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Influence of melt flow on the formation of defect band in high pressure die casting of AZ91D magnesium alloy
Accepted Manuscript Influence of melt flow on the formation of defect band in high pressure die casting of AZ91D magnesium alloy
Shoumei Xiong, Xiaobo Li, Zhipeng Guo PII: DOI: Reference:
S1044-5803(16)30401-6 doi: 10.1016/j.matchar.2017.05.009 MTL 8670
To appear in:
Materials Characterization
Received date: Revised date: Accepted date:
28 September 2016 15 March 2017 9 May 2017
Please cite this article as: Shoumei Xiong, Xiaobo Li, Zhipeng Guo , Influence of melt flow on the formation of defect band in high pressure die casting of AZ91D magnesium alloy, Materials Characterization (2017), doi: 10.1016/j.matchar.2017.05.009
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ACCEPTED MANUSCRIPT Title page Title: Influence of melt flow on the formation of defect band in high pressure die casting of AZ91D magnesium alloy Corresponding author:
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Prof. Shoumei Xiong School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China State Key Laboratory of Automobile Safety and Energy, Tsinghua University, Beijing 100084, China Tel.: 0086-10-62773793 Fax: 0086-10-62773793 E-mail: [email protected]
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Co-authors are:
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Mr. Xiaobo Li School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Tel.: 0086-10-62789448 Fax: 0086-10-62773637 E-mail: [email protected]
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Dr. Zhipeng Guo School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Tel.: 0086-10-62789448 Fax: 0086-10-62773637 E-mail: [email protected]
ACCEPTED MANUSCRIPT Title page Title: Influence of melt flow on the formation of defect band in high pressure die casting of AZ91D magnesium alloy Abstract: The characteristics of defect band under different melt flow patterns in high
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pressure die casting of AZ91D magnesium alloy were investigated. Results showed that the distribution of defect band followed the contour of melt flow. On the side
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close to the skin layer, the porosity in defect band was uniformly distributed and
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followed the contour of melt flow, while on the other side close to the casting center,
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the porosity in defect band was nonuniformly distributed and had a poor consistency with the contour of melt flow. During filling process, the flow speed and solid fraction
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inside and outside the contour of melt flow were much different. Thus, the crystals in the contour of the melt flow would rotate and fragment under the flush of melt flow,
in the defect band.
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resulting in a larger gap between crystals and the formation of an aggregated porosity
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flow
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Keywords: High pressure die casting, AZ91D magnesium alloy, Defect band, Melt
ACCEPTED MANUSCRIPT 1 Introduction As the lightest structural alloy, magnesium alloys have been used increasingly in the industry, because of the potential application for energy saving and environment protection [1, 2]. Due to the faster prototyping, better casting dimensional accuracy
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and less requirement for post-processing, high-pressure die casting (HPDC) becomes
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the key manufacturing process for the magnesium components. On the other hand,
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though the magnesium components have been successfully manufactured by HPDC, the formation mechanism of microstructure in HPDC magnesium alloy is still not
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completely understood.
A typical microstructure of HPDC magnesium alloys comprised of porosities,
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externally solidified crystals (ESCs), α-Mg grains, divorced eutectics, porosity and
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defect band [3-7], of which the defect band was the most special microstructural feature. Studies had been performed in many aspects to investigate the
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characterization and formation mechanism of the defect band in HPDC. The
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characteristics of the defect band in Mg-Al and Al-Si die castings were studied by Gourlay et al [8]. It was found that the defect band comprised of macrosegregation or porosity, commonly followed the surface contour of the die castings, and could form near to and relatively far from the surface layer, which was similar to the results of the study [9].
The formation mechanism of defect band was correlated with the rheological behavior
ACCEPTED MANUSCRIPT of the mushy zone in the study [10]. The shear stresses would be developed during the filling and the subsequent feeding processes during solidification [11]. Two transition points, the dendrite coherency point and the maximum packing solid fraction, divided the mushy zone into three regions of different mechanical and feeding behaviors. The
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resulting defect band were rationalized by considering the governing local shear stress
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and shear rate, local strength and time available for fluid flow. The shear bands
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initiated after reaching the peak stress and then increased in thickness during strain softening from 10 to 16 mean grains thick theoretically [12, 13]. The formation
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mechanism of defect band in Al-Si die castings was studied by Gourlay et al [14],
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results of which were considered with reference to the rheological properties of the filling semisolid metal. The study of thickness of defect bands in HPDC showed that
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the band thicknesses were measured to be in the range 7-18 mean grains size
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experimentally, which was believed to be the substantial evidence that defect band formed due to strain localization in partially solidified alloys [15]. Based on the
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theory of rheological behavior of the mushy zone, the effects of Si content on defect
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band formation in hypoeutectic Al-Si die castings were discussed [16]. Results showed that with decreasing Si content, the defect bands formed closer to the casting surface, became more prevalent and also the width of the bands decreased, because of the steeper solidification gradient in the lower Si-containing alloys.
For the factors influencing the defect band, the results of the studies were in conflict to some extent. In the study conducted by Cao et al [17], there were two types of
ACCEPTED MANUSCRIPT segregation bands, of which the first type far from the surface was determined by the level and magnitude of ESCs present in the die castings, whereas, the second type near surface was less common and found to have no obvious relation with ESCs. While in the study [8], results showed that microstructure characterization revealed
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that ESCs were not necessary for defect band formation. In the study on the feeding
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mechanism in HPDC [18], shear band-like features existed through the gate in the
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castings produced with a high intensification pressure (IP) or thick gate. When a low IP is combined with a thin gate, no shear band is observed in the gate. The influence
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of plunger velocities on the defect band had been studied by Bladh et al [19], results
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showed that ingate velocities influence the location and characteristics of the shear bands. At slower phase I filling velocities, multiple defect bands were observed in the
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components. While in the study [17], it was discovered that intensification pressure
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had the strongest influence on the appearance of segregation bands inside the castings. By applying high intensification pressure, the risk of tearing along the bands and the
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segregation levels inside the band were both dramatically decreased.
It was clear that the defect band comprised of macrosegregation or porosity, and could form near to and relatively far from the surface layer. Currently, most of these studies were performed on the formation mechanism based on the theory of rheological behavior of the mushy zone in a laboratory scale, and the dilatant shear was believed to be the major factor during the formation of defect band. However, very limited studies have been performed to study the influence mechanism of melt flow on the
ACCEPTED MANUSCRIPT defect band formation. On the other hand, the characterization of microstructure around and in defect band of HPDC magnesium alloys was insufficient.
In this study, the influence of melt flow on defect band was investigated. Different
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melt flow patterns were obtained by using different gating systems, and the resulted
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defect band was characterized, including the evolution of location and distribution,
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porosity, chemical composition distribution and grain orientation, based on which the
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formation mechanism of defect band was discussed.
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2 Experimental
In this study, a specific casting with three tensile test bars and one plate (see Fig. 1a)
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was produced by a TOYO BD–350V5 cold chamber die casting machine using the
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AZ91D magnesium alloy. Four different types of ingates were desighed to obtain different flow patterns in the tensile test bar and plate samples [20, 21]. In the Figs. 1b
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and 1e, the locations of ingates A1 and B2 were in the center of the samples, and the
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contours of ingates A1 and B2 were similar to those of the tensile test bar and plate samples, respectively. While in the Figs. 1c and 1d, the locations of ingates A2 and B1 were not in the center of the samples, and the contours of ingates A2 and B1 were different from those of the tensile test bar and plate samples.
The key parameters adopted during experiment are listed in Table 1, in which the plunger was first moved in a constant speed of 0.2 m/s for 270 mm in the slow shot
ACCEPTED MANUSCRIPT stage, and then changed to the speed of 2.5m/s for 50mm in the fast shot stage. The maximum gate velocity was 75m/s. The whole shot sleeve had a length of 340 mm and a cross section radius of 35 mm. The composition of the commercial AZ91D magnesium alloy used in the experiment is shown in Table 2, and the solidus and
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liquidus temperatures are 470 and 595 ℃, respectively.
