstructure observed above, it can be seen that fine or coarse grains can be formed in welding seams during porthole die extrusion of aluminum alloy profiles. The formation of the fine grains may be attributed to recrystallization, while the formation of the coarse grains may be due to grains growth.
3.1.2. EBSD results
Fig. 5 compares the morphology, misorientation and orientation of the grains on the cross-sections of the profiles extruded by E1 and E3. The grains morphology, welding zone and welding line observed by means of OM are shown in Fig. 5(a) and (e). The grains morphology and orientation observed by means of EBSD are shown in Fig. 5(b) and (f). The thin-grey and thick-black lines respectively indicate the low (the misorientation angle of 2° ≤
9
θ < 15°) and high (the misorientation angle of θ ≥ 15°) angles grain boundaries. The misorientation angles along the lines A and B are respectively shown in Fig. 5(c) and (g). The {100} pole figures are shown in Fig. 5(d) and (h).
For the welding seam extruded by E1, the grains morphology shown in Fig. 5(b) further demonstrates that there are many fine grains in and around the welding zone. The two black arrows in Fig. 5(b) show the position of the bonding interface. It can be seen that many grains with large misorientation angles (θ ≥ 15°) are separated by the bonding interface. Furthermore, the {100} pole figure shown in Fig. 5(d) demonstrates that the main orientation of grain in this welding seam is [001]//LD.
For the welding seam extruded by E3, it can be seen from Fig. 5(e) and (f) that the grains boundaries obtained by OM and EBSD are almost consistent, but the weldling line is not detected by EBSD. A further analysis of the grain G1 shown in Fig. 5(f) indicates that the arrangement of atoms in the grain G1 has a very small misorientation angle, as shown in Fig. 5(g). Therefore, it can be concluded that the two grains with the very well matched boundaries on both sides of the welding line are a newly formed grain. It should be noticed that although the new grains are formed, a visible welding line still exists. In addition, it can be seen from Fig. 5(h) that the main orientation of the grains around the welding line is [001]//LD.
3.1.3. TEM results
TEM results shows that there are many micro-voids on the bonding interfaces of the welding seams extruded by E1-E4. The typical morphological features of these micro-voids were shown in Fig. 6. It can be seen that the shape of the micro-voids is irregular. Overall, with the incease of billet heating temperature and extrusion speed, the length and width of the micro-voids on the bonding interface decrease, more and more material on both sides of the bonding interface contact to each other, and the atomic bonding degree of the interface increases. In
10
addition, compared to the micro-voids on the bonding interface of the welding seam extruded by E3, the microvoids on the bonding interface of the welding seam extruded by E4 have a very small size. The maximum size of the micro-voids on the bonding interface of the welding seam extruded by E4 is only several nanometers in width and dozens of nanometers in length, as shown in Fig. 6(d). Therefore, the bonding interface of the profile extruded by E4 has a higher atomic bonding degree than that extruded by E3. It should be pointed out that although the micro-voids on the bonding interface of the welding seam extruded by E4 are very small, the welding seam is still visible under OM. Therefore, the wedling quality of the welding seam extruded by E4 is still imperfect.
Fig. 7 shows the TEM images of the precipitates in the welding seams extruded by experiments E2, E3 and E4. It can be seen that with the increase of billet heating temperature and extrusion speed, the density of dotlike and needle-shaped precipitates increases, as indicated by the arrows in Fig. 7(a)-(c). Fig. 7(d) shows the highresolution electron micrograph of the dotlike and needle-shaped precipitates imaged along <100> zone axis of the matrix Al. According to the studies of Sato et al. (1999) and Edwards et al. (1998), it can be concluded that the dotlike and needle-shaped precipitates in this study are the β’’ phase.
3.2. Mechanical properties 3.2.1. Micro-hardness
Micro-hardness tests were conducted at the positions with the distances of 100 μm, 200 μm, 300 μm, 400 μm and 500 μm from the welding seam. At the same distance from the welding seam, three points were chosen to test micro-hardness, and the average value and standard deviation of the hardness were calculated. The hardness distributions on the cross-sections of the different profiles extruded by experiments E1-E4 were given in Fig. 8.
