Ph.D Thesis

Ph.D StudentAbd El Majid Suzan
SubjectMicrostructures,Mechanical Properties and Castability of
Aluminum Alloys(A201)with Additions of Si,Ti, and
DepartmentDepartment of Materials Science and Engineering
Supervisor PROFESSOR EMERITUS Menachem Bamberger
Full Thesis textFull thesis text - English Version


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