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Samples for further analysis were extracted at locations near the ingate of both tensile test bar and plate samples, as shown in Fig. 1a. For metallography observation, the
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specimens were sectioned, mounted, polished, and then etched with a diluted acetic
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acid solution of 50 ml distilled water, 150 ml anhydrous ethyl alcohol and 1 ml glacial acetic acid. The microstructure was observed with a ZEISS scope A1 optical
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microscope (OM) and a ZEISS MERLIN Compact scanning electron microscope
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(SEM). The composition of the microstructure was characterized with JXA-8230 electron probe microanalysis (EPMA). To characterize the grain orientation in the
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samples, the specimens were sectioned, mounted, polished, and then etched with a
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GATAN Model 682 precision etching coating system (PECS), and the applied voltage and current were 5 kV and 250 μA, respectively. The electron backscattered diffraction (EBSD) experiments were performed with a ZEISS MERLIN Compact SEM with HKL Channel5 system. An X-Ray computer tomography (CT) machine named Phoenix nanotom|m was applied to scan the porosity distribution inside the samples near the ingates. The applied voltage and current in the tube were 110 kV and 100 μA, respectively, and the image resolution was 1.167 μm. The 3D morphology of
ACCEPTED MANUSCRIPT the porosity was reconstructed using the software named VGStudio Max 3.0.
3 Results 3.1 Characterization of the microstructure
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Figure 2 shows the microstructure with defect band in the section perpendicular to the
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flow direction in tensile test bar samples. From Figs. 2a-2d, it can be observed that
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there was a porosity aggregation in the defect band, which was similar to the characterization of defect band in the studies [8, 17]. On the other hand, the defect
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band showed a good match with the contour and location of the ingates A1 (Figure 1b)
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and A2 (Figure 1c) respectively, which was different from the characterization of following the contour of castings in studies [8-10, 14, 17]. The etched microstructure
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in Figs. 2c and 2d revealed that the defect band divided the microstructure of cross
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section into three zones, i.e. I (inside defect band), II (defect band), III (outside of defect band). In I and III zone, the microstructure was bright, and contained large
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numbers of ESCs, which was similar to the results in study [8]. While in II zone, the
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microstructure was dark, and contained small and broken ESCs (see Figs. 2e and f).
Fig. 3 shows the microstructure in the transverse section along the flow direction near the two different ingates in the plate samples. Similar to the tensile test bar samples, there was also a porosity aggregation in the defect band, and which followed the contour of the ingates B1 and B2, respectively. In Fig. 3a, the distribution of defect band firstly followed the contour and position of ingate B1, and then gradually
ACCEPTED MANUSCRIPT developed along the contour of plate sample far from the ingate B1. Similar behavior could also be observed in Fig. 3b, though in this case the contour of ingate B2 closely matched to that of the sample (see Fig. 1e). On the other hand, the pattern of melt flow from ingates B1 and B2 could also be observed in Figs. 3c and d, respectively. It
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was clear that the zone I (inside defect band) was the melt flow, the zone III (outside
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of defect band) was the pre-filled melt, and the zone II (defect band) was the gap
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between zones I and III. Thus, it can be concluded that the distribution of defect band was between the melt flow and pre-filled melt, and the development of defect band
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followed the contour of melt flow, not the castings or ingates.
According to Figs. 2 and 3, the different microstructure in the three zones indicated
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that the zone I (inside defect band) was the melt flow, the zone III (outside of defect
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band) was the pre-filled melt, and between the zones I and III, the zone II was the defect band. Thus, the distribution and development of defect band were determined
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by the melt flow, which was influenced by the ingate.
3.2 Characterization of the porosity in defect band Fig. 4 shows the morphology of porosity in defect band in longitudinal section and transverse section in tensile test bar and plate samples respectively. The porosity in the defect band was irregular and connected with one another. The porosity on the side of defect band near the skin layer of casting was uniform and distributed in such a way that could match approximately the contour of ingates and melt flow (see Figs.
ACCEPTED MANUSCRIPT 4a and 4c). On the other hand, the porosity on the side close to the casting center was nonuniform and distributed inconsistently with the contour of ingates and melt flow for both tensile test bar (see Fig. 4a) and plate samples (see Fig. 4c). Further investigation revealed that in Fig. 4b, the outline on two sides of porosity in defect
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band mostly could stitch seamlessly. While in Figure 4d, the grains in defect band had
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signs of rotation and breaking along the flow direction. Between those rotated or
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broken grains, the gap existed inevitably. Under the solidification contraction, the gap
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would finally change to porosity in defect band due to the less melt left.
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Fig. 5 shows the morphology of shrinkage pores in longitudinal section and transverse section in tensile test bar and plate samples respectively. It can be observed that the
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shrinkage pores in both tensile test bar and plate samples all randomly located at the
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grain boundaries, especially the ESCs, and were independent with each other. Besides, the morphology of shrinkage pores was complex and showed no characterizations
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related to the melt flow. In this respect, the formation mechanism of porosity in defect
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band was different from that of shrinkage pore: the solidification contraction could not lead to the formation of defect band, though the defect band exhibited a an aggregation of porosity with complex morphology.
Fig. 6 shows the 3D morphology and distribution of porosity in the tensile test bar samples with ingates A1 and A2. It can be observed that there was a porosity aggregation in defect band, which firstly followed the contour of ingates A1 and A2,
ACCEPTED MANUSCRIPT then developed in the same pattern as the melt flow, which confirmed the finding in Fig. 2. While in zones I and III, both shrinkage pore and gas pore randomly distributed. As for the morphology of porosity, the porosity in defect band was large, complex and connected with one another, while the porosity in zones I and III was
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relatively small, simple and mutually independent, which confirmed the finding in
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Figs. 4 and 5.
Fig. 7 shows the 3D distribution of porosity along the normalized thickness
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(X/T×100%, where X was the distance from the casting surface, and T was the
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thickness of the casting) in the plate sample with ingate B2. Similar to Fig. 6, a porosity aggregation in defect band followed the contour of ingate B2 could be
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observed, and the porosity in defect band was large and complex, which confirmed
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the finding in Fig. 3. According to Figs. 7c and 7d, the average sphericity and the number of porosity in defect band were lower than those in zones I and III, whereas
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the volume, either entire or average, was the largest in defect band, indicating that the
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porosity in defect band was complex, large and mutually connected. The average length of porosity in X (width direction), Y (melt flow direction) and Z (thickness direction) directions was shown in Fig. 7c. The average length of porosity was larger in both X and Y directions than that in Z direction. Besides, the average length of porosity in defect band was significantly larger in X and Y directions than that in zones I and III, indicating that the melt flow had a close correlation with the formation of defect band.
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According to Figs. 4-7, it can be concluded that the porosity in zone II (defect band) was much different from that in zones I (inside defect band) and III (outside of defect band), this is because of the different formation mechanisms. The porosity in zones I
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(inside defect band) and III (outside of defect band) was the result of solidification
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contraction, while the porosity in zone II (defect band) was the result of melt flow and
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solidification contraction.
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3.3 Composition and grain orientation in the defect band
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Fig. 8 show the EPMA map scanning results of microstructure in defect band near the skin layer and in the center of plate sample with ingate B1. Similar to Figure 4, the
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map scanning results of Mg (Fig. 8b) and Al (Fig. 8c) revealed that the outline on two
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sides of porosity in defect band could stitch seamlessly, and the composition, especially Al element, of the microstructure on both sides around porosity had a high
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degree of agreement. On the other hand, there was a segregation of Al element in the
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microstructure around the edge of porosity in Fig. 8c. Thus, it can be deduced that the porosity in defect band near skin layer was induced by the detachment of the solidified crystals.