11
From Fig. 8, it can be seen that, no matter which extrusion process parameters were chosen for experiment, on the cross-sections of the extruded profiles, the values of hardness of the material on the welding seams are always less than those of the material near the welding seam. In addition, under the same extrusion speed, the hardness of the extruded profile increases with the increase of billet heating temperature, as shown as the curves E1, E2 and E3 in Fig.8. Under the same billet heating temperature, the hardness of the extruded profile also increases with the increase of extrusion speed, as shown as the curves E2 and E4 in Fig. 8. In conclusion, the hardness of extruded profiles increases with the increase of billet heating temperature and extrusion speed.
3.2.2. Tensile properties
Fig. 9 gives the ultimate tensile strengths and elongation percentages in the directions of 90°, 45° and 0° of the profiles extruded by E1-E4. For each extrusion experiment, the extruded profile’s ultimate tensile strengths in the directions of 90°, 45° and 0° are almost identical, but the elongation percentages have a relatively obvious difference. Overall, the elongation percentages are minimal in the direction of 90°, and maximal in the direction of 0°. The elongation percentages in the direction of 45° are close to those in the direction of 90°.
With the increase of billet heating temperature and extrusion speed, both the ultimate tensile strengths and elongation percentages in the directions of 90°, 45° and 0° increase. According to Fig. 9(b), it can be calculated that, for each extruded profile, the difference of the elongations in the directions of 0°and 90° is 7.5%, 5.6%, 4.7% and 4.0%, respectively. It can be seen that, with the increase of billet heating temperature and extrusion speed, the difference of the elongations in the directions of 0°and 90° decreases.
12
3.2.3. Fracture morphologies
Fig. 10 shows the fracture positions, fracture morphologies and gauge lengths of the 90° tensile specimens after tensile tests. It can be seen that all the tensile specimens were fractured on the welding seams, but the fracture morphologies are different. For the tensile specimens cut from the profiles extruded by E1 and E2, there are some dimples on the fracture surfaces, and some areas of these fracture surfaces are smooth, as shown in Fig. 10(a) and (b). For the tensile specimen cut from the profile extruded by E3, there are many dimples and some smooth facets on the fracture surface, as shown in Fig. 10(c). A closer examination of the smooth facets at a high magnitude shows that there are some very shallow grooves. For the tensile specimen cut from the profile extruded by E4, there are many dimples and some intergranular cracking surfaces on the fracture surface, as shown in Fig. 10(d). A further observation of the intergranular cracking surfaces at a high magnitude indicates that there are some very shallow and small dimples.
4. Discussion
According to the above-mentioned characterization of the microstructure of welding seams by means of OM, EBSD and TEM, it can be seen that fine or coarse grains and micro-voids can be formed in welding seams in the porthole die extrusion process of aluminum alloy profiles. Although the new grains through the bonding interface have been formed, there are still many micro-voids in these new grains. Therefore, the formation of the new grains through the bonding interfaces does not mean that the welding seam is sound. Increasing billet heating temperature and extrusion speed not only contributes to the formation of the new grains through the bonding interface, but also promotes the closure of the micro-voids on the bonding interface, and thereby improves the atomic bonding degree of the material on both sides of the bonding interface.
13
As reported by Jo et al. (2002), the increase of billet heating temperature results in the increase of the ratio of the welding pressure to material’s flow stress in welding chamber during porthole die extrusion. This increase contributes to the closure of the micro-voids by means of plastic deformation. In addition, as reported by Chen et al. (2007), the increase of temperature leads to a great improvement of the diffusion speed of the atoms on the bonding interface. Therefore, in this study, the increase of the closure degree of the micro-voids at the elevated billet heating temperature may be attributed to the increases of plastic deformation and atomic diffusion.
