Ph.D Thesis
Ph.D Student Abd El Majid Suzan Microstructures,Mechanical Properties and Castability of Aluminum Alloys(A201)with Additions of Si,Ti, and B Department of Materials Science and Engineering Professor Emeritus Menachem Bamberger

Abstract

The alloys investigated in this study are the aluminum alloys A201: Al-4.97 wt. % Cu - 0.56 wt. % Ag based alloys. These alloys are commercially important because of their high mechanical properties, excellent machinability and good formability, shown to be suitable for aerospace and automobile parts. These alloys have a particularly high response to age hardening (especially T6 heat-treated conditions (and as a result offer good mechanical properties.

The additions of silicon, titanium and boron to the A201 alloy (Al - 4.97wt. % Cu - 0.56 wt. % Ag) were used to modify the microstructure and to improve the castability and mechanical properties of the alloy. The four alloys       A201, A201-1 wt. % Si, A201-1.33 wt. %B- 3.17 wt. % Ti and A201-1 wt. % Si -1.33 wt. %B- 3.17 wt. % Ti alloys were investigated in the as-cast, solution treated at 550˚C for ~20 hours and aged at 170˚C up to 32 days conditions. The effects of precipitation on the alloys properties were investigated by a combination of HRSEM?, TEM and microhardness test.   The as-cast alloys contained α-Al as a matrix and eutectic structure (a-Al/θ-Al2Cu) along grain boundaries.
Addition of 1 wt. % of Si improved the microhardness; additions of 2, 4 and 6 wt. %Si improved the fluidity, prevented hot tears, but generated a lot of pores along GBs, then decreased the mechanical properties of the alloys. Additions of Ti and B caused the grain refinement, prevented hot tears, but caused a large amount of Al3Ti phase which makes the melt more viscose, and less fluid and castable. After solution treatment the eutectic structure at the GBs dissolved and an intermetallic phase α-Al15(Fe,Mn)3Si2 with faceted {112} planes, appeared    .

The precipitation sequence during aging was the following: supersaturated solid solution (SSSS) à GP zones à θ'' Ω à θ' Ω à θ. At the maximum hardness, there are two kinds of semi-coherent precipitates: θ' (Al2Cu-tetragonal) and Ω (Al2Cu-orthorhombic).

The Si addition induced a large amount of GP zones at the initial stage of aging and, by these means, increased the microhardness. The additions of Ti and B (without Si) caused a large amount of semi-coherent Ω precipitates in the expense of θ'' and θ' phases. Additions of 1 wt. % Si or Ti and B (without Si) improved the corrosion resistance of A201 alloys after ST and aging by adding a passivation plateau.

The A201 wt. % Si showed the optimal combination of mechanical properties and corrosion resistance