Figs. 8d-8f show the EPMA map scanning results of microstructure in defect band near the center of the plate sample with ingate B1. It can be observed that the contour of the porosity was complex and the two sides could not stitch seamlessly. On the
ACCEPTED MANUSCRIPT other hand, the distribution of elements Mg (Fig. 8e) and Al (Fig. 8f) on the two sides around porosity had a poor agreement. This is because there was melt left in the gap induced by the rotation or fragment of crystals, and the left melt would adjoin to one side of crystals. Because the heat transfer was perpendicular to the die wall, the Al
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element could only get segregated, or accumulated on one side of the porosity, while
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no aggregation of Al element could found at the other edge, detail of which was
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shown in Figs. 8e and 8f.
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Fig. 9 shows the EPMA map scanning results of the microstructure surrounding the
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shrinkage pores in the skin layer and center of plate sample with ingate B1. Similar to Fig. 5, the shrinkage pore in Fig. 9 showed a complex morphology, and the outline
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showed no stitching ability. According to Figs. 9b and 9c, there was a low segregation
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degree of elements in microstructure surrounding the shrinkage pore near skin layer. While in the center of plate sample, a clear segregation of Al element in
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microstructure surrounding the shrinkage pore could be observed (see Figs. 9e and 9f).
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This is because the solidification rate near the skin layer in HPDC was much higher than that in the casting center, accordingly there was a lower solidification segregation degree in the skin layer than that in the center of plate sample.
According to the Figs. 8 and 9, it can be concluded that the shrinkage pore in skin layer formed at the earliest stage of solidification process, the shrinkage pore in the center formed at the latest stage of solidification process, while the porosity in defect
ACCEPTED MANUSCRIPT band formed at the middle stage of solidification process.
Fig. 10 shows the EBSD analysis results of the grain orientation at different locations in the plate sample. As shown in Fig. 10a, no twinning occurred in the α-Mg grains in
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the skin layer. This is because the α-Mg grains in the skin layer mostly nucleated in
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the die cavity, no significant external forces could exert on these α-Mg grains.
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According to Fig. 10b, there was no twinning in these large complete ESCs in the center of the plate sample. However, for these broken ESCs in the center of the plate
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sample, twinning could be clearly observed (see Fig. 10c). This is because the ESCs
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nucleated in shot sleeve, and grew up in both shot sleeve and die cavity. During the filling process, the ESCs were pushed into the die cavity at a very fast speed through
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the ingate, and the turbulent melt flow would force the ESCs to rotate and fragment.
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The complete ESCs survived during the filling process would continue to grow up and show no twins, while the broken ESCs would exhibt twins due to the induced
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stress under the flush of melt flow. Similar to the Figs. 2-4, the ESCs around the
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porosity in defect band were mostly broken and exhibited twins, as shown in Fig. 10d, indicating the presence of the induced stress. In this respect, it can be concluded that the formation of defect band could be attributed to the stress induced by the melt flow.
3.4 Formation mechanism of the defect band A schematic illustration of the formation mechanism of defect band was shown in Fig. 11. During filling process, the melt with ESCs was pushed into die cavity through the
ACCEPTED MANUSCRIPT ingate by plunger [23]. Because of the rapid solidification, the melt near the wall of die cavity would immediately solidify, then the so-called skin layer (i.e. III, fs~1, V~0) formed. Whereas in the center of die cavity, the melt flow from the ingate still remained liquid (i.e. I, fs<<1, V=Vmax). As a consequence, a semi-solid zone II (fs<1,
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V=0~Vmax) between zones I and III developed.
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According to Fig.11b, the ESCs would rotate and fragment under the flush of the turbulent melt flow from ingate, leading to the formation of gap between the crystals.
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Because the solid fraction (fs) in zone I was low, the gap caused by the rotated or
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broken crystals could be easily filled by the melt flow due to efficient feeding [10]. As solidification proceeded, the ESCs (both broken and complete) in zone I would grow
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up, then the shrinkage pore would form at the boundaries of the aggregated large and
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dendritic ESCs because of the solidification contraction [20], see Fig. 5.
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As shown in Fig. 11d, zone III could be further divided into two parts based on the
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local solidification rate. The first section was the skin layer, no porosity and only a few small ESCs existed due to the chilling effect (see Figs. 2 and 3). Next to the skin layer, the second part was the solidification layer, formed during the filling process due to continuous solidification after the formation of skin layer. In zone III, the melt flow speed was close to zero, i.e. melt feeding could hardly occur. In this respect, the presence of porosity was inevitable (see Fig. 9a). Besides, because of the fast solidification rate, the segregation degree was very low, e.g. for the Al element as
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As shown in Fig. 11c, in zone II, the melt flow speed was approximately zero on the side close to zone III. However, on the other side of zone II, i.e. close to zone I, the
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melt flow speed was much higher. The uneven melt flow speed on the two sides of
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zone II would cause the crystals to rotate and fragment (see Figs. 4d and 10d), leading
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to small and broken ESCs (see Figs. 2 and 3) and gap between crystals. On the other hand, because the fs in zone II was much higher than zero, the melt feeding in this area
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was insufficient [10], leading to the formation of aggregated porosity in defect band.
On the side of defect band close to the casting center, because of the turbulent melt
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flow and a low solidification fraction fs, the crystals would randomly rotate, leading to
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a poor matching between the distribution of porosity in defect band and the contour of melt flow. While on the side of defect band close to the casting surface, because of the
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relatively slow melt flow speed and a high solidification fraction fs, the crystals would
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roughly rotate along the contour of melt flow, leading to a better consistency of the distribution of porosity in defect band and the contour of melt flow (see Figs. 4a and c). Because the rotation and fragment of crystals concentrately occurred in zone II, the gap between the crystals would easily get connected with one another and subsequently formed the aggregated porosity in defect band. On the contrary, the shrinkage pore in zones I and III formed rather randomly, and accordingly these shrinkage pores could hardly get connected. In this respect, the porosity in defect
ACCEPTED MANUSCRIPT band was larger in size and more complex in morphology than these in zones I and III (see Figs. 6 and 7).
4 Conclusions
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In this paper, the characteristics of defect band under different melt flow patterns in
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HPDC AZ91D magnesium alloy were investigated, based on which the formation
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mechanism of defect band was discussed, and the following conclusions can be drawn:
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(1) The distribution of defect band followed the contour of melt flow. On the side
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close to the casting surface, the porosity in the defect band was uniformly distributed and followed the contour of melt flow, while on the other side close to the casting
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center, the porosity in defect band was nonuniformly distributed and had a poor
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consistency with the contour of melt flow. (2) During filling process, the flow speed and solid fraction inside and outside the
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contour of melt flow were much different. Thus, the crystals in the contour of the melt
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flow would rotate and fragment under the flush of melt flow, resulting in a larger gap between crystals and the formation of an aggregated porosity in the defect band.
Acknowledgements The authors would like to thank the National Key Research and Development Program of China (No.2016YFB0301001) for financial support.
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transfer in die casting process. J. Mater. Sci. Technol. 2000; 16: 269-72.
ACCEPTED MANUSCRIPT Figure captions Figure 1 Configuration of (a) the specific casting including three tensile test bars (diameter at the center was 6.4mm) and one plate (thickness was 2.5mm), (b) ingate A1 and (c) ingate A2 in the tensile test bar, (d) ingate B1 and (e) ingate B2 in the plate
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sample.
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Figure 2 Characterization of the microstructure in the cross section perpendicular to
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the flow direction of the tensile test bar. (a), (c) and (e) were near ingate A1, while (b), (d) and (f) were near ingate A2. (a) and (b) were polished microstructure, while (c)
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and (d) were etched microstructure. (e) and (f) were zoom-in visualizations of (c) and
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(d), respectively.
Figure 3 Characterization of microstructure in the transverse section along the flow
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direction of the plate sample. (a), (c) and (e) were near ingate B1, while (b), (d) and (f)
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were near ingate B2. (a) and (b) were polished microstructure, while (c) and (d) were etched microstructure. (e) and (f) were zoom-in visualizations of (c) and (d),
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respectively.