As reported by Zhang et al. (2012), in porthole die extrusion process, the increase of extrusion speed can obviously increase the temperature of the deforming material. As stated previously, increasing temperature can improve the diffusion speed of the atoms on the bonding interface. However, the increase of extrusion speed means the decrease of welding time. Therefore, the influence of extrusion speed on the atomic diffusion seems to be difficult to determine. In addition, the study of Donati and Tomesani (2004) has shown that the increase of extrusion speed will decrease the ratio of the welding pressure to material’s flow stress in welding chamber. This decrease is detrimental to the closure of the micro-voids under the action of plastic deformation. However, according to the experimental results in this work, increasing extrusion speed indeed promotes the closure of the micro-voids. Therefore, it is incomplete to evaluate the closure degree of the micro-voids only according to the ratio of the welding pressure to material’s flow stress in welding chamber. According to the welding criterion J proposed by Yu et al. (2016a), the closure process of the micro-voids under the action of plastic deformation is not only related to the ratio of the welding pressure to material’s flow stress in welding chamber, but also related to the effective strain surrounding the micro-voids. Increasing effective strain contributes to the closure of the micro-voids under the action of plastic deformation, as reported by Zhang et al. (2009). Therefore, for the extrusion process investigated in this work, the reason why the increase of extrusion speed can promote the
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closure of the micro-voids may be that the increase of extrusion speed causes the increase of the effective strain of the material around the bonding interface.
In order to confirm the influence of extrusion process parameters on the effective strain of the material around the bonding interface, finite element analysis was carried out by using Deform-3D software. The detailed method of establishing the finite element model can be seen in the previous research of Yu et al. (2016a). Fig. 11 shows the distribution of effective strain field on the cross-section of the profiles extruded by E1-E4. It can be seen that, with the increase of billet heating temperature and extrusion speed, the effective strain of the material around the bonding interface indeed increases, though the increase of the effective strain is not significant.
According to the aforementioned test results of mechanical properties, it can be seen that the increases of billet heating temperature and extrusion speed can improve the hardness, ultimate tensile strength and elongation percentage of the extruded profiles. On the one hand, the increases of billet heating temperature and extrusion speed can increase the temperature of the deforming material, and therefore promote second-phase particles to dissolve into the aluminum matrix, as reported by Ma et al. (2015). During subsequent aging, more fine precipitates can be formed, as reported by Fan et al. (2015). As a result, the hardness and strength of profiles were improved. On the other hand, the increase of billet heating temperature and extrusion speed will enhance the material’s plastic deformation and atomic diffusion, and therefore promote the closure of the micro-voids on the bonding interface to improve welding quality. Furthermore, with the increase of billet heating temperature and extrusion speed, the grains of welding seams gradually become uniform. As a result, the elongation percentage of the extruded profiles increases in the directions of 90°, 45° and 0°, and the difference of the elongations in the directions of 0°and 90° decreases. It should be noticed that, for the welding seam extruded by E4, although the micro-voids on the bonding interface are very small, the welding seam is still visible under OM. 15
Therefore, the elongation in the direction 90° is still lower than that in the direction of 0°. In addition, it was found that the hardness of the material on the bonding interface is always lower than that of the material outside the bonding interface, regardless of whether the grains around the bonding interface are fine or coarse. This phenomenon may be due to the existence of micro-voids on the bonding interface.
