The Heat Treat Doctor

Daniel H. Herring | 630-834-3017 | [email protected]

Innovations in Aluminum Heat Treatment

heh heat treater is always looking to improve his pro- decrease in yield and tensile strength. Double aging this alloy to ccessing techniques, shorten cycle times and lower costs peak hardness results in a signifi cantly reduced processing time wwithout sacrifi cing production quality. Recent innova- – from 48 hours to 2 hours – and such processing can lead to re- T ttions in aluminum heat treating suggest this is possible. duced energy usage and reduced production costs resulting from Let’s learn more. the accelerated kinetics of precipitation. Again, there is an optimum time, which varies with both the Double Aging (DA) composition of the alloy and temperature of fi rst aging treatment. Double aging (Fig. 1) is a technique used for reducing total ag- A higher second aging temperature accelerates the precipitation ing time for various aluminum alloys. The effects of double ag- in 7075, but it cannot be so high as to cause signifi cant reduction ing and thermomechanical double aging in volume fraction of precipitates or dissolution of GP-zones. (i.e. a combination of work hardening and Thermomechanical double aging (Fig. 2) causes further accel- heat treatment) on enhanced mechanical eration of precipitation, reducing the total heat-treatment time properties of a 7075 aluminum alloy have to only 80 minutes. This results in an increase in hardness and recently been reported.[1] Results illustrate strength in 7075 alloy beyond the conventional aging procedure, the effects of accelerating the kinetics of but the ductility is reduced relative to conventional single age or precipitation on hardness and strength (via the double-aging heat treatments. tensile testing). By applying the appropri- ate heat-treatment cycle, the time to peak Rapid Heat Treatment (RHT)[2,3] aging can be reduced by a factor of up to 36 When cast components for structural applications such as alloy without a substantial decrease in hardness or tensile properties. wheels are manufactured using Al-Si-Mg-based casting alloys As heat treaters we know age-hardenable aluminum alloys can (typically A356 and A357), T6 heat treatment is in most cases an be hardened or strengthened by natural or artifi cial aging. The essential step in the manufacturing process. The T6 heat treat- science tells us that a supersaturated solid solution is formed after ment provides two benefi cial effects: an improved ductility and the solution heat treatment. Aging at temperatures well below the fracture toughness through spheroidization of the eutectic silicon solutionizing temperature produces Guinier-Preston (GP) zones particles in the microstructure and a higher alloy yield strength and metastable coherent precipitates that harden the alloy up to (YS) through the formation of a large number of fi ne precipitates the peak condition. However, stable and incoherent precipitates that strengthen the soft aluminum matrix. The fi rst benefi t is start forming in the overaged condition. The type of precipitation realized through the solution treatment, normally at a tempera- varies with alloy composition. ture around 1005°F (540°C), while the second benefi t is achieved For example, the most important factor infl uencing the DA through the combination of solution treatment, quenching and treatment in an Al-Zn-Mg alloy such as 7075 is the time of fi rst artifi cial aging in the temperature range of 285-340°F (140-170°C). aging. Test results on 7075 show positive effects of the DA treat- In the casting industry, it is often specifi ed that a cast compo- ment, with the ductility being higher with slightly less than a 6% nent should be solution treated for six hours at 1005°F (540°C).

Solutionizing Solutionizing

High- High- temperature temperature Low- aging Low- aging

temperature Temperature temperature

Temperature aging aging

Deformation

Time Time Fig. 1. Double-aging heat-treatment process cycle Fig. 2. Double aging and thermomechanical heat-treatment process cycle

22 February 2011 - IndustrialHeating.com rial He st ati u ng d ' In s

s s

T

H R E O T H C EA O T TREAT D

60

50 6061-O

40 6061-T4 6063-T1 6063-T832 30 6061-T6, 6063-T6 D Tube diameter, D diameter, Tube Wall thickness, T Wall 20

10 6063-T5 T R 0 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5

Bend radius, R Tube diameter, D

Fig. 4. Bending relationship Note: In the above fi gure, if the tube diameter to wall thickness ratio lies above the respective lines for the given tempers, the Fig. 3. Typical induction coils used in the RHT process tube generally cracks during bending.