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Figure 4 Morphology of porosity in defect band in the longitudinal section of the tensile test bar and transverse section of the plate sample, respectively. (a) and (b) were near ingate A1, while (c) and (d) near ingate B1. (b) and (d) were zoom-in visualizations of (c) and (d), respectively. Figure 5 Morphology of the shrinkage pores in the longitudinal section of the center tensile test bar and transverse section of the plate sample. (a) and (b) were near ingate A1, while (c) and (d) near ingate B1. (b) and (d) were zoom-in visualizations of (c) and
ACCEPTED MANUSCRIPT (d), respectively. Figure 6 The 3D distribution of the porosity in the tensile test bar. (a) and (c) showed the plate sample with ingate A1 and A2, respectively. The view direction of (a) and (b) was perpendicular, while (b) and (d) was parallel to the flow.
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Figure 7 The 3D distribution of the porosity in the plate sample with ingate B2. (a)
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and (b) were front and top view of the sample, respectively. (c) the average length in
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X, Y, Z axis and the sphericity of the porosity. (d) the number, volume and average diameter of the porosity as a function of the normalized thickness of the specimen.
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Figure 8 The EPMA map scanning results of the microstructure around the porosity.
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(a), (b) and (c) corresponded to the side of defect band near the skin layer, while (d), (e) and (f) corresponded to the side of defect band near center. (a) and (d) showed the
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position of the scanning areas. (b) and (e) corresponded to the element Mg, while (c)
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and (f) corresponded to the element Al. Figure 9 The EPMA map results of the microstructure surrounding a shrinkage pore
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of the plate sample with ingate B1. (a), (b) and (c) were close to the skin layer, while
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(d), (e) and (f) were close to the specimen center. (a) and (d) showed the scanning area, while (b) and (e), (c) and (f) corresponded to Mg, and Al elements respectively. Figure 10 EBSD analysis results of the grain orientation, showing (a) α-Mg grains in the skin layer, (b) complete ESCs in the center, (c) broken ESCs in center and (d) broken ESCs in the defect band. Figure 11 Schematic illustration of the formation mechanism of defect band, showing (a) the flow pattern in the specimen. (b)-(d) were zoom-in areas according to zones I,
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II and III, respectively.
ACCEPTED MANUSCRIPT Table 1 Processing parameters adopted during die casting Initial mold
Casting
Slow shot
Fast shot
temperature,
temperature,
pressure,
velocity,
velocity,
℃
℃
MPa
m/s
m/s
680
150
79
0.2
2.5
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Pouring
Table 2 Chemical compositions of the AZ91D magnesium alloy used in the study
Zn
Si
8.80
0.18
0.61
0.06
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Cu
Fe
Mg
0.007
0.003
Bal
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Mn
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Al
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(mass %).
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Figure 1 Configuration of (a) the specific casting including three tensile test bars (diameter at the center was 6.4mm) and one plate (thickness was 2.5mm), (b) ingate
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A1 and (c) ingate A2 in the tensile test bar, (d) ingate B1 and (e) ingate B2 in the plate
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sample.
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Figure 2 Characterization of the microstructure in the cross section perpendicular to the flow direction of the tensile test bar. (a), (c) and (e) were near ingate A1, while (b), (d) and (f) were near ingate A2. (a) and (b) were polished microstructure, while (c)
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and (d) were etched microstructure. (e) and (f) were zoom-in visualizations of (c) and (d), respectively.
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Figure 3 Characterization of microstructure in the transverse section along the flow direction of the plate sample. (a), (c) and (e) were near ingate B1, while (b), (d) and (f)
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were near ingate B2. (a) and (b) were polished microstructure, while (c) and (d) were etched microstructure. (e) and (f) were zoom-in visualizations of (c) and (d),
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respectively.
Figure 4 Morphology of porosity in defect band in the longitudinal section of the tensile test bar and transverse section of the plate sample, respectively. (a) and (b) were near ingate A1, while (c) and (d) near ingate B1. (b) and (d) were zoom-in visualizations of (c) and (d), respectively.
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Figure 5 Morphology of the shrinkage pores in the longitudinal section of the center
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tensile test bar and transverse section of the plate sample. (a) and (b) were near ingate A1, while (c) and (d) near ingate B1. (b) and (d) were zoom-in visualizations of (c) and
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(d), respectively.
Figure 6 The 3D distribution of the porosity in the tensile test bar. (a) and (c) showed the plate sample with ingate A1 and A2, respectively. The view direction of (a) and (b) was perpendicular, while (b) and (d) was parallel to the flow.
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Figure 7 The 3D distribution of the porosity in the plate sample with ingate B2. (a) and (b) were front and top view of the sample, respectively. (c) the average length in
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X, Y, Z axis and the sphericity of the porosity. (d) the number, volume and average
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diameter of the porosity as a function of the normalized thickness of the specimen.
Figure 8 The EPMA map scanning results of the microstructure around the porosity. (a), (b) and (c) corresponded to the side of defect band near the skin layer, while (d), (e) and (f) corresponded to the side of defect band near center. (a) and (d) showed the position of the scanning areas. (b) and (e) corresponded to the element Mg, while (c) and (f) corresponded to the element Al.
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Figure 9 The EPMA map results of the microstructure surrounding a shrinkage pore
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of the plate sample with ingate B1. (a), (b) and (c) were close to the skin layer, while (d), (e) and (f) were close to the specimen center. (a) and (d) showed the scanning area,
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while (b) and (e), (c) and (f) corresponded to Mg, and Al elements respectively.
Figure 10 EBSD analysis results of the grain orientation, showing (a) α-Mg grains in the skin layer, (b) complete ESCs in the center, (c) broken ESCs in center and (d) broken ESCs in the defect band.
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Figure 11 Schematic illustration of the formation mechanism of defect band, showing
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(a) the flow pattern in the specimen. (b)-(d) were zoom-in areas according to zones I,
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II and III, respectively.
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Highlights 1. The melt flow would cause the rotation and fragment of crystals. 2. The rotation and fragment of crystals would lead to formation of defect band. 3. The distribution of defect band was strongly dependent on the local melt flow.
Shoumei Xiong, Xiaobo Li, Zhipeng Guo PII: DOI: Reference:
S1044-5803(16)30401-6 doi: 10.1016/j.matchar.2017.05.009 MTL 8670
To appear in:
Materials Characterization
Received date: Revised date: Accepted date:
28 September 2016 15 March 2017 9 May 2017
Please cite this article as: Shoumei Xiong, Xiaobo Li, Zhipeng Guo , Influence of melt flow on the formation of defect band in high pressure die casting of AZ91D magnesium alloy, Materials Characterization (2017), doi: 10.1016/j.matchar.2017.05.009
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ACCEPTED MANUSCRIPT Title page Title: Influence of melt flow on the formation of defect band in high pressure die casting of AZ91D magnesium alloy Corresponding author:
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Prof. Shoumei Xiong School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China State Key Laboratory of Automobile Safety and Energy, Tsinghua University, Beijing 100084, China Tel.: 0086-10-62773793 Fax: 0086-10-62773793 E-mail: [email protected]
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Co-authors are:
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Mr. Xiaobo Li School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Tel.: 0086-10-62789448 Fax: 0086-10-62773637 E-mail: [email protected]
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Dr. Zhipeng Guo School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Tel.: 0086-10-62789448 Fax: 0086-10-62773637 E-mail: [email protected]
ACCEPTED MANUSCRIPT Title page Title: Influence of melt flow on the formation of defect band in high pressure die casting of AZ91D magnesium alloy Abstract: The characteristics of defect band under different melt flow patterns in high
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pressure die casting of AZ91D magnesium alloy were investigated. Results showed that the distribution of defect band followed the contour of melt flow. On the side
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close to the skin layer, the porosity in defect band was uniformly distributed and
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followed the contour of melt flow, while on the other side close to the casting center,
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the porosity in defect band was nonuniformly distributed and had a poor consistency with the contour of melt flow. During filling process, the flow speed and solid fraction
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inside and outside the contour of melt flow were much different. Thus, the crystals in the contour of the melt flow would rotate and fragment under the flush of melt flow,
in the defect band.
