Heat Treating Aluminum for Aerospace Applications

aero alum.qxp 6/14/2005 11:04 AM Page 2 HEAT TREATING ALUMINUM FOR AEROSPACE APPLICATIONS

It is of low weight. Its alloys can be heat treated to relatively high strengths. It is of reasonable cost, and it is easy to bend and machine. Because of these advantages, aluminum is the most common material used in aerospace today. This article discusses typical applications of aluminum heat treating in the aerospace industry. An overview of the physical metallurgy of the various alloys is given, as is a description of common heat treating defects in aluminum. The benefits and advantages of polymer quenchants are also examined.

D. Scott MacKenzie, PhD Houghton International, Inc. Valley Forge, PA aero alum.qxp 6/5/2005 8:32 PM Page 3

n the aerospace industry, the Copper, atomic % most common aluminum al- 1 2 3 4

loys used are 7XXX wrought L products. The thickness of 1200 parts used range from 0.6 mm 600 to 250 mm. Aluminum alloy Al+L I7075, 7050 extrusions are used exten- 548 °C sively for stringers, and other struc- 1000 5.65 tural components. Some extrusions 500 of 2024, and 2014 are used. Sheet, both Al °C clad and non-clad, of 2024, 7475 and °F 7075 is used for wing and fuselage 800 skins. Sheet is also formed and built- 400 up to use in bulkheads and other structural components. For heavy 600 loading, large forgings of 7050 and 300 7040 are typical, particularly in mili- Al+CuAl2 tary aircraft. 400 Aluminum Heat Treating 200 Al 1 2 3 4 5 6 7 8 9 10 Aluminum Solution Heat Treat- Copper, wt % ing — Aluminum alloys are classified as either heat treatable or not heat Figure 1 — Partial aluminum-copper phase diagram. treatable, depending on whether the alloy responds to precipitation hard- Table 1 — Solution heat treatment temperature range and ening. In heat treatable alloy systems eutectic melting temperature for 2XXX alloys. like 7XXX, 6XXX, and 2XXX, the al- Solution Heat Treatment Initial Eutectic Melting loying elements show greater solu- Alloy Temperature Range, °C Temperature, °C bility at elevated temperatures than at room temperature. This is illus- 2014 496-507 510 trated for the Al-Cu phase diagram 2017 496-507 513 (Figure 1). 2024 488-507 502 The Al-Cu phase diagram shows that holding a 4.5% alloy at 515 to to maintain the solute in supersatu- 550ºC will cause all the copper to go rated solid solution. Because the so- completely into solution. This is typ- lution heat treatment temperature is ically known as the “solution heat so close to the eutectic melting tem- treating temperature.” If the alloy is perature, temperature control is crit- slowly cooled, then the equilibrium ical. This is especially true for 2XXX structure of Al + CuAl2 will form. The series alloys. In this alloy group, the CuAl2 that forms is large, coarse, and initial eutectic melting temperature incoherent. However, if the alloy is is only a few degrees above the max- cooled rapidly, there will be inade- imum recommended solution heat quate time for the CuAl2 to precipi- treatment temperature. tate. All the solute is held in a super- Solution heat treating and saturated condition. Controlled quenching of these alloys is typically precipitation of the solute in finely dis- accomplished in large high tempera- persed particles at room temperature ture furnaces. Aluminum is also com- (natural aging) or at elevated temper- monly heat treated in salts. In some atures (artificial aging) is used to de- applications, the furnace is supported velop the optimized mechanical and above the quench tank, which moves corrosion properties of these alloys. under the furnace on rails. Sometimes Solution heat treatment develops there is more than one quench tank, the maximum amount of solute into each containing a different quen- solid solution. This requires heating chant. the material near the eutectic temper- Quenching — An understanding of ature and holding it there long heterogeneous precipitation during enough to allow nearly complete quenching can be understood by nu- solid solution. After solution heat cleation theory applied to diffusion treatment, the material is quenched controlled solid-state reactions. The HEAT TREATING PROGRESS • JULY 2005 37 aero alum.qxp 6/5/2005 8:32 PM Page 4

urally aged to T3 or T4 tempers exhibit high ratios of ultimate tensile strength/yield strength. They also have excellent fatigue and fracture toughness properties. Natural aging and the increase in properties occur by the rapid formation of GP (Guinier-Preston) Zones from the supersaturated solid solution and from quenched-in vacancies. Strength increases rapidly, with properties becoming stable after approximately 4-5 days. The T3 and T4 tem-

