Effect of solidified structure on hot tear in Al-Cu alloy

Transcription

1 IOP Conference Series: Materials Science and Engineering PAPER OPEN ACCESS Effect of solidified structure on hot tear in Al-Cu alloy To cite this article: Y Yoshida et al 2015 IOP Conf. Ser.: Mater. Sci. Eng View the article online for updates and enhancements. Related content - Effect of liquid volume on the evolution of solidified structure in horizontal centrifugal casting H Esaka, Y Kataoka and K Shinozuka - Effect of the dendritic morphology on hot tearing of carbon steels M R Ridolfi - Study of hot tearing and macrosegregation through ingot bending test and its numerical simulation T Koshikawa, M Bellet, C A Gandin et al. This content was downloaded from IP address on 05/04/2019 at 19:31

2 Effect of solidified structure on hot tear in Al-Cu alloy Y Yoshida 1, H Esaka 2 and K Shinozuka 2 1 Graduate student, National Defense Academy, Hashirimizu, Yokosuka, , Japan 2 Department of Materials Science and Engineering, National Defense Academy, Hashirimizu, Yokosuka, , Japan boudai.kakashi@gmail.com Abstract. Hot tear is the one of the biggest problems of cast products of aluminum alloy. The effect of solidified structure on the hot tear has not been clear. Therefore, this study has been carried out to correlate solidified structure and hot tear. Al-2.0 wt% Cu alloy was cast at 750 ºC in a mold cavity, which could intentionally form hot tear. To change solidified structure, some amount of refiner was added to the molten alloy. Length of hot tear decreased with increasing the amount of refiner. Further, the area of fine eutectic Al 2 Cu increased as amount of refiner increased. These may indicate that probability of healing increased in case of equiaxed structure. Therefore, the length of hot tear decreased with increasing the amount of refiner. great care should be taken in constructing both. 1. Introduction Hot tear is one of the biggest problems of cast products of steel, copper and aluminum alloys. Continuous casting of steel is widely popular process in industries, though, there are some problems, such as surface and internal cracks. Once these cracks form, they propagate and usually remain in steel products[1]. Therefore, cracks decrease productivity. It is regarded that some surface cracks and most of internal cracks initiate during solidification. For example, longitudinal surface crack is believed to initiate in the mold and increase its size in secondary cooling zone[2]. Hence, mild and uniform cooling in the mold may be a countermeasure to reduce the longitudinal surface cracks. In order to reduce crack or hot tear, it is important to know where and when it initiates and how it propagates. There have been many researches in order to understand the formation mechanism of hot tear[3-5]. However, it is still uncertain when a hot tear initiates and how it propagates. Furthermore, the effect of solidified structure on the hot tear has not been clear. Therefore, this study has been carried out to correlate solidified structure and hot tear. 2. Experimental procedure Figure 1 shows a schematic view of a mold. One of four side plates was a steel plate. The steel plate had two grooves in order to intentionally make hot tear. In addition to this, a ceramic insulator was set in the middle of two grooves to slowly solidify in local. The other three side plates as well as bottom plate were alumina brick. The size of ingot was 30 mm x 100 mm x 40 mm. Al-2.0 wt% Cu alloy was cast at 750 ºC in a mold cavity. The liquidus and eutectic temperatures of this alloy are 650 ºC and 550 ºC, respectively. To change solidified structure, some amount of refiner was added to the molten alloy. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by Ltd 1

3 The refiner includes TiB2 particles and promotes heterogeneous nucleation. The amount of refiner were 0, 0.03, 0.1 and 0.3 wt%. The plane for characterization was 10 mm from the ingot bottom. The solidified structure in the middle of two grooves were observed using a FE-SEM and an optical microscope(om). In order to characterize the hot tear, the observation with low magnification was used. A length of hot tear in the cross section was measured vertically to a plane set a ceramic insulator. To characterize the solidified structure near the hot tear, backscattered electron(bse) image was used with high magnification. The grain size was measured using the BSE image near a hot tear. Here, the lineal analysis was adopted. Figure 1. Schematic view of steel mold. 3. Results and Discussions 3.1. Relationship between grain size and length of hot tear Figure 2 shows OM images of near the hot tear. Top of these photographs is on the steel plate surface and the direction of solidification is from the top to the bottom. Black and wavy lines are hot tears. It became thinner as the distance from the steel plate increased. In case of w = 0 wt% or 0.03 wt%, two major hot tears were recognized on the observation plane. The length and width of hot tears are long and large. On the other hand, in case of w = 0.1 wt% or w=0.3 wt%, there was one small hot tear on the observation plane. The length and width of it is short and small. Especially when the addition of refiner(w) was 0.3 wt%, the hot tear is discontinuous. Here, L is defined as the length between the surface and the hot tear tip. Figure 2. OM images of hot tears and the definition of hot tear length (L). 2