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resulting in a larger gap between crystals and the formation of an aggregated porosity
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flow
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Keywords: High pressure die casting, AZ91D magnesium alloy, Defect band, Melt
ACCEPTED MANUSCRIPT 1 Introduction As the lightest structural alloy, magnesium alloys have been used increasingly in the industry, because of the potential application for energy saving and environment protection [1, 2]. Due to the faster prototyping, better casting dimensional accuracy
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and less requirement for post-processing, high-pressure die casting (HPDC) becomes
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the key manufacturing process for the magnesium components. On the other hand,
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though the magnesium components have been successfully manufactured by HPDC, the formation mechanism of microstructure in HPDC magnesium alloy is still not
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completely understood.
A typical microstructure of HPDC magnesium alloys comprised of porosities,
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externally solidified crystals (ESCs), α-Mg grains, divorced eutectics, porosity and
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defect band [3-7], of which the defect band was the most special microstructural feature. Studies had been performed in many aspects to investigate the
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characterization and formation mechanism of the defect band in HPDC. The
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characteristics of the defect band in Mg-Al and Al-Si die castings were studied by Gourlay et al [8]. It was found that the defect band comprised of macrosegregation or porosity, commonly followed the surface contour of the die castings, and could form near to and relatively far from the surface layer, which was similar to the results of the study [9].
The formation mechanism of defect band was correlated with the rheological behavior
ACCEPTED MANUSCRIPT of the mushy zone in the study [10]. The shear stresses would be developed during the filling and the subsequent feeding processes during solidification [11]. Two transition points, the dendrite coherency point and the maximum packing solid fraction, divided the mushy zone into three regions of different mechanical and feeding behaviors. The
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resulting defect band were rationalized by considering the governing local shear stress
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and shear rate, local strength and time available for fluid flow. The shear bands
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initiated after reaching the peak stress and then increased in thickness during strain softening from 10 to 16 mean grains thick theoretically [12, 13]. The formation
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mechanism of defect band in Al-Si die castings was studied by Gourlay et al [14],
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results of which were considered with reference to the rheological properties of the filling semisolid metal. The study of thickness of defect bands in HPDC showed that
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the band thicknesses were measured to be in the range 7-18 mean grains size
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experimentally, which was believed to be the substantial evidence that defect band formed due to strain localization in partially solidified alloys [15]. Based on the
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theory of rheological behavior of the mushy zone, the effects of Si content on defect
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band formation in hypoeutectic Al-Si die castings were discussed [16]. Results showed that with decreasing Si content, the defect bands formed closer to the casting surface, became more prevalent and also the width of the bands decreased, because of the steeper solidification gradient in the lower Si-containing alloys.
For the factors influencing the defect band, the results of the studies were in conflict to some extent. In the study conducted by Cao et al [17], there were two types of
ACCEPTED MANUSCRIPT segregation bands, of which the first type far from the surface was determined by the level and magnitude of ESCs present in the die castings, whereas, the second type near surface was less common and found to have no obvious relation with ESCs. While in the study [8], results showed that microstructure characterization revealed
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that ESCs were not necessary for defect band formation. In the study on the feeding
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mechanism in HPDC [18], shear band-like features existed through the gate in the
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castings produced with a high intensification pressure (IP) or thick gate. When a low IP is combined with a thin gate, no shear band is observed in the gate. The influence
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of plunger velocities on the defect band had been studied by Bladh et al [19], results
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showed that ingate velocities influence the location and characteristics of the shear bands. At slower phase I filling velocities, multiple defect bands were observed in the
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components. While in the study [17], it was discovered that intensification pressure
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had the strongest influence on the appearance of segregation bands inside the castings. By applying high intensification pressure, the risk of tearing along the bands and the
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segregation levels inside the band were both dramatically decreased.
It was clear that the defect band comprised of macrosegregation or porosity, and could form near to and relatively far from the surface layer. Currently, most of these studies were performed on the formation mechanism based on the theory of rheological behavior of the mushy zone in a laboratory scale, and the dilatant shear was believed to be the major factor during the formation of defect band. However, very limited studies have been performed to study the influence mechanism of melt flow on the
ACCEPTED MANUSCRIPT defect band formation. On the other hand, the characterization of microstructure around and in defect band of HPDC magnesium alloys was insufficient.
In this study, the influence of melt flow on defect band was investigated. Different
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melt flow patterns were obtained by using different gating systems, and the resulted
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defect band was characterized, including the evolution of location and distribution,
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porosity, chemical composition distribution and grain orientation, based on which the
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formation mechanism of defect band was discussed.
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2 Experimental
In this study, a specific casting with three tensile test bars and one plate (see Fig. 1a)
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was produced by a TOYO BD–350V5 cold chamber die casting machine using the
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AZ91D magnesium alloy. Four different types of ingates were desighed to obtain different flow patterns in the tensile test bar and plate samples [20, 21]. In the Figs. 1b
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and 1e, the locations of ingates A1 and B2 were in the center of the samples, and the
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contours of ingates A1 and B2 were similar to those of the tensile test bar and plate samples, respectively. While in the Figs. 1c and 1d, the locations of ingates A2 and B1 were not in the center of the samples, and the contours of ingates A2 and B1 were different from those of the tensile test bar and plate samples.
The key parameters adopted during experiment are listed in Table 1, in which the plunger was first moved in a constant speed of 0.2 m/s for 270 mm in the slow shot
ACCEPTED MANUSCRIPT stage, and then changed to the speed of 2.5m/s for 50mm in the fast shot stage. The maximum gate velocity was 75m/s. The whole shot sleeve had a length of 340 mm and a cross section radius of 35 mm. The composition of the commercial AZ91D magnesium alloy used in the experiment is shown in Table 2, and the solidus and
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liquidus temperatures are 470 and 595 ℃, respectively.
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Samples for further analysis were extracted at locations near the ingate of both tensile test bar and plate samples, as shown in Fig. 1a. For metallography observation, the
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specimens were sectioned, mounted, polished, and then etched with a diluted acetic
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acid solution of 50 ml distilled water, 150 ml anhydrous ethyl alcohol and 1 ml glacial acetic acid. The microstructure was observed with a ZEISS scope A1 optical
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microscope (OM) and a ZEISS MERLIN Compact scanning electron microscope
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(SEM). The composition of the microstructure was characterized with JXA-8230 electron probe microanalysis (EPMA). To characterize the grain orientation in the
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samples, the specimens were sectioned, mounted, polished, and then etched with a
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GATAN Model 682 precision etching coating system (PECS), and the applied voltage and current were 5 kV and 250 μA, respectively. The electron backscattered diffraction (EBSD) experiments were performed with a ZEISS MERLIN Compact SEM with HKL Channel5 system. An X-Ray computer tomography (CT) machine named Phoenix nanotom|m was applied to scan the porosity distribution inside the samples near the ingates. The applied voltage and current in the tube were 110 kV and 100 μA, respectively, and the image resolution was 1.167 μm. The 3D morphology of
ACCEPTED MANUSCRIPT the porosity was reconstructed using the software named VGStudio Max 3.0.
3 Results 3.1 Characterization of the microstructure
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Figure 2 shows the microstructure with defect band in the section perpendicular to the
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flow direction in tensile test bar samples. From Figs. 2a-2d, it can be observed that
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there was a porosity aggregation in the defect band, which was similar to the characterization of defect band in the studies [8, 17]. On the other hand, the defect
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band showed a good match with the contour and location of the ingates A1 (Figure 1b)
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and A2 (Figure 1c) respectively, which was different from the characterization of following the contour of castings in studies [8-10, 14, 17]. The etched microstructure
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in Figs. 2c and 2d revealed that the defect band divided the microstructure of cross
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section into three zones, i.e. I (inside defect band), II (defect band), III (outside of defect band). In I and III zone, the microstructure was bright, and contained large
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numbers of ESCs, which was similar to the results in study [8]. While in II zone, the
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microstructure was dark, and contained small and broken ESCs (see Figs. 2e and f).