Temperature pers are based on natural aging for 4 days. For 2XXX alloys, improve- ments in properties after 4-5 days are relatively minor, and become stable after one week. The Al-Zn-Mg-Cu and Al-Mg-Cu alloys (7XXX & 6XXX) also harden by Precipitation Supersaturation the mechanism of GP Zone forma- Diffusion Rate tion. However, the properties from natural aging are less stable. These alloys still exhibit significant changes Figure 2 — Schematic showing the interrelationship between the amount of solute supersat- in properties even after many years. uration and diffusion rate on the amount of heterogeneous precipitation occurring during Natural aging characteristics quenching. change from alloy to alloy. The most notable differences are the initial in- Table 2 — Typical time and temperature limits for cubation time for changes in proper- refrigerated parts stored in the as-quenched ties to be observed, and the rate of condition. change in properties. Aging effects are suppressed with lower than am- Maximum Storage Time for Retention of the AQ Condition bient temperatures. In many alloys, such as 7XXX alloys, natural aging Maximum Delay Time -12°C (10°F) -18°C (0°F) -23°C (-10°F) can be almost completely suppressed Alloy after Quenching Max. Max. Max. by holding at -40°C. 2014 15 minutes 1 day 30 days 90 days Because of the very ductile and 2024 formable nature of as-quenched al- 2219 loys, retarding natural aging increases 6061 30 minutes 7 days 30 days 90 days scheduling flexibility for forming and 7075 straightening operations. It also al- lows for uniformity of properties kinetics of heterogeneous precipita- This is shown schematically in Figure during the forming process, con- tion occurring during quenching de- 2. The amount of time spent in this tributing to a quality part. However, pends on the degree of solute super- critical temperature range is governed refrigeration at normal temperatures saturation and the diffusion rate, as by the quench rate. does not completely suppress natural a function of temperature. As an alloy The amount of precipitation formed aging. Some precipitation still occurs. is quenched, there is greater super- during quenching reduces the amount Table 2 shows typical temperature saturation (assuming no solute pre- of subsequent possible hardening. This and time limits for refrigeration. cipitates). But the diffusion rate in- is because solute precipitated from so- Artificial Aging — After quench- creases as a function of temperature, lution during quenching is unavail- ing, most aerospace aluminum alloys become greatest at elevated temper- able for any further precipitation reac- are artificially aged by precipitation atures. When either the supersatura- tions. This results in lower tensile hardening. Precipitation hardening tion or the diffusion rate is low, the strength, yield strength, ductility and is the mechanism by which the hard- precipitation rate is low. At interme- fracture toughness. ness, yield strength, and ultimate diate temperatures, the amount of su- Natural Aging — Some heat treat- strength dramatically increase with persaturation is relatively high, as is able alloys, especially 2XXX alloys, time at a constant temperature (the the diffusion rate. Therefore, the het- harden appreciably at room temper- aging temperature) after rapidly erogeneous precipitation rate is ature to produce the useful tempers cooling from the much higher solu- greatest at intermediate temperatures. T3 and T4. Alloys that have been nat- tion heat treat temperature. This rapid 38 HEAT TREATING PROGRESS • JULY 2005 aero alum.qxp 6/15/2005 2:33 PM Page 5

