Fig. 1. Typical furnaces used for solution heat treating aluminum


Fig. 2. Extreme example of a large aerospace forging exhibiting twisting due to improper racking during solution heat treatment

Aluminum is used in a variety of different products, from sporting goods to aircraft, because of its light weight and high strength. It has excellent formability, and can be provided in wrought or cast forms. To achieve high strength, it must be solution heat treated, quenched and aged. Typical alloys heat-treated are the 2XXX, 6XXX, 7XXX wrought alloys, and many cast alloys (e.g. 319, A356 and A357).

Quenching is one of the most critical operations in the heat-treating process. Proper quench system design and practice will yield parts that meet the design properties and result in minimal residual stress and low distortion. There are many factors that contribute to a good quench system design for aluminum heat-treating.

Fig. 3 - Schematic representation of temperature effects on factors that determine the heterogeneous precipitation rate during quenching (after Hatch [1])

Review of Aluminum Heat Treating

Typical aluminum heat-treating operations consist of solution heat treatment, quenching, deformation or straightening, and finally, aging to the desired temper.

Solution heat treating involves heating the aluminum alloy to a temperature slightly below the eutectic melting temperature and holding at temperature long enough to allow close to complete solid solution. Solution heat treatment develops the maximum amount of solute into solid solution. After solution heat treatment, the material is quenched to maintain the solute in a supersaturated solid solution. Furnaces used can either be circulated air or salt (Fig. 1).

Rapid quenching in water or a water solution with a polymer additive such as polyalkylene glycol follows solution heat treatment. The quenching is performed rapidly in order to produce a supersaturated solid solution. The aluminum in the as-quenched (AQ) condition is soft but is very uniform in mechanical characteristics. Immediately after quenching, AQ Temper alloys are nearly as ductile as the "O" or annealed condition. Because of this, as-quenched alloys are often formed after quenching, but before artificial aging.

Fig. 4 - Heterogeneous precipitation and formation of precipitate-free zones in 7050 wrought aluminum

Straightening after quenching is used to correct distortion. Usually this is accomplished by placing the part (in the as-quenched condition) on a die and hitting the part with leather straps or mallets. Generally, it is best to heat treat the parts and then form in the as-quenched condition. However, this is often not practical. In that case, straightening should only be performed in areas of uniform section. Straightening across thickness transitions and areas containing holes or cutouts should not be done because the holes maybe be warped by straightening. In heavy sections, twist is very difficult to remove and is usually impractical. A good example of twist in a large heat-treated forging is shown in Figure 2. Unfortunately, because of the extent of twisting in this forging, it was not possible to salvage it.

Aging involves heating the alloyed aluminum to a temperature in the 200°-450°F (90° - 230°C) range. Natural aging, or aging at room temperature is also used to obtain properties. During aging, the supersaturated solid solution created by quenching from the solution heat-treating temperature begins to decompose. Initially there is a clustering of solute atoms near vacancies. Once sufficient atoms have diffused to these initial vacancy clusters, coherent precipitates form. Because the clusters of solute atoms have a mismatch to the aluminum matrix, a strain field surrounds the solute clusters. As more solute diffuses to the clusters, eventually the matrix can no longer accommodate the matrix mismatch. A semi-coherent precipitate forms. Finally, after the semi-coherent precipitate grows to a large enough size, the matrix can no longer support the crystallographic mismatch, and the equilibrium precipitate forms.

Fig. 5 - Tensile strengths of six alloys as a function of average cooling rate during quenching

Design of Aluminum Quench Systems

Quenching is arguably the most important step in the aluminum heat-treating process. Without proper quenching, parts will be distorted, contain high levels of residual stress, or not meet properties. If the part is quenched too fast, excessive distortion will occur. If quenched too slowly, excessive heterogeneous precipitation will occur, removing solute from subsequent precipitation.