Fig. 3 shows the microstructure in the transverse section along the flow direction near the two different ingates in the plate samples. Similar to the tensile test bar samples, there was also a porosity aggregation in the defect band, and which followed the contour of the ingates B1 and B2, respectively. In Fig. 3a, the distribution of defect band firstly followed the contour and position of ingate B1, and then gradually
ACCEPTED MANUSCRIPT developed along the contour of plate sample far from the ingate B1. Similar behavior could also be observed in Fig. 3b, though in this case the contour of ingate B2 closely matched to that of the sample (see Fig. 1e). On the other hand, the pattern of melt flow from ingates B1 and B2 could also be observed in Figs. 3c and d, respectively. It
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was clear that the zone I (inside defect band) was the melt flow, the zone III (outside
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of defect band) was the pre-filled melt, and the zone II (defect band) was the gap
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between zones I and III. Thus, it can be concluded that the distribution of defect band was between the melt flow and pre-filled melt, and the development of defect band
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followed the contour of melt flow, not the castings or ingates.
According to Figs. 2 and 3, the different microstructure in the three zones indicated
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that the zone I (inside defect band) was the melt flow, the zone III (outside of defect
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band) was the pre-filled melt, and between the zones I and III, the zone II was the defect band. Thus, the distribution and development of defect band were determined
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by the melt flow, which was influenced by the ingate.
3.2 Characterization of the porosity in defect band Fig. 4 shows the morphology of porosity in defect band in longitudinal section and transverse section in tensile test bar and plate samples respectively. The porosity in the defect band was irregular and connected with one another. The porosity on the side of defect band near the skin layer of casting was uniform and distributed in such a way that could match approximately the contour of ingates and melt flow (see Figs.
ACCEPTED MANUSCRIPT 4a and 4c). On the other hand, the porosity on the side close to the casting center was nonuniform and distributed inconsistently with the contour of ingates and melt flow for both tensile test bar (see Fig. 4a) and plate samples (see Fig. 4c). Further investigation revealed that in Fig. 4b, the outline on two sides of porosity in defect
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band mostly could stitch seamlessly. While in Figure 4d, the grains in defect band had
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signs of rotation and breaking along the flow direction. Between those rotated or
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broken grains, the gap existed inevitably. Under the solidification contraction, the gap
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would finally change to porosity in defect band due to the less melt left.
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Fig. 5 shows the morphology of shrinkage pores in longitudinal section and transverse section in tensile test bar and plate samples respectively. It can be observed that the
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shrinkage pores in both tensile test bar and plate samples all randomly located at the
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grain boundaries, especially the ESCs, and were independent with each other. Besides, the morphology of shrinkage pores was complex and showed no characterizations
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related to the melt flow. In this respect, the formation mechanism of porosity in defect
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band was different from that of shrinkage pore: the solidification contraction could not lead to the formation of defect band, though the defect band exhibited a an aggregation of porosity with complex morphology.
Fig. 6 shows the 3D morphology and distribution of porosity in the tensile test bar samples with ingates A1 and A2. It can be observed that there was a porosity aggregation in defect band, which firstly followed the contour of ingates A1 and A2,
ACCEPTED MANUSCRIPT then developed in the same pattern as the melt flow, which confirmed the finding in Fig. 2. While in zones I and III, both shrinkage pore and gas pore randomly distributed. As for the morphology of porosity, the porosity in defect band was large, complex and connected with one another, while the porosity in zones I and III was
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relatively small, simple and mutually independent, which confirmed the finding in
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Figs. 4 and 5.
Fig. 7 shows the 3D distribution of porosity along the normalized thickness
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(X/T×100%, where X was the distance from the casting surface, and T was the
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thickness of the casting) in the plate sample with ingate B2. Similar to Fig. 6, a porosity aggregation in defect band followed the contour of ingate B2 could be
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observed, and the porosity in defect band was large and complex, which confirmed
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the finding in Fig. 3. According to Figs. 7c and 7d, the average sphericity and the number of porosity in defect band were lower than those in zones I and III, whereas
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the volume, either entire or average, was the largest in defect band, indicating that the
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porosity in defect band was complex, large and mutually connected. The average length of porosity in X (width direction), Y (melt flow direction) and Z (thickness direction) directions was shown in Fig. 7c. The average length of porosity was larger in both X and Y directions than that in Z direction. Besides, the average length of porosity in defect band was significantly larger in X and Y directions than that in zones I and III, indicating that the melt flow had a close correlation with the formation of defect band.
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According to Figs. 4-7, it can be concluded that the porosity in zone II (defect band) was much different from that in zones I (inside defect band) and III (outside of defect band), this is because of the different formation mechanisms. The porosity in zones I
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(inside defect band) and III (outside of defect band) was the result of solidification
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contraction, while the porosity in zone II (defect band) was the result of melt flow and
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solidification contraction.
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3.3 Composition and grain orientation in the defect band
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Fig. 8 show the EPMA map scanning results of microstructure in defect band near the skin layer and in the center of plate sample with ingate B1. Similar to Figure 4, the
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map scanning results of Mg (Fig. 8b) and Al (Fig. 8c) revealed that the outline on two
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sides of porosity in defect band could stitch seamlessly, and the composition, especially Al element, of the microstructure on both sides around porosity had a high
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degree of agreement. On the other hand, there was a segregation of Al element in the
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microstructure around the edge of porosity in Fig. 8c. Thus, it can be deduced that the porosity in defect band near skin layer was induced by the detachment of the solidified crystals.
Figs. 8d-8f show the EPMA map scanning results of microstructure in defect band near the center of the plate sample with ingate B1. It can be observed that the contour of the porosity was complex and the two sides could not stitch seamlessly. On the
ACCEPTED MANUSCRIPT other hand, the distribution of elements Mg (Fig. 8e) and Al (Fig. 8f) on the two sides around porosity had a poor agreement. This is because there was melt left in the gap induced by the rotation or fragment of crystals, and the left melt would adjoin to one side of crystals. Because the heat transfer was perpendicular to the die wall, the Al
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element could only get segregated, or accumulated on one side of the porosity, while
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no aggregation of Al element could found at the other edge, detail of which was
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shown in Figs. 8e and 8f.
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Fig. 9 shows the EPMA map scanning results of the microstructure surrounding the
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shrinkage pores in the skin layer and center of plate sample with ingate B1. Similar to Fig. 5, the shrinkage pore in Fig. 9 showed a complex morphology, and the outline
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showed no stitching ability. According to Figs. 9b and 9c, there was a low segregation
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degree of elements in microstructure surrounding the shrinkage pore near skin layer. While in the center of plate sample, a clear segregation of Al element in
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microstructure surrounding the shrinkage pore could be observed (see Figs. 9e and 9f).
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This is because the solidification rate near the skin layer in HPDC was much higher than that in the casting center, accordingly there was a lower solidification segregation degree in the skin layer than that in the center of plate sample.
According to the Figs. 8 and 9, it can be concluded that the shrinkage pore in skin layer formed at the earliest stage of solidification process, the shrinkage pore in the center formed at the latest stage of solidification process, while the porosity in defect
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Fig. 10 shows the EBSD analysis results of the grain orientation at different locations in the plate sample. As shown in Fig. 10a, no twinning occurred in the α-Mg grains in
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the skin layer. This is because the α-Mg grains in the skin layer mostly nucleated in
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the die cavity, no significant external forces could exert on these α-Mg grains.
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According to Fig. 10b, there was no twinning in these large complete ESCs in the center of the plate sample. However, for these broken ESCs in the center of the plate
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sample, twinning could be clearly observed (see Fig. 10c). This is because the ESCs
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nucleated in shot sleeve, and grew up in both shot sleeve and die cavity. During the filling process, the ESCs were pushed into the die cavity at a very fast speed through
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the ingate, and the turbulent melt flow would force the ESCs to rotate and fragment.