The observations of the fracture morphologies in this work have shown that some areas of the fracture surfaces of the tensile specimens cut from the profiles extruded by experiments E1 and E2 are smooth, as shown in Fig. 10(a) and (b). This is because that there are many micro-voids on the bonding interfaces of the profiles and most of the grains in the welding zones are separated by the bonding interfaces. With the increase of billet heating temperature, the new grains are formed, but some micro-voids shown in Fig. 6(c) still exist in the newly formed grains. In the process of tensile test, some cracks originated at the micro-voids and propagated along the bonding interface. As a result, there are some smooth facets on the fracture surface of the tensile specimen cut from the profile extruded by experiments E3, as shown in Fig. 10(c). With the increase of extrusion speed, the temperature of the deforming material will increase obviously, and the grains boundaries may weaken under the combined action of second-phase particles’ solution and recrystallization, as reported by Zhou et al. (2014). As a result, there are some intergranular cracking surfaces on the fracture surface of the tensile specimen cut from the profile extruded by experiments E4, as shown in Fig. 10(d). 5. Conclusions
In this work, the extrusion experiments for a plate-shaped profile under different billet heating temperatures and extrusion speeds were performed. The microstructure of welding seams in extruded profiles were characterized by means of optical microscopy (OM), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM), and the mechanical properties of welding seams were analyzed through microhardness and tensile tests. The influence of billet heating temperature and extrusion speed on the microstructure and mechanical properties of welding seams was studied. The following conclusions were drawn: (1) In the porthole die extrusion processes of aluminum alloy profiles, fine or coarse grains and micro-voids can be formed in welding seams and have a significant influence on the mechanical properties of the extruded profiles. (2) Although the new grains through the bonding interface have been formed, there are still many micro-voids in these 16
new grains. Therefore, the formation of the new grains through the bonding interface does not mean that the welding seam is sound. (3) Increasing billet heating temperature and extrusion speed not only contributes to the formation of the new grains through the bonding interface, but also promotes the closure of the micro-voids on the bonding interface, and thereby improves the atomic bonding degree of the material on both sides of the bonding interface. (4) The hardness, strength and ductility of the extruded profiles can be improved by increasing billet heating temperature and extrusion speed. Acknowledgements
The authors would like to acknowledge the financial support from National Natural Science Foundation of China (51375270, 51575315 and 51405268).
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Fig. 1. The extrusion die for experiments.
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Fig. 2. The positions and orientations of different samples as well as the dimensions of tensile specimens: (a) the positions of the welding seam and the observed cross-section for microstructure, (b) the orientations of tensile samples, (c) the dimensions of tensile samples.
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Fig. 3. The optical microstructure on the cross-sections of the profiles extruded by experiments E1 and E4.
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Fig. 4. The optical microstructure of the zone A on the cross-sections of the profiles extruded by different experiments: (a) E1, (b) E2, (c) E3, (d) E4.
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Fig. 5. The comparison of morphology, misorientation and orientation of the grains on the cross-sections of the profiles extruded by experiments E1 and E3: (a) and (e) optical microstructure, (b) and (f) EBSD images of grains morphology, (c) and (g) misorientation angles along the lines A and B, (d) and (h) {100} pole figures.
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Fig. 6. Bright field TEM images of the micro-voids on the bonding interfaces of the welding seams extruded by different experiments: (a) E1, (b) E2, (c) E3, (d) E4.
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Fig. 7. The TEM images of the precipitates: (a), (b), (c) bright field TEM images of the precipitates in the welding seams extruded by experiments E2, E3, E4, (d) high-resolution electron micrograph of the dotlike and needleshaped precipitates imaged along <100> zone axis of the matrix Al.
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Fig. 8. Hardness distributions on the cross-sections of the profiles extruded by experiments E1-E4.
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Fig. 9. Tensile properties of the profiles extruded by E1-E4: (a) ultimate tensile strength, (b) elongation percentage.
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Fig. 10. The fracture positions, fracture morphologies and gauge lengths of the 90° tensile specimens after tensile tests: (a) E1, (b) E2, (c) E3, (d) E4.
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Fig. 11. Distribution of effective strain field on the cross-section of the profiles extruded by different experiments: (a) E1, (b) E2, (c) E3, (d) E4.
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Table 1 Chemical compositions of the as-homogenized AA6063 aluminum alloy billets. Element
Si
Fe
Cu
Mn
Mg
Cr
Zn
Ti
Ni
Al
(wt %)
0.3942
0.1221
0.0844
0.0063
0.5563
0.0035
0.0085
0.0103
0.0048
Bal.
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Table 2 Extrusion process parameters for experiments E1-E4. Experiment No.
E1
E2
E3
E4
Billet heating temperature (℃)
460
490
520
490
Extrusion speed (mm/s)
1.0
1.0
1.0
4.0
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