טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentLarianovsky Natalya
SubjectStructure and Properties of Al 7075 Prepared by
Rapid Solidification
DepartmentDepartment of Materials Science and Engineering
Supervisors Professor Emeritus Menachem Bamberger
Research Professor E Dan Shechtman


Abstract

Although Al 7075 is a most widely used high-strength aluminium alloy, substantial research activity is still being undertaken in an effort to improve its performance.

The purpose of this study was to improve the mechanical properties of Al 7075 by using Rapid Solidification Processing (RSP) followed by their comminution and extrusion.

Rapidly Solidified (RS) Al 7075 was produced by melt spinning technique in the form of long, thin high quality ribbons, 50-170 mm thick and 5-10 mm wide.

An electron microscopy and X-ray study of ribbons show that they have a crystalline structure with increased solubility of alloying elements in solid aluminium. Their microstructure consists of a fine matrix and submicron intermetallic precipitates.

Mechanical properties of the ribbons were estimated by microhardness measurements performed on the cross section specimens. The microhardness of the precipitation hardened ribbons increased by 8% compared with to the conventional alloy. The potential benefit of the RSP is thus evident.

Hot extrusion of comminuted ribbons can be used in order to keep the high strength Al 7075 alloy in bulk form for further practical applications. The RS extruded aluminium rods were subjected to heat treatments: annealing (O-stage) and precipitation hardening (T6 stage).

The extruded RS O-tempered Al 7075 alloy bar demonstrated an increase of 27 MPa (12%) and 34 MPa (10%) in yield strength and ultimate tensile strength, respectively. The improved mechanical properties were achieved by grain refinement and improved levels of chemical homogeneity, which in turn results from the reduced solidification time. In addition, the coherent precipitates increase alloy strength by distorting the crystal lattice and creating resistance to dislocation motion.

The strength of the T6-treated alloy was lower than that of the conventional alloy, but its tensile elongation was higher.

The tensile properties of the RS extruded material depend strongly on the consolidation process. Failure in the tensile test occurred because of voids and weld cracks between comminuted ribbons that formed during extrusion process. Therefore, in order to obtain the high strength bulk material from the RS ribbons an improved process is essential.