Abstract: The quench sensitivity of 6351 alloy was determined by the time−temperature-transformation (TTT) curves and time−temperature-property (TTP) curves by an interrupted quench technique with measurement of as-aged hardness and as-quenched electro-conductivity. The microstructure transformation during isothermal treatment was studied by the transmission electron microscopy (TEM) and Avrami equation. The results showed that the electro-conductivity of the 6351 alloy increased and the hardness decreased with prolonging the holding time at a certain isothermal temperature. The TEM observation indicated that the supersaturated solid solution decomposed and needles β″ precipitated at the initial stage of isothermal holding. With the prolongation of holding time at the nose temperature, rod β′ and plate β phases formed. The isothermal transformation rate at 360 °C was the fastest, and became slow at 280 °C and reached the slowest at 440 °C. The nose temperatures of the TTT and TTP curves were about 360 °C and the high quench sensitive temperature range was 230−430 °C. The quench factor analysis indicated that the cooling rate should be more than 15 °C/s in the quench sensitive areas in order to get optimal mechanical properties.
1 Introduction
6xxx Al−Mg−Si alloys are widely used as medium-strength heat-treatable alloys for structural applications due to their excellent formability,weldability and good corrosion resistance. In Al−Mg−Si alloys, Mg and Si are added either in a balanced amount to form binary Al−Mg2Si alloys or with excess amount of Si to form the Al−Mg2Si quasi-binary composition, to enhance the kinetics of the precipitation process without changing the nature of precipitates [1−3]. Mn addition is generally used to decrease the grain size and restrain recrystallization [4]. In order to achieve opt-imal mechanical properties, three steps including solution,quenching and aging are generally used in heat-treatable aluminum alloys [5,6]. For alloys with quench sensitivity,quenching is a key step because the mechanical properties strongly depend on the cooling rate during quenching. Inadequate cooling rate often leads to the drop in strength and hardness after aging. This is attributed primarily to the loss of solute by heterogeneous nucleation and growth of quench precipitates which do not provide strengthening during subsequent aging [7−9]. However, extremely rapid cooling rate will increase the tendency for thin productto distort and thick pieces to develop high levels of residual stress and warping [9]. Consequently, an appropriate quenching rate is necessary in order to minimize the residual stress while maximizing the mechanical properties of the alloy.
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