• PDF / 1,961,633 Bytes
• 9 Pages / 593.972 x 792 pts Page_size
• 86 Downloads / 199 Views

DOWNLOAD

REPORT

---

020-06052-0  The Minerals, Metals & Materials Society and ASM International 2020

ABDUL WAHID SHAH, and SHAE K. KIM are with the University of Science and Technology, Daejeon, 34113, South Korea and with the Advanced Process and Materials R&D Group, Korea Institute of Industrial Technology, Incheon, 21999, Korea. SEONG-HO HA, BONG-HWAN KIM, YOUNG-OK YOON, and HYUN-KYU LIM are with the Advanced Process and Materials R&D Group, Korea Institute of Industrial Technology. Contact e-mail: [email protected] Manuscript submitted May 20, 2020; accepted September 28, 2020.

METALLURGICAL AND MATERIALS TRANSACTIONS A

I.

INTRODUCTION

AL-MG-BASED wrought alloys have been extensively used in the aerospace and the automobile industries owing to their attractive corrosion resistance, sufficient tensile properties, good machinability, attractive appearance when anodized, and ample weldability, all accompanied by low density. However, Al-Mg-based

cast alloys have very limited applications, primarily due to their poor fluidity compared to Al-Si-based alloys. Therefore, during casting, Al-Mg-based alloys need more care in the gating system and the placement of risers in the mold. In addition, the higher oxidation tendency of Al-Mg melt demands more closely controlled melting and pouring practices.[1–5] Therefore, for the wider use of Al-Mg-based cast alloys in the automobile and aerospace industries, further work is needed to improve their oxidation resistance and flow length. Fluidity is defined as the distance traveled by the molten metal when it is forced to flow through a channel of small cross section. This covered distance is called fluidity length (Lf). Fleming’s mathematical model for fluidity Lf of an alloy in a cylindrical channel in a mold is described as follows[2]:

Lf ¼ qav H þ c T  Tliq =2hðT  T0 Þ ½1 In this equation, liquid density (q), heat of fusion (H), and specific heat of the liquid metal (c) are metal-related variables. These factors depend only on the alloy composition and, therefore, can be altered by altering the composition. For example, in aluminum alloys, the fluidity of Al-Si binary alloys is much higher than for Al-Cu and Al-Mg binary alloys. The channel radius a, flow velocity v, and melt superheat (T  Tliq) are test variables. For a particular casting part, mold cross-sectional area and mold channel circumference are constant. Increase in flow velocity leads to turbulent flow and causes an increase in the amount of oxide inclusion in casting part. Conversely, melt superheat can easily be increased by increasing the melt temperature, provided that the melt surface is protected from severe oxidation. The heat transfer coefficient h between mold and metal is a mold-related factor, as is the mold temperature (To). The fluidity of an alloy is always higher in a sand mold than in a permanent mold, due to metal’s higher thermal conductivity. In the case of a permanent mold, the higher the mold temperature, the higher the fluidity. However, higher temperature causes a deterioration in the mechanical properties of fina