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The complete ESCs survived during the filling process would continue to grow up and show no twins, while the broken ESCs would exhibt twins due to the induced
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stress under the flush of melt flow. Similar to the Figs. 2-4, the ESCs around the
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porosity in defect band were mostly broken and exhibited twins, as shown in Fig. 10d, indicating the presence of the induced stress. In this respect, it can be concluded that the formation of defect band could be attributed to the stress induced by the melt flow.
3.4 Formation mechanism of the defect band A schematic illustration of the formation mechanism of defect band was shown in Fig. 11. During filling process, the melt with ESCs was pushed into die cavity through the
ACCEPTED MANUSCRIPT ingate by plunger [23]. Because of the rapid solidification, the melt near the wall of die cavity would immediately solidify, then the so-called skin layer (i.e. III, fs~1, V~0) formed. Whereas in the center of die cavity, the melt flow from the ingate still remained liquid (i.e. I, fs<<1, V=Vmax). As a consequence, a semi-solid zone II (fs<1,
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V=0~Vmax) between zones I and III developed.
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According to Fig.11b, the ESCs would rotate and fragment under the flush of the turbulent melt flow from ingate, leading to the formation of gap between the crystals.
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Because the solid fraction (fs) in zone I was low, the gap caused by the rotated or
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broken crystals could be easily filled by the melt flow due to efficient feeding [10]. As solidification proceeded, the ESCs (both broken and complete) in zone I would grow
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up, then the shrinkage pore would form at the boundaries of the aggregated large and
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dendritic ESCs because of the solidification contraction [20], see Fig. 5.
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As shown in Fig. 11d, zone III could be further divided into two parts based on the
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local solidification rate. The first section was the skin layer, no porosity and only a few small ESCs existed due to the chilling effect (see Figs. 2 and 3). Next to the skin layer, the second part was the solidification layer, formed during the filling process due to continuous solidification after the formation of skin layer. In zone III, the melt flow speed was close to zero, i.e. melt feeding could hardly occur. In this respect, the presence of porosity was inevitable (see Fig. 9a). Besides, because of the fast solidification rate, the segregation degree was very low, e.g. for the Al element as
ACCEPTED MANUSCRIPT shown in Fig. 9c.
As shown in Fig. 11c, in zone II, the melt flow speed was approximately zero on the side close to zone III. However, on the other side of zone II, i.e. close to zone I, the
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melt flow speed was much higher. The uneven melt flow speed on the two sides of
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zone II would cause the crystals to rotate and fragment (see Figs. 4d and 10d), leading
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to small and broken ESCs (see Figs. 2 and 3) and gap between crystals. On the other hand, because the fs in zone II was much higher than zero, the melt feeding in this area
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was insufficient [10], leading to the formation of aggregated porosity in defect band.
On the side of defect band close to the casting center, because of the turbulent melt
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flow and a low solidification fraction fs, the crystals would randomly rotate, leading to
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a poor matching between the distribution of porosity in defect band and the contour of melt flow. While on the side of defect band close to the casting surface, because of the
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relatively slow melt flow speed and a high solidification fraction fs, the crystals would
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roughly rotate along the contour of melt flow, leading to a better consistency of the distribution of porosity in defect band and the contour of melt flow (see Figs. 4a and c). Because the rotation and fragment of crystals concentrately occurred in zone II, the gap between the crystals would easily get connected with one another and subsequently formed the aggregated porosity in defect band. On the contrary, the shrinkage pore in zones I and III formed rather randomly, and accordingly these shrinkage pores could hardly get connected. In this respect, the porosity in defect
ACCEPTED MANUSCRIPT band was larger in size and more complex in morphology than these in zones I and III (see Figs. 6 and 7).
4 Conclusions
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In this paper, the characteristics of defect band under different melt flow patterns in
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HPDC AZ91D magnesium alloy were investigated, based on which the formation
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mechanism of defect band was discussed, and the following conclusions can be drawn:
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(1) The distribution of defect band followed the contour of melt flow. On the side
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close to the casting surface, the porosity in the defect band was uniformly distributed and followed the contour of melt flow, while on the other side close to the casting
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center, the porosity in defect band was nonuniformly distributed and had a poor
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consistency with the contour of melt flow. (2) During filling process, the flow speed and solid fraction inside and outside the
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contour of melt flow were much different. Thus, the crystals in the contour of the melt
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flow would rotate and fragment under the flush of melt flow, resulting in a larger gap between crystals and the formation of an aggregated porosity in the defect band.
Acknowledgements The authors would like to thank the National Key Research and Development Program of China (No.2016YFB0301001) for financial support.
ACCEPTED MANUSCRIPT References [1] H. Friedrich, S.J. Schumann. Research for a new age of magnesium in the automotive industry. J. Mater. Process. Technol. 2001; 117 (3): 276-81. [2] B.L. Mordike, T. Ebert. Magnesium properties-application-potential. Mater. Sci.
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Eng. A. 2001; 302(1): 37-45.
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[3] A.K. Dahle, S. Sannes, D.H. John, H. Westengen. Formation of defect band in
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high pressure die cast magnesium alloys. J. Light Metals. 2001; 1: 99-103. [4] B. Wang, S. Xiong. Effects of shot speed and biscuit thickness on externaly
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solidified crystals of high-pressure diet cast AM60B magnesium alloy. Trans.
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Nonferrous Met. Soc. China. 2011; 21: 767-72.
[5] M. Wu, S. Xiong. Microstructure characteristics of the eutectics of die cast
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AM60B magnesium alloy. J. Mater. Sci. Technol. 2011; 27: 1150-6.
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[6] W. Xiao, S. Zhu, M.A. Easton, M.S. Dargusch, M.S. Gibson, J. Nie. Microstructural characterization of high pressure die cast Mg-Zn-Al-RE alloys.
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Mater Charact. 2012; 65: 28-36.
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[7] J. Song, S.M. Xiong. The correlation between as–cast and aged microstructures of high–vacuum die–cast Mg–9Al–1Zn magnesium alloy. J. Alloys Compd. 2011; 509: 1866–69. [8] C.M. Gourlay, H.I. Laukli, A.K. Dahle. Defect band characteristics in Mg-Al and Al-Si high-pressure die castings. Metall. Mater. Trans. A. 2007; 38: 1833-44. [9] S. Otarawanna, C.M. Gourlay, H.I. Laukli, A.K. Dahle. Microstructure formation in AlSi4MgMn and AlMg5Si2Mn high-pressure die castings. Metall. Mater.
ACCEPTED MANUSCRIPT Trans. A. 2009; 40: 1645-59. [10] A.K. Dahle, D.H. StJohn. Rheological behaviour of the mushy zone and its effect on the formation of casting defects during solidification. Acta. Mater. 1998; 47: 31-41.
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[11] A.K. Dahle, L. Arnberg. Development of strength in solidifying aluminium
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alloys. Acta. Mater. 1997; 45: 547-59.
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[12] B. Meylan, S. Terzi, C.M. Gourlay, M. Suery, A.K. Dahle. Development of shear band during deformation of partially solid alloys. Scr. Mater. 2010; 63: 1185-8.
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[13] C.M. Gourlay, A.K. Dahle. Dilatant shear band in solidifying metals. Nature.
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2007; 445: 70-73.
[14] C.M. Gourlay, H.I. Laukli, A.K. Dahle. Segregation band formation in Al-Si die
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castings. Metall. Mater. Trans. A. 2009; 35: 2881-91.
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[15] S. Otarawanna, C.M. Gourlay, H.I. Laukli, A.K. Dahle. The thickness of defect band in high-pressure die castings. Mater. Charact. 2009; 60: 1432-41.
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[16] H.I. Laukli, C.M. Gourlay, A.K. Dahle, O. Lohne. Effects of Si content on defect
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band formation in hypoeutectic Al-Si die castings. Mater. Sci. Eng. A. 2005; 413: 92-97.
