The effect of extrusion conditions on yield strength of 6060 Aluminium Alloy
The objective of this investigation was to quantify whether T5 temper is equal to T6 temper in yield strength of 6060 extrusion alloys. Extrusion conditions achieving uniform yield strength along the whole length of extrudate were examined. The optimal ageing conditions relating to thicknesses of 1.1 mm and 1.5 mm were studied. Firstly peak age hardening conditions were determined for thicknesses of 1.1 mm and 1.5 mm of both T5 temper and T6 temper at mid-length of extrudate. For T6 temper all samples were solution treated at 540c for one hour in the furnace, followed by water quenching. Then all the specimens for T6 temper and T5 temper were artificially aged at 185c or 205c in the oven for varying times: 0.5, 1, 3, 5, 7, 9, 11, 13 and 15 hours, and then tensile tested to compare 0.2% proof strength values. Yield strength of extrudate along the whole length was investigated for thickness 1.1 mm. Samples were treated at 205c for two and four hours, and at 185c for six and twelve hours, for both T5 and T6 temper. The average cooling rate of thickness 1.1 mm extrudate was estimated. Chemical composition analysis using ICP-AES was performed. This was to determine Mg concentration distribution in pre-heated billet before extrusion, and in butt and extrudate after extrusion. Results have shown that a cooling rate of greater than 5c / s in 6060 alloys is sufficient to prevent Mg2Si forming during press quenching. Optimal yield strength for thickness 1.1 mm and 1.5 mm can be achieved above 180 MPa by press quenching, when homogenizing conditions were held at 575c for two hours. Ageing at 205c reached peak yield strength faster than ageing at 185c for both T5 and T6 temper. When subjected to T5 temper at 205c held for two hours, thickness 1.5 mm reached above 180 MPa but thickness 1.1 mm did not. It required ageing at 205c for three hours, or at 185c for five hours. Constant ram speed of 5 mm/s, when pre-heat temperature is 380 - 390c, gives a nearly constant exit temperature of 530 - 545c.