February 2023 Volume 5

EQUIPMENT & TECHNOLOGY

Undergoing of Different Cooling Systems for Aluminum Alloys By Francisco J. Flores Gatica

Recently, aluminum alloys have become an important part in the supply chain of lightweight materials, especially in the automotive industry because of several factors that include decreased body weight, fabrication of internal parts, or increased power efficiency. Regardless of the situation, one important matter must be kept in mind: the material’s mechanical performance needs to qualify automotive standards. One way to keep these standards is by increasing the material’s properties by heat treatment applications. Wrought aluminum alloys can be heat-treated, which is accomplished in a three-stage cycle: solution, cooling, and aging (Figure 1).

have enough time to precipitate during cooling, which will affect the properties required for the work that will be submitted for the piece, in other words, the solid solution will not remain at room temperature, hence the controlled precipitation will not be effective as we would like. On the other hand, it will be possible for alloys of complex geometry to exhibit distortion or, in some cases, fracture when a high-cooling rate is applied, so a true understanding of heat flux is needed to predict and understand the physical andmechanical behavior of workpieces at different cooling rates. Air quenching is used to limit the residual stresses as well as the brittleness that occurs during the quenching process. To improve fatigue life, some residual stress can play a significant role, and this can be achieved through the comparatively slower cooling rate of air quenching. Cooling rates are ruled by the Heat Transfer coefficient ( h ), which is a function of the heat flux and the temperature gradient. Since the h is a critical factor in achieving desired material properties while air quenching, it is necessary to understand some of its influences. Some of these influences include size and material that is to be quenched, air velocity, air temperature, the type of gas being used for quenching, and the orientation of the material in the stream of air. However, using air quenching is not very useful because of the lower h gather (100–200 w/m2K), which results in very slow cooling rates that allow enough time for precipitation to occur during the cooling process, restraining the aging step. To achieve better cooling rates and higher h , Nutec Bickley, developed an early stage design of a cooling system that gathers the benefits of an air quenching while minimizing distortion and residual stress but also introduces water in the form of mist to increase the cooling rates by means of heat flux extraction through the tiny water droplets. The present case of study seeks to evaluate the results carried out to evaluate the h that arises when a wrought aluminum alloy is cooled by a developed air-mist cooling system prototype and calculate the h by means of finite element analysis using computer simulations in 2-D transient analysis, and to determine the minimum cooling rate needed to ensure adequate physical and mechanical properties of an aluminum alloy in order to establish good cooling strategies with the air-mist cooling system to achieve higher cooling rates that will provide better understanding for more complex geometries. Design of Experiments and Heat Treatment Process for h Analysis To make the comparison straightforward, the experimental and numerical set-ups were kept simple. The cooling system prototype consists of an enclosure where air is coming up from nozzles with atomized water. Solution heating for a 6000-series aluminum

Figure 1: Heat treatment cycle for a wrought aluminum alloy Solution treatment is conducted at temperatures above or close to the solvus, allowing intermetallic to enter in the aluminum matrix and form what’s called a solid solution; in the case of the aluminum alloy 6000 series, it is above 500°C. After solution, a fast cooling or quenching is performed to ensure that the supersaturated solid solution formed in the solution process, may stay at room temperature. The last stage is where we try to obtain the desirable mechanical properties by means of controlling the mechanism of precipitation hardening . This is when the intermetallic at solid solution, starts to appear out of the aluminum matrix with a heat treatment that is called aging that depends on both time and temperature since the mechanical properties of the material depend on the size and distribution of the precipitates being formed. The improvement of mechanical properties in aluminum alloys by heat treatment is restricted to precipitation as no solid allotropic transformation takes place in the alloy. So, why the interest in cooling systems? One of the factors that affects aging response is the cooling rate that a piece previously experienced after the solution, a phenomenon that is associated with the supersaturation of elements inside the aluminum matrix. The cooling rate to which the heat-treated parts are subjected is a critical parameter. If this rate is below a critical rate (depending of the alloy series), the dissolved elements will

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FIA MAGAZINE | FEBRUARY 2023