Effects of Cu and Si Contents on the Fluidity, Hot Tearing, and Mechanical Properties of Al-Cu-Si Alloys
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ALUMINUM alloys have been widely used in the automotive industry due to their low density and good fluidity.[1–3] An increasing number of automotive parts are being replaced with aluminum alloys due to their lower weight, but crucial parts requiring high strength and ductility are still made of steel alloys. To improve the mechanical properties of aluminum alloys so that they can compete with those of high-strength steel alloys, many methods, including alloy design, addition of nanoparticles, heat treatment, and semi-solid processing, have been studied.[4–7] The addition of Cu to aluminum alloys is one way to achieve high strength, but the 2xxx series of Al-Cu alloys are generally used in forging and not for casting. Since the cast products are less expensive and time-consuming than forged products, developing a casting process and product for these Al-Cu-based alloys is advantageous for cost-effective production of complex products. However, producing these alloys by casting require improved fluidity, lower hot tearing susceptibility and mechanical behavior comparable to the forged products.
BYUNG KEUN KANG and IL SOHN are with the Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea. Contact e-mail: [email protected] Manuscript submitted January 15, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Fluidity is also an important factor for ensuring the quality of the cast product. During casting, the fluidity is defined by the ability of a molten metal to flow before termination by solidification.[8] The fluidity is affected by various factors, such as the alloy composition, solidification mode, superheat of the melt, viscosity, pouring temperature, and cooling rate.[9–13] The fluidity decreases with the addition of alloying elements such as Cu and Si to pure Al due to the extension of the solidification range by the alloying elements. Flemings[8] indicated that melts with a large solidification range typically result in lower fluidity. The minimum fluidity is reached near the composition resulting in the maximum solidification range, and as the solidification range decreases, the fluidity increases up to the eutectic composition of the alloy. In Al alloys, the fluidity decreases with increasing Si content up to 5 to 6 pct, which corresponds to the region of the maximum solidification range in which the solid and liquid coexist. The fluidity then increases with increasing Si content, reaching a maximum fluidity at 20 wt pct Si.[8] The high latent heat of primary Si extends the solidification time of the liquid state during solidification, which increases the fluidity.[10] In addition, a higher superheat can enhance the fluidity,[11] and the addition of grain refiners has also been shown to increase the fluidity.[14] Hot tearing is a well-known fracture phenomenon that occurs during the solidification process, which must be inhibited during casting. As solidification progresses, the solid phase nucleates, grows, and forms network structures resulting in a characteristic
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