Optimization of Spray Quenching for Aluminum Extrusion, Forging, or Continuous Casting

I N T R O D U C T I O N Mechanical and metallurgical requirements impose limits on the rates at which aluminum alloys can be quenched by water sprays. The upper limit is set by the occurrence of plastic deformation which causes warping of the product and the lower limit is set by an inability to develop the required metallurgical properties in subsequent heat treatment operations. In addition, if a product contains sections of differing thicknesses, it is unlikely that the optimum properties can be obtained throughout the entire cross section using a single spray density. At present a method for determining the nozzle configuration and spray density required for the spray cooling of a given alloy of given geometry is not available and this causes much guesswork and trial and error in establishing an acceptable configuration for a production run. The transformation of bulk liquid into sprays and other physical dispersions of small particles in a gaseous atmosphere has importance in several industrial processes such as the application of chemicals to agricultural crops, paint spraying, spray drying of wet solids, food processing, cooling of nuclear cores, and dispersions of liquid fuels for combustion [1]. Of specific interest in the present work is the application of water sprays to the quenching of aluminum alloys. T.A. Deiters is Graduate Research Assistant and I. Mudawar is Assistant Professor and Director of the Boiling and Two-Phase Flow Laboratory, School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907, USA. Accu ra t e con t ro l o f the coo l i ng rates dur ing quenching requires a careful characterization of the spray. One means of characterizing the effectiveness of sprays is the boiling curve shown in Figure 1 as a loglog plot of the variation of the heat flux from a hot surface, q, with the difference between the temperature of the hot surface and the saturation temperature of the cooling fluid, ATs. If, in a spray quenching process, the initial value of ATs is high enough, the liquid droplets in the spray do not make contact with the surface. The phenomenon occurs in the film boiling regime of the boiling curve, corresponding to the location of point 1 in Figure 1. In this regime heat transfer to the spray is relatively poor because of the insulating properties of the thin blanket of vapor which is produced on the hot surface. With continued cooling both the heat flux and the surface temperature decrease until a minimum in the former is reached at the Leidenfrost point, located at position 2 in Figure 1. At this point liquid droplets in the spray begin to penetrate to the metal surface and a transition to nucleate boiling within the drops commences. With further decrease in ATs the heat flux increases as the regime of nucleate boiling is entered, and at the onset of full nucleate boiling the heat flux from the surface reaches a maximum value, called the critical heat flux, at point 3 in Figure 1. With further cooling of the surface, nucleate boiling ceases and heat transfer occurs by single-phase convection to the cooling water flowing over the surface. Consequently, in this regime of forced convection, a decrease in ATs causes a decrease in the heat flux, as shown at location 4 in Figure 1. J. Heat Treating, Voi. 7, No. 1, 1989 9 9 T.A. Deiters & I. M u d a w a r 9 O p t i m i z i n g Spray Quench ing Process