cooling (quenching) results in a su- 80 persaturated solid solution and is the 2014-T4 driving force for precipitation. 300°F 275°F 225°F This phenomenon was discovered 70 250°F by Wilm, who found the hardness of aluminum alloys with minute quan- 60 tities of copper, magnesium, silicon, and iron increased with time, after 50 quenching from a temperature just 350°F below the melting point. In general, 400°F 40 the sequence of precipitation occurs Vickers Hardness, 30 kg. Vickers by the clustering of vacancies, forma- 500°F 450°F tion of GP Zones; nucleation of a co- 30 herent precipitate; precipitation of an incoherent precipitate; and the coars- 20 0.01 0.1 1 10 100 1000 10000 ening of the precipitates. Natural Aging Time, Days Precipitation hardening (aging) in- volves heating the alloyed aluminum 50 to a temperature in the 200°- 450°F 6061-T4 250°F range. At this temperature, the super- 275°F saturated solid solution created by 400°F quenching begins to decompose. Ini- 450°F 300°F 40 tially there is a clustering of solute 225°F 350°F atoms near vacancies. Once sufficient atoms have diffused to these vacancy clusters, coherent precipitates form. Because the clusters of solute atoms 30

are a mismatch to the aluminum ma- Hardness, 30 kg. Vickers trix, a strain field surrounds them. As 500°F more solute diffuses to the clusters,

eventually the matrix can not accom- 20 modate the matrix mismatch and a 0.01 0.1 1 10 100 1000 10000 semi-coherent precipitate forms. Natural Aging Time, Days Heating the quenched material in Figure 3 — Aging curves for 2014 and 6061 aluminum alloys. the range of 95-205°C accelerates precipitation in heat treatable alloys. This tion of small GP Zones that are below Generally, the higher acceleration is not completely because the critical size. Similar aging curves of changes in reaction rate. Structural have been developed for 7075 and the aging temperature changes occur that are dependent on casting alloys. the shorter the time time and temperature. In general, the Generally, the higher the aging increase in yield strength that occurs temperature the shorter the time re- required to attain during artificial aging increases faster quired to attain maximum properties. maximum properties. than the ultimate tensile strength. For this reason, aging temperatures This means the alloys lose ductility are usually lower to assure the entire and toughness. T6 tensile properties load is brought to the required aging are higher than T4 properties, but temperature without risk of reduced ductility is reduced. Over-aging de- properties caused by over-aging of creases the tensile strength and in- the fastest heating aluminum. creases the resistance to stress-corrosion-cracking. It also enhances Defects Occurring resistance to fatigue crack growth and During Heat Treatment imparts dimensional stability to the Defects can occur during a part’s part. production. These may come from Precipitation hardening curves operations before heat treatment, have been developed for all the most such as midline porosity, inclusions common alloys. Figure 3 shows aging formed as the ingot is cast. Further curves for 2024 and 6061. Both alloys defects can form during homogeniza- show evidence of reversion of GP tion of the ingot, such as segregation Zones by initial reductions in hard- — the formation of hard inter- ness. This is caused by the destruc- metallics and second phase particles. HEAT TREATING PROGRESS • JULY 2005 39 aero alum.qxp 6/5/2005 8:33 PM Page 6

Most thermally induced aluminum It is also important to be sure load defects occur during solution heat racks are also dry. However, it is not treatment or during quenching. So- always possible to eliminate high hu- lution heat treating defects include midity in the air to prevent surface oxidation, incipient melting, and blistering. Often, the ambient relative under-heating. Quenching defects are humidity is high, so other measures typically distortion or inadequate may have to be taken. properties caused by a slow quench, Ammonium fluoroborate is typi- which result in precipitation during cally used to prevent blistering on quenching, and inadequate supersat- 7XXX extrusions and forgings. An uration. amount equivalent to 5g/m3 of work- Oxidation — If parts are exposed load space is used. This is applied as to temperature too long, high temper- a powder in a shallow pan hanging ature oxidation can become a from the furnace load rack. The ma- problem. The term high temperature terial is corrosive and requires oper- 200 µm oxidation is really a misnomer. The ators to wear appropriate personal culprit is actually moisture in the air protective equipment. Additionally, Figure 4 — Oxidation evident on a 7050 during solution heat treatment. The because the material is corrosive at extrusion exposed to extended times at the so- moisture is a source of hydrogen, temperature, it is recommended the lution heat-treating temperature. which diffuses into the base metal. furnace’s inside panels be of stainless Voids form at inclusions or other dis- steel. continuities. The hydrogen gas Anodizing parts prior to solution gathers and forms a surface blister on heat treatment is an alternative to am- the part. In general, 7XXX alloys are monium fluoroborate. This is gener- the most susceptible (particularly ally practical for larger extrusions and 7050), followed by the 2XXX alloys. forgings, where the cost of anodizing Extrusions are most prone to blis- is small compared with the cost of the tering, followed by forgings. An ex- part. However, for small parts, the ad- ample of surface blistering that can ditional added cost does not gener- occur is shown in Figure 4. ally justify the possible benefit of an- Eliminating moisture minimizes odizing prior to solution heat the surface blistering problem. This treatment.