Nucleation theory applied to diffusion controlled solid-state reactions [1] is used to understand what occurs during the quenching process. The kinetics of precipitation occurring during quenching is dependent on the degree of solute supersaturation and the diffusion rate, as a function of temperature. So, as an alloy is quenched, there is greater supersaturation (assuming no solute precipitates). But the diffusion rate increases as a function of temperature. The diffusion rate is greatest at elevated temperature. When either the supersaturation or the diffusion rate is low, the precipitation rate is low. At intermediate temperatures, the amount of supersaturation is relatively high, as is the diffusion rate. Therefore the heterogeneous precipitation rate is the greatest at intermediate temperatures. This is shown schematically in Figure 3. The amount of time spent in this critical temperature range is governed by the quench rate. This critical temperature range is typically about 750°F (400°C) to 550°F (290°C) for most wrought aluminum alloys.

Fig. 6 - Comparison of the cooling rates of distilled water and normal "hard" water

The amount of precipitation occurring during quenching reduces the amount of subsequent hardening possible. This is because as solute is precipitated from solution during quenching, it is unavailable for further precipitation reactions. This results in lower tensile strength, yield strength, ductility and fracture toughness. It can also contribute to intergranular corrosion. An illustration of the heterogeneous precipitation of grain boundaries and the formation of precipitate free zones (PFZ) is shown in Figure 4.

Quantifying quenching and the cooling effect of quenchants have been extensively studied [2] [3] [4] [5]. At quench rates exceeding 840°F/sec (450°C/s), it was determined that maximum strength and corrosion resistance were obtained [6]. At intermediate quench rates of 840°F/sec (450°C/s) to 212°F/sec (100°C/s), the strength obtained is lowered (using the same age treatment), but the corrosion resistance is unaffected. Between 212°F/sec (100°C/s) and 70°F/sec (20°C/s), the strength decreased rapidly, and the corrosion resistance is at a minimum. At quench rates below 70°F/sec (20°C/s), the strength decreases rapidly, but the corrosion resistance is improved. However, for a given quenching medium, the cooling rate through the critical temperature range was invariant no matter the solution heat treat temperature. An illustration of the effect of average cooling rate from the solution heat-treating temperature on tensile strength is shown in Figure 5.

Fig. 7 - Comparison of the distortion occurring between sheet materials quenched in either cold water or a polyalkylene quenchant such as Aqua-Quench 260

To avoid excessive precipitation during quenching, three requirements must be met. First, the transfer time from the solution heat-treat furnace into the quench tank must be minimized. Second, the quenchant properties must be fast enough to ensure that proper supersaturation is achieved and that desired properties could be achieved. Third, the quench tank must have adequate thermal inertia so that the quenchant does not heat excessively, causing an interrupted quench. In addition, the quenching system must extract heat uniformly to minimize property variations.

Proper quench tank design is difficult and not well understood. The fluid used, temperature, flow, and parts configuration all interact during the quench cycle. The load configuration, the quenchants used, agitation rate, total load weight, and the density of the load on the racks all impact successful design of a quench system. It is very difficult to exactly predict the outcome in the production environment. The basic quench tank design takes the following considerations into account:

  • Quenchant
  • Agitation
  • Part racking and baskets
  • Hoist speed
  • Quench delay time
  • Material selection
  • Heat load
  • Cooling/heating
  • Fluid maintenance
  • Concentration control and separation methods


Quenchants

There are two types of quenching commonly used for commercially heat-treating aluminum: direct immersion and spray quenching. Direct immersion quenching requires that the workbasket be completely immersed into a quenchant bath. Spray quenching is a specialized form of quenching, where a stream of quenchant is directed onto the part.

Immersion quenching is controlled by specifying the quenchant (and concentration if appropriate) and temperature of quenching. There are two types of quenchants used for aluminum heat-treating: water and polyalkylene glycol (polymer) quenchants.

Cold water quenching is the most severe of commonly used quenchants. In an early study using cooling curves [7], it showed that quenching into still water caused rapid heat transfer. This study showed that heat transfer at the surface of the part was very turbulent at the metal/water interface. This study also showed that there was a marked difference between hard water and distilled water. Distilled water showed an extensive vapor blanket that extended to very low temperatures (Fig. 6). The cooling rate of water quenching is independent of material properties like thermal conductivity and specific heat. It is primarily dependent on water temperature and agitation [8]. Water temperature is the largest primary variable controlling the cooling rate. With increasing water temperature, the cooling rate decreases. The maximum cooling rate also decreases as the water temperature is increased. In addition, the temperature of maximum cooling decreases with increasing water quench temperature. The length of time and stability of the vapor barrier increases, with increasing water temperature. This is shown in Table 1.