4 Figure 3. BSE images of enlarged views of hot tear in case of w = 0.03 wt% and 0.3 wt%. An enlarged views of hot tear are shown in figure 3 in case of w = 0.03 wt% and 0.3 wt%. In case of w = 0.03 wt%(figure 3(a)), the width of hot tear is wide. Since white or bright region is Cu concentrated or eutectic region and they should be interdendritic region, the equiaxed grains can be judged. As w increases, equiaxed grain size decreases, and the hot tear was discontinuous and turned much. Figure 4 shows the relationship between w and the length of hot tear(l). The hot tear was severe and L is long when no refiner was added to the molten aluminum alloy. L decreased much when the refiner was added and L decreased as w increased. However the slope with respect to w decreased with increasing w. The reasons of these changes may be due to macroscopic solidified structure and grain size. Figure 5 shows the relationship between w and the grain size. In case of w = 0 wt%, since the solidified structure was columnar dendrite, the experimental data was omitted. The grain size gradually decreased as w increased. Figure 4. Relation between amount of refiner and length of hot tear. Figure 5. Relation between amount of refiner and size of equiaxed grain. Figure 6 shows the image of propagation of hot tear along columnar dendrites or equiaxed grains. The solidified structure near a hot tear was columnar dendrites in case of without refiner. Therefore hot tear propagates along interdendritic region easily and rather long and relatively straight hot tear results(figure 6(a)). Thus, the sensitivity to hot tear is high when the solidified structure is columnar dendrite. On the other hand, in case of with refiner, the hot tear propagates along the inter-grain region. Since the packing of equiaxed grains are usually complex, the path of hot tear is wavier than that in case of columnar dendrite. Therefore, the hot tear tends not to initiate nor propagate when the solidified structure is equiaxed(figure 6(b)). 3

5 Figure 6. Image of propagation of hot tear along columnar dendrites(a) or equiaxed grains(b) Observation of Eutectic structure Figure 7 shows the BSE image of the solidified structure in case of w = 0.3 wt% near the hot tear. As described before, white regions refer to area where copper concentration was high and it is found that some hot tears are mended. Enlarged views of figure 7 are shown in figure 8. Here, lamellar eutectic structure is observed[6]. The morphologies of lamellar eutectic structure are rod like(i) as well as spherical(ii). The "rod like" lamellar eutectic structure is often observed in the hot tear. This may mend or heal the hot tear. On the contrary, a white and round particle(iii) is recognized. This is a divorced eutectic Al2Cu[7]. As far as the observation concerned, the morphologies of divorced Al2Cu were spherical and there is no divorced eutectic Al2Cu in the hot tear. Comparing the size of eutectic Al2Cu, it in divorced eutectic is larger than that in lamellar eutectic. Both lamellar and divorced eutectic are sometimes observed in the same area and this indicates that they may have the same cooling rate. However, the structures are different. This indicates that lamellar eutectic structure solidified rapidly and this suggests that the remaining liquid in the interdendritic region may move to the hot tear due to negative pressure. Figure 7. BSE image of the solidified structure in case of w = 0.3 wt% near a hot tear. Figure 8. BSE image of lamellar eutectic structure and divorced eutectic Al2Cu. (i) Rod like lamellar eutectic structure. (ii) Spherical lamellar eutectic structure. (iii) Divorced eutectic structure. Here, the areal ratio of lamellar eutectic structure(σ) is defined as a following equation: (1) 4

6 σ was measured using an image analysis software. At around middle of hot tear, σ was characterized and indicated in figure 9 as a function of w. As shown, all data are over 70 %. σ increased with increasing w. In other words, σ gradually increased as the equiaxed grain size decreased. A grain boundary may be a route for remaining liquid due to suction. Since number of grain boundaries increases with decreasing grain size, it is reasonable that σ increased as the amount of refiner increased. Figure 9. Relation between amount of refiner and areal ratio of lamellar eutectic structure near the middle point of hot tear. Then, σ was measured along a line indicated in figure 10, where 1 mm apart from the steel plate surface. Figure 11 shows the relation between the distance from hot tear and σ. In case of x 0 mm, the values of σ are over 70 %. σ rapidly decreased with increasing x. But, in case of x > 6 mm, the values of σ are 0 %. These results indicate that the area where the lamellar eutectic structures are found is limited near the hot tear. In normal solidification condition, a divorced eutectic structure is found in Al-2.0 wt%cu alloy. Therefore, lamellar eutectic structures are attributed to the suction due to negative pressure in hot tear formation. As shown in figure 11, the range of suction may spread approximately 3 mm from the hot tear. Figure 10. A schematic view of horizontal cross section and the line for measuring σ. Figure 11. Relation between distance from hot tear and areal ratio of lamellar eutectic structure. 5

7 4. Conclusions In order to characterize the hot tear, casting test using Al-2.0 wt%cu alloy with and without refiner has been performed. Solidified structure has been metallographically observed and following conclusions have been derived. (1) It was found that the length of hot tear decreased with decreasing the equiaxed grain size. (2) Hot tear was easily healed as size of equiaxed grain decreased. (3) The range of suction may spread approximately 3 mm from the hot tear. References [1] Morishita M, Abe M, Tokuda K and Yoshida M 2008 J. Japan Inst Metals 59 pp [2] Brimacombe J K and Sorimachi K 1977 Metall.Trans.B 8 pp [3] Watanabe T, Kimura R, Nakazawa T, Chiba H, Tanaka S, Ueki T, Toriyama T and Yoshida M 2008 J. Japan Inst Metals 58 pp [4] Li S, Sadayappan K, and Apelian D 2011 Int. J Cast Metal Research 24 pp [5] Monroe C and Beckeramann C 2014 JOM 66 pp [6] Kurz W and Fisher J D 1992 Funfamentals of Solidification (Aedermannsdorf : Trans Tech Publication) chapter 5 [7] Bramfitt B L and Benscoter A O 2002 Metallographer's Guide (Ohio : ASM International) chapter 9 6