20 µm is accomplished by the sequencing of Incipient Melting — Non-equi- the door over quench tanks, and thor- librium conditions occur because of Figure 5 — Incipient melting observed in oughly drying and cleaning furnace localized solute concentrations, a 2024 alloy. loads prior to solution heat treatment. which can decrease the eutectic temperature and cause localized 500 melting. This is often called incipient ! 6061-T6 Sy melting. An example is shown in ! 6061-T6 Su Figure 5. When this occurs, signifi- 450 ! 2024-T4 Sy cant decreases in properties result. ! ! 2024-T4 Su Toughness, ductility, and tensile ! properties are most affected. ! ! Local melting can also occur if the 400 part is heated too quickly. This is particularly true of 2XXX alloys. In this alloy system, there are local concentrations of Al2Cu. At slow heating

Strength, MPa 350 ! rates, the Al2Cu dissolves slowly into ! the matrix. At high heating rates, ! ! there is inadequate time for the Al2Cu ! 300 ! to dissolve. Local concentrations ! cause the local eutectic temperature to drop, resulting in localized melting. ! ! ! If inadequate time is allowed for this 250 ! metastable liquid to dissolve into the 470 480 490 500 510 520 530 Solution Heat Treatment Temperature matrix there is generally no decre- ment in properties. Figure 6 — Tensile strength as a function of solution heat treating temperature for 6061 and Under-heating — Under-heating 2024. during solution heat treatment can 40 HEAT TREATING PROGRESS • JULY 2005 aero alum.qxp 6/5/2005 8:33 PM Page 7

cause problems by not allowing enough solute to go into solid solution. This means less solute is avail- able during subsequent precipitation hardening reactions. As an illustra- tion of this, Figure 6 shows the effect of solution heat-treating temperature on yield strength and ultimate tensile strength. As the temperature is increased for both alloys, the tensile strength is also increased. For 2024- T4, it can be seen that there is a change in slope and rapid rise in properties as the temperature is increased past about 488°C. Distortion during Quenching — Of all possible defects occurring Figure 7 — Distortion of a large aerospace rib resulting from improper quenching and racking practice. during heat treatment, distortion during quenching is the most common. It is probably responsible for most of the non-value added work (straightening) and costs associated with aluminum heat treating. An ex- treme case of distortion is shown in Figure 7. Distortion during quenching is caused by differential cooling and differential thermal strains developed during quenching. These strains can occur center-to-surface, or surface-to- surface. Differential cooling can be caused by high quench rates, so that the center is cooled much slower than the surface (non-Newtonian cooling), Figure 8 — Distortion from inadequately supporting the workload. or by non-uniform heat transfer across the surface of the part. changes in length or volume as a Distortion during Aluminum is more prone to function of temperature, and in- quenching distortion than steel be- creases the probability that distortion quenching is caused by cause solution heat treating temper- will occur. differential cooling and atures are so close to the liquidus tem- Racking of the parts is critical. The perature. Aluminum exhibits less parts should be fully supported, with differential thermal strains strength and greater plasticity than the loads spread out over a large area developed during steel at the solution heat-treating tem- since the creep strength of aluminum perature (or austenitizing tempera- is poor. The effect of this is illustrated quenching. ture for steel). Much higher quench in Insert Figure 8. Parts should be rates are necessary in aluminum to wired loosely to prevent them from prevent premature heterogeneous hitting each other during solution precipitation occurring during heat treatment. If wired too tightly, quenching, and to maintain supersat- the wire could cut the parts. The use uration of the solute. of pure aluminum wire minimizes In steel, there is a coupled phase this problem. transformation of austenite to Because of the poor strength of the martensite, which causes a 3% heat treated aluminum parts, distor- volume change during quenching. tion can occur as they enter the quen- There is no coupled phase transfor- chant. As a general rule, parts should mation in aluminum that can cause enter the quenchant aerodynamically cracking or distortion. However, the to avoid distorting the part before it coefficient of linear expansion of alu- even enters the bath. It should enter minum is approximately twice that smoothly – it should not “slap” the of steel. This causes much greater quenchant. This is shown in Figure 9. HEAT TREATING PROGRESS • JULY 2005 41 aero alum.qxp 6/5/2005 8:33 PM Page 8