Fig. 8 - Effect of PAG concentration on cooling rate

Quenching into water at < 120° - 140°F (50-60°C) often produces non-uniform quenching. This non-uniformity manifests itself as spotty hardness, distortion, and cracking and is caused by relatively unstable vapor blanket formation. Because of this difficulty, it was necessary to develop an alternative to water quenching. Polyalkylene Glycol quenchants (PAG) were developed to provide a quench rate between that of water and oil. By control of agitation, temperature, and concentration, quench rates similar to water and thick oil can be achieved.

For the past 40 years, polyalkylene glycol (PAG) has been extensively used to reduce distortion in wrought and cast aluminum parts (Fig. 7). It is a copolymer of ethylene oxide and propylene oxide. It exhibits an inverse solubility with water. In other words, as the water temperature is increased, the solubility of PAG quenchants in water is decreased. If a solution of typical polyalkylene glycol quenchant is heated in a beaker to approximately 180°F (80°C), a two-phase system results, as the temperature of the water is raised. The lighter phase is water, which floats to the top. A second phase, denser than water, sinks to the bottom. Each region contains a bit of the other in solution. In other words, the glycol-rich region contains some water, while the water-rich region contains some PAG quenchant. However, as the temperature is increased, the partitioning of PAG and water increases. The temperature at which separation occurs is called the cloud point. The cloud point is affected by pH, %PAG, and other contaminates in the system. As the pH is increased, the cloud point decreases. As the contaminate level (such as salt from salt baths) increases, the cloud point is decreased.

During actual quenching, the presence of polyalkylene glycol quenchants modifies the conventional three-stage quenching mechanism and provides great flexibility of controlling the cooling rate.

Fig. 9 - Illustration showing the sequence of quenching in a polyalkylene glycol quenchant

When a hot component is first immersed, the solution in the immediate vicinity of the metal surface is heated to above the inverse solubility temperature. The polymer becomes insoluble, and a uniform polymer-rich vapor film encapsulates the surface. This is a period of slow cooling, analogous to the vapor phase in oil quenching. The stability and cooling rate depend upon the temperature, concentration, and degree of agitation of the solution (Fig. 8).

The stable polymer-rich vapor film eventually ruptures, and cool quenchant comes into contact with the hot metal surface, resulting in nucleate boiling and high heat extraction rates. Because the polymer-rich film ruptures uniformly across the surface of the part, at virtually the same time, residual stresses and distortion is substantially reduced. Properties are maintained because of the high rate of cooling throughout the critical temperature range for heterogeneous precipitation.

As the active boiling period subsides, cooling occurs by conduction and convection into the liquid. When surface temperatures fall below the inversion temperature, the polymer re-dissolves, forming a homogeneous solution once again. The overall sequence of events during quenching in a polyalkylene glycol quenchant is illustrated in Figure 9.

Slow cooling minimizes temperature differences between the surface and center of a part, as well as surface-to-surface differences in heat transfer. This reduces the residual stresses since residual stresses are the result of large differential thermal strains. However, slow cooling also results in heterogeneous precipitation during quenching. This decreases properties by decreasing the amount of supersaturated solute. So a balancing act between residual stresses and acceptable properties occurs. To achieve the best balance, it is necessary to use the slowest possible quench that will still achieve properties with an appropriate safety factor.

References:

1. Hatch,J.E., Aluminum: Properties and Physical Metallurgy, ASM Intl., 154, 1984
2. Grossman, M.A., Metal Progress, 4 (1938) 373.
3. Scott, H., "Quenching Mediums," Metals Handbook, ASM Intl., (1948): 615.
4. Wever, F., Archiv für das Eisenhüttenwesen, 5 (1936): 367.
5. Dakins, M., Central Scientific Laboratory, Union Carbide, Report CSL 226A.
6. Fink, W. L., Wiley, L. A., Trans. AIME 175 (1948): 414.
7. Speith, K., Lange, H., Mitt. Kaiser Wilhelm Inst. Eisenforssch, 17 (1935): 175.
8. Rose, A., Arch. Eisenhullennes, 13 (1940): 345.
9. ASM Handbook, Vol. 4: Heat Treating, ASM Intl., 51 1991

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