While the benefi t of T6 heat treatment is 505°C), quenching and then artifi cial high strength elsewhere. Although the accepted, the additional cost and produc- aging at temperatures in the range of 300- method of heating in RHT can vary, in- tion time associated with such a treatment 425°F (150-220°C). duction heating has been found to be most is also substantial. Positive results have reliable and consistent in applying the been reported for permanent mold-cast Retrogression Heat Treatment (RHT)[4] RHT process to aluminum alloys. test bars of a modifi ed A356 alloy where a Retrogression heat treatment (Fig. 3) is solution treatment of 50 minutes at 1005°F used for improving formability of alumi- Summing Up (540°C) is suffi cient to attain more than num extrusions. It is defi ned as a means While most heat-treatment processes for 90% of the maximum YS, more than 95% of rapidly heat treating 6xxx and 7xxx aluminum alloys are dictated by specifi - of the ultimate tensile strength (UTS) and aluminum alloy extrusions in various tem- cations (e.g., AMS 2770 or AMS 2771) it nearly 90% of the maximum elongation for pers, either locally or throughout their is important to understand that ongoing a given aging condition. length, in order to return to or approach research is looking into ways of reducing Other studies have investigated the their highly ductile state in their solution- cycle times and minimizing costs. Stay effect of even shorter (0-30 minute) solu- ized and freshly quenched state. tuned. IH tion-treatment time at 1005°F (540°C) or For these extrusions in some tempers – 1020°F (550°C) on the microstructure and such as T4, T5, T6, and even T7, T8, and References 1. Emani, S. V., and J. Benedyk, P. Nash and properties of fully modifi ed Al-7wt.%Si- T9 – RHT involves heating into the sol- D. Chen, “Double Aging and Thermome- 0.3wt.%Mg alloys. In addition, it has re- vus temperature range and quenching. chanical Heat Treatment of AA7075 Alumi- cently been demonstrated that common Unlike conventional solutionizing heat num Alloy Extrusions,” J. Mater. Sci. (2009) high-pressure diecasting (HPDC) alloys, treatments that involve long times, RHT 44:6384-6391. 2. Lumley, R. N., and I. J. Polmer and P. R. Cur- such as those based on the Al-Si-Cu and can be conducted within a matter of sec- tis, “Rapid Heat Treatment of Aluminum Al-Si-Mg-(Cu) systems, may be success- onds and synchronized with a given form- High-Pressure Diecastings,” Metallurgical fully heat treated without causing surface ing operation. Several commercial appli- and Materials Transactions A, Volume 40A, blistering or dimensional instability. The cations of the RHT process have already July 2009, pp. 1716 – 1726. potential exists therefore to develop and been demonstrated, including forming 3. Zhang, D. L., and L. H. Zheng, and D. H. St. John, “Effect of a Short Solution Treatment evaluate secondary HPDC alloys designed of aluminum extrusions (Fig. 4) and the Time on Microstructure and Mechanical specifi cally for rapid heat treatment while forming of heat-treated automotive sheet. Properties of Modifi ed Al- 7 wt.%, Si- 0.3 still displaying high castability. The localized heating and quenching wt% Mg Alloy,” Journal of Light Metals 2 Signifi cant improvements in the ten- in RHT is applied to materials in high- (2002), pp. 27 – 36. 4. Benedyk, Joseph, Retrogression Heat Treat- sile properties for HPDC alloys may be strength tempers only in regions that will ment as a Means of Improving Formability achieved by a truncated solution treat- subsequently experience heavy deforma- of Aluminum Extrusions, ET’08 Conference, ment of 15 minutes at 915-940°F (490- tion during forming, thus maintaining 2008.

24 February 2011 - IndustrialHeating.com