[17] H. Cao, M. Wessen. Characteristics of microstructure and banded defects in die cast AM50 magnesium components. Int. J. Cast Met. Res. 2005; 18: 377-84. [18] S. Otarawanna, C.M. Gourlay, H.I. Laukli, A.K. Dahle. Feeding mechanisms in high-pressure die castings. Metall. Mater. Trans. A. 2010; 41: 1836-46. [19] M. Bladh, M. Wessen, A.K. Dahle, Shear band formation in shaped rheocast
ACCEPTED MANUSCRIPT aluminium component at various plunger velocities. Trans. Nonferrous Met. Soc. China. 2010; 20: 1749-1755. [20] W. Zhang, S. Xiong, Study on a CAD/CAE system of die casting. J. Mater. Process. Technol. 1997; 63: 707-11.
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[21] L. Jia, S. Xiong, B. Liu, Study on numerical simulation of mold filling and heat
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transfer in die casting process. J. Mater. Sci. Technol. 2000; 16: 269-72.
ACCEPTED MANUSCRIPT Figure captions Figure 1 Configuration of (a) the specific casting including three tensile test bars (diameter at the center was 6.4mm) and one plate (thickness was 2.5mm), (b) ingate A1 and (c) ingate A2 in the tensile test bar, (d) ingate B1 and (e) ingate B2 in the plate
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sample.
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Figure 2 Characterization of the microstructure in the cross section perpendicular to
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the flow direction of the tensile test bar. (a), (c) and (e) were near ingate A1, while (b), (d) and (f) were near ingate A2. (a) and (b) were polished microstructure, while (c)
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and (d) were etched microstructure. (e) and (f) were zoom-in visualizations of (c) and
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(d), respectively.
Figure 3 Characterization of microstructure in the transverse section along the flow
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direction of the plate sample. (a), (c) and (e) were near ingate B1, while (b), (d) and (f)
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were near ingate B2. (a) and (b) were polished microstructure, while (c) and (d) were etched microstructure. (e) and (f) were zoom-in visualizations of (c) and (d),
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respectively.
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Figure 4 Morphology of porosity in defect band in the longitudinal section of the tensile test bar and transverse section of the plate sample, respectively. (a) and (b) were near ingate A1, while (c) and (d) near ingate B1. (b) and (d) were zoom-in visualizations of (c) and (d), respectively. Figure 5 Morphology of the shrinkage pores in the longitudinal section of the center tensile test bar and transverse section of the plate sample. (a) and (b) were near ingate A1, while (c) and (d) near ingate B1. (b) and (d) were zoom-in visualizations of (c) and
ACCEPTED MANUSCRIPT (d), respectively. Figure 6 The 3D distribution of the porosity in the tensile test bar. (a) and (c) showed the plate sample with ingate A1 and A2, respectively. The view direction of (a) and (b) was perpendicular, while (b) and (d) was parallel to the flow.
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Figure 7 The 3D distribution of the porosity in the plate sample with ingate B2. (a)
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and (b) were front and top view of the sample, respectively. (c) the average length in
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X, Y, Z axis and the sphericity of the porosity. (d) the number, volume and average diameter of the porosity as a function of the normalized thickness of the specimen.
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Figure 8 The EPMA map scanning results of the microstructure around the porosity.
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(a), (b) and (c) corresponded to the side of defect band near the skin layer, while (d), (e) and (f) corresponded to the side of defect band near center. (a) and (d) showed the
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position of the scanning areas. (b) and (e) corresponded to the element Mg, while (c)
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and (f) corresponded to the element Al. Figure 9 The EPMA map results of the microstructure surrounding a shrinkage pore
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of the plate sample with ingate B1. (a), (b) and (c) were close to the skin layer, while
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(d), (e) and (f) were close to the specimen center. (a) and (d) showed the scanning area, while (b) and (e), (c) and (f) corresponded to Mg, and Al elements respectively. Figure 10 EBSD analysis results of the grain orientation, showing (a) α-Mg grains in the skin layer, (b) complete ESCs in the center, (c) broken ESCs in center and (d) broken ESCs in the defect band. Figure 11 Schematic illustration of the formation mechanism of defect band, showing (a) the flow pattern in the specimen. (b)-(d) were zoom-in areas according to zones I,
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II and III, respectively.
ACCEPTED MANUSCRIPT Table 1 Processing parameters adopted during die casting Initial mold
Casting
Slow shot
Fast shot
temperature,
temperature,
pressure,
velocity,
velocity,
℃
℃
MPa
m/s
m/s
680
150
79
0.2
2.5
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Pouring
Table 2 Chemical compositions of the AZ91D magnesium alloy used in the study
Zn
Si
8.80
0.18
0.61
0.06
MA D PT E CE AC
Cu
Fe
Mg
0.007
0.003
Bal
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Mn
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Al
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(mass %).
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Figure 1 Configuration of (a) the specific casting including three tensile test bars (diameter at the center was 6.4mm) and one plate (thickness was 2.5mm), (b) ingate
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A1 and (c) ingate A2 in the tensile test bar, (d) ingate B1 and (e) ingate B2 in the plate
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sample.
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Figure 2 Characterization of the microstructure in the cross section perpendicular to the flow direction of the tensile test bar. (a), (c) and (e) were near ingate A1, while (b), (d) and (f) were near ingate A2. (a) and (b) were polished microstructure, while (c)
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and (d) were etched microstructure. (e) and (f) were zoom-in visualizations of (c) and (d), respectively.
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Figure 3 Characterization of microstructure in the transverse section along the flow direction of the plate sample. (a), (c) and (e) were near ingate B1, while (b), (d) and (f)
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were near ingate B2. (a) and (b) were polished microstructure, while (c) and (d) were etched microstructure. (e) and (f) were zoom-in visualizations of (c) and (d),
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CE
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respectively.
Figure 4 Morphology of porosity in defect band in the longitudinal section of the tensile test bar and transverse section of the plate sample, respectively. (a) and (b) were near ingate A1, while (c) and (d) near ingate B1. (b) and (d) were zoom-in visualizations of (c) and (d), respectively.
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Figure 5 Morphology of the shrinkage pores in the longitudinal section of the center
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tensile test bar and transverse section of the plate sample. (a) and (b) were near ingate A1, while (c) and (d) near ingate B1. (b) and (d) were zoom-in visualizations of (c) and
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D
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(d), respectively.
Figure 6 The 3D distribution of the porosity in the tensile test bar. (a) and (c) showed the plate sample with ingate A1 and A2, respectively. The view direction of (a) and (b) was perpendicular, while (b) and (d) was parallel to the flow.
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Figure 7 The 3D distribution of the porosity in the plate sample with ingate B2. (a) and (b) were front and top view of the sample, respectively. (c) the average length in
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X, Y, Z axis and the sphericity of the porosity. (d) the number, volume and average
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diameter of the porosity as a function of the normalized thickness of the specimen.
Figure 8 The EPMA map scanning results of the microstructure around the porosity. (a), (b) and (c) corresponded to the side of defect band near the skin layer, while (d), (e) and (f) corresponded to the side of defect band near center. (a) and (d) showed the position of the scanning areas. (b) and (e) corresponded to the element Mg, while (c) and (f) corresponded to the element Al.
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Figure 9 The EPMA map results of the microstructure surrounding a shrinkage pore
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of the plate sample with ingate B1. (a), (b) and (c) were close to the skin layer, while (d), (e) and (f) were close to the specimen center. (a) and (d) showed the scanning area,
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while (b) and (e), (c) and (f) corresponded to Mg, and Al elements respectively.
Figure 10 EBSD analysis results of the grain orientation, showing (a) α-Mg grains in the skin layer, (b) complete ESCs in the center, (c) broken ESCs in center and (d) broken ESCs in the defect band.
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Figure 11 Schematic illustration of the formation mechanism of defect band, showing
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(a) the flow pattern in the specimen. (b)-(d) were zoom-in areas according to zones I,
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II and III, respectively.
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Highlights 1. The melt flow would cause the rotation and fragment of crystals. 2. The rotation and fragment of crystals would lead to formation of defect band. 3. The distribution of defect band was strongly dependent on the local melt flow.