Papers by Keyword: Surface Crack

Abstract: Al-Si-Mg alloy strips with a Si content ranging from 0.5% to 12% were cast using a vertical type high-speed twin-roll caster at a roll speed of 60 m/min. The effect of Si content on ripple marks and cracks on the cast surface, as well as center cracks (crocodile cracks), was investigated. The results show a relationship between the Si content and these defects. When the Si content was higher than 4%, surface cracks and center cracks did not occur. However, ripple marks worsened when the Si content ranged from 3% to 5%.

Abstract: Cracks occur on the surface of Al-Mg strips cast using a high-speed twin-roll caster. Surface cracking of the as-cast strip decreases with decreasing roll load. However, the difficulty of roll casting increases as the roll load is decreased because the strip becomes brittle and is easily broken because the strip is not completely solidified. In the present study, an unequal-diameter twin-roll caster, which is capable of high-speed casting, was used with a lip attached to a back-dam plate in an attempt to decrease surface cracking without deceasing the roll load. The thickness of the solidified layer was found to decrease as the length of the solidified region decreased. The thickness of the solidified layer at the center of the width direction of a strip on the lower roll decreased by shortening the solidification length by a lip attached to a back dam-plate. The solidified layer at the edges was thicker than that inside the cast strip. The thickness of semisolid metal inside the cast strip increased more than that at the edges of the roll bite. The roll load at the width of the lip decreased. The effect of the lip length on surface cracks was investigated.

Abstract: A roll with a groove was used to decrease surface cracking on strips cast using a vertical-type high-speed twin roll caster. The roll speed was 30 m/min. Al-4.6%Mg alloy was used because surface cracking easily occurs during roll casting of this alloy. The effect of roll load on the occurrence of surface cracks in Al-4.6%Mg was investigated before the grooved roll was evaluated. Importantly, surface cracking decreased as the roll load decreased. The roll load between the edges, that is, at the groove, became smaller than that at the edges corresponding to the flanges that formed the groove. The relationship between the depth of the groove and the occurrence of cracking was investigated. The groove on the roll was useful for reducing the number of cracks. However, cracking occurred at the center of the width direction. The cause of this cracking is discussed.

Abstract: Semisolid Al–Mg alloy strips were cast using high-speed twin-roll casting under very low roll loads to investigate the effect of low roll loads on surface cracking and center segregation of Mg. In the conventional twin-roll caster for aluminum alloys, the roll speed is usually less than 2 m/min, and the roll load is typically greater than 1 kN/mm to solidify the aluminum alloy and reduce casting defects. In the vertical type high-speed twin-roll caster, the roll speed can range from 10 to 90 m/min, and strips can be cast at roll loads below 500 N/mm, down to loads as low as 2 N/mm. Strips cast at 2 N/mm in this study did not completely solidify when released from the rolls; this means that the strips were semisolid. Al–Mg strips can be continuously cast without breaking when they are semisolid. The surface cracking and center segregation of these strips were compared with those of strips cast at a higher load of 88 N/mm. The effect of the small load on the presence of Mg at cross sections of the strip was investigated using etching with Weck’s reagent.

Abstract: Effect of casting conditions on ripple mark and surface crack of as-cast Al-4.7%Mg strip was investigated. A vertical type high speed twin roll caster was used. A casting speed was 30 m/min. Casting conditions were tip-shape of a back-dam plate, gap between a roll and the back dam plate and texture on the roll-surface. The texture on the roll-surface was useful to reduce the ripple mark. In bending test conducted to investigate surface crack of as-cast strip, the texture on the roll surface was effective to the reduce the crack. The position of the crack was not coin side with the position of the ripple mark. This result shows that there is no correlation between the ripple mark and the ductility or the strength of as-cast strip. Small roll load was effective to reduce the crack and degree of the ripple mark. Effect of the roll load on the ripple mark was discussed including solidification delay and seepage of Magnesium.

Abstract: When a strip of Al-Si alloy with an Si content of 1% was cast using a vertical-type high-speed twin-roll caster, cracks form in its surface. The effects of the pouring method, the shape and position of the nozzle, and the roll surface texture on surface crack formation were evaluated with a roll caster. The rolls were made of a copper alloy, and the roll speed was 30 m/min. The as-cast strips were bent to investigate the degree of crack formation, and the outer surface of the strips was observed without magnification and with a stereomicroscope to determine the influence of the pouring method, the shape and position of the nozzle, and the roll surface. A roll machined to form V-shaped grooves 0.4 mm deep on the surface of the strips was most useful for reducing surface cracking. Changing the shape of the nozzle tip was second-most effective. There was a clear correlation between the roll surface condition and surface cracking in the Al-Si strip.

Abstract: Al-Mg alloy strips were cast by an unequal-diameter twin-roll caster. It was found that cracks formed on the surface at grain boundaries. The grains near the surface were small in size, which likely contributed to crack formation. The use of a molten metal pouring method to increase the grain size near the surface is proposed to reduce cracks. In the previous method, molten metal is poured into a pool, which is on the lower roll surrounded by side-dam plates, a back-dam plate, and the upper roll. In this study, molten metal was directly poured onto the roll surface at a shallow angle using a launder. When the angle was smaller than 20°, cracks did not form. With the proposed method, the heat transfers between the molten metal and the roll surface decreased, as determined from the grain size and strip thickness. The cracks on the strip surface were color-checked and visually inspected.

Abstract: In order to investigate the effect of pre-crack lengths on strength of silicon nitride balls under cyclic pressure loads, growth behavior of 600~700μm pre-cracks were compared to those of 200μm~300μm and 400~500μm pre-cracks. Furthermore, the change in initial threshold limit of the maximum stress intensity factor was discussed. It was found that the increasing ratio of stress intensity factor during N=0 and N=1000 distinguished the failure and non-failure, and pre-crack length had strong effect on the threshold limits of the increasing ratio.

Abstract: In order to investigate the effect of pre-crack lengths on silicon nitride balls under cyclic pressure loads, the pre-crack lengths ranging from 400μm to 500μm were observed. Their growth behavior was compared to that of 200μm to 300μm pre-cracks. Furthermore, the initial threshold limits of their maximum stress intensity factors were measured.

Abstract: Silicon chip has been widely used in solar cell recently. The thinning of silicon chip, easily inducing surface defects, becomes necessary to produce solar cells more efficiently. The surface defects resulting in stress concentration on the silicon chip surface would be the source of chip failure. In this study, the finite element analysis was used to investigate the stress distribution near the surface crack of a solar cell on which the nanostructures were introduced to alleviate the induced stress. For the solar cell model, positive silver and negative aluminum electrodes were added on the top and bottom sides of silicon chip. The solar cell under four-point bending was simulated in analysis with and without nanostructures. The results show that the stresses reduce more than 50 % for the solar cell model with nanostructures. When the crack depth is deeper enough, the stress at crack tip is higher than that at junction near the electrode and the crack leads to the failure of solar cell. The effect of different section length of nanostructures on the stress distribution caused by the surface crack was also discussed.