Aging of an aluminum alloy resulting from variations in the cooling rate
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Aging of an Aluminum Alloy Resulting from Variations in the Cooling Rate J.L. Cavazos and R. Colás (Submitted 30 September 1998; in revised form 25 May 1999) The effect that the rate of cooling after solubilization exerts on the aging behavior of an aluminum heat treatable alloy was studied. Bars of the alloy were heated in a box furnace for solubilization, and after this was achieved they were cooled to room temperature by placing one end in a shallow tank of water. Thermal evolution along the bar was registered with the aid of thermocouples connected to a PC-based data logging system. Small samples were cut from the bars and aged for different times and temperatures. Results from microhardness tests indicate that peak hardness, at a given aging temperature, augments with the increase of the cooling rate until a certain value is achieved, above which the hardness remains constant. This feature was found to be due to precipitation taking place at the lower cooling rates. Keywords
aging, aluminum alloys, cooling rate, hardness, heat treating
1. Introduction Heat treating is based on the possibility of controlling solidstate transformation reactions taking place during heating or cooling commercial alloys. Most of the reactions depend on the temperature/time path, and, due to their kinetic nature, it is possible to suppress or accelerate a given reaction by changing either the heating or cooling rate of the material (Ref 1). Heat treating of aluminum alloys rely on the possibility of achieving a supersaturated solid solution, which will improve the mechanical properties of the alloy as it ages (Ref 2-4). The steps involved are solubilization, quenching, and aging. Solubilization is conducted at a temperature high enough to put in solution the components of the alloy which, in the case of silicon and magnesium, is normally done at temperatures above 500 °C. Quenching is carried out either in cold water or, when distortion control is critical, in air or hot water (Ref 5). The aging behavior depends on both the time and the temperature at which the treatment is made (Ref 2-4), since the mechanical properties of the material depend on the size and distribution of the precipitates formed. Another factor that affects this response is the rate at which the piece or part cools from the solubilization temperature, a feature associated with the amount of supersaturation attained in the alloy. The aim of this article is to present the results obtained during the study of the aging behavior of an aluminum type 6063 alloy solubilized and then cooled at different rates by unidirectional cooling.
mm in diameter and 320 mm long. Small, 1.6 mm diam holes were drilled at different distances (measured from one of their ends) up to their center to introduce eight type K (chromelalumel) thermocouples. The thermocouples were connected to a data logging system based on a PC computer (Ref 6), to record the thermal evolution during heating and cooling. The bars were soaked for 4 h at 520 °C in a box furnace; then, o
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