Figure 9 — Typical racking procedures for aerospace parts. The parts are racked vertically so heat transfer occurs evenly on all sides.

Racking a part so that it enters the quenchant smoothly makes it more likely to have uniform heat transfer across the part. Distortion is more likely to occur because of horizontal changes in heat transfer than by ver- tical differences in heat transfer. Polyalkylene glycol (PAG) polymer quenchants are used in the aerospace industry to control and minimize quenching distortion. Typically, quenchants are governed by AMS 3025, and are either Type I or Type II. Type I quenchants are single polyalkylene glycol polymers. Type II quenchants are multiple molecular weight polyalkylene glycol polymers. Each offers different benefits. Because of the higher molecular weight of the Type II quenchants, lower concentrations can be used. However, Type II polymers have a lower cloud point temperature, which can cause higher drag-out if parts are removed from the quenchant before they reach the quenchant temperature (typically 80- 100ºF). The benefit of quenching aluminum in a PAG quenchant is shown in Figure 10. Polyalkylene glycols exhibit inverse solubility in water — they are completely soluble in water at room temperature, but insoluble at elevated temperatures. The inverse solubility temperature can range from 60ºC to 90ºC depending on the molecular weight and structure of the polymer. Inverse solubility modifies the con- ventional three-stage quenching mechanism and provides great flexibility in controlling the cooling rate. When a component is first im- mersed, 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 film en- capsulates the surface of the part. The stability and duration of this film is de- pendant on the temperature, concentration, and level of agitation present. The stable polymer-rich film eventually uniformly collapses, and cool Figure 10 — Comparison of the distortion occurring when quenching in cold water and a quenchant comes into contact with the 20% Type I polyalkylene glycol quenchant. hot metal surface. Nucleate boiling re- 42 HEAT TREATING PROGRESS • JULY 2005 aero alum.qxp 6/5/2005 8:33 PM Page 9

Figure 11 — Various stages during quenching using a PAG based polymer quenchant.

sults, with its attendant high heat ex- rate of cooling, and the cooling rate This article is edited from a paper originally traction rates, as shown in Figure 11. in the convection phase decrease. delivered at the 4th International Conference The cooling rate of these polymers Agitation has an important effect on Quenching and the Control of Distortion, can be varied to suit the specific ap- on the quenching characteristics of Nov. 22-25, 2003, Beijing, China. Author D. plication by changing the concentra- the polymer quenchant. It ensures Scott MacKenzie, Ph.D., is a technical spe- cialist, heat treating products, for Houghton tion, quenchant temperature, and the uniform temperature distribution International Inc., Valley Forge, PA. amount of agitation. Typically, for within the quench tank, and it also af- most applications, the agitation is fects the quench rate. As the severity usually fixed, while the concentration of agitation increases, the duration of is changed. the polymer-rich phase decreases and The polymer concentration influ- eventually disappears, and the max- ences the thickness of the polymer imum rate of cooling increases. Ag- film deposited on the surface of the itation has comparatively little effect part during quenching. As the con- on the cooling rate during the centration increases, the maximum convection stage.

Circle 24 or visit www.adinfo.cc 43