1.1 HEAT TREATING of aluminum alloyHeat treatment in itsbroadest sense, refers to any of the heating and cooling methods that areperformed for the purpose of changing the mechanical properties and themetallurgical structure, or the residual stress state of a metal product38.The marketableheat-treatable aluminum alloys are, with few exceptions, based on ternary orquaternary systems with regard to the solutes involved in establishing strengthby precipitation. Commercial alloys that hardness and strength can besignificantly increased by heat treatment include 2xxx, 6xxx, and 7xxx series,wrought alloys except 7072 and 2xx. Some of these have only copper, or have copperand silicon, as their primary strengthening alloy addition(s). Most of theseheat-treatable alloys however may contain combinations of magnesium with one ormore of the elements copper, zinc, and silicon.
Characteristically, even litleamounts of magnesium I concert that have these elements accelerate andaccentuate precipitation hardening, while alloys in the 6xxx series have magnesiumand silicon approximately in the proportions required for the formulation ofmagnesium silicide (MgESi). Although not as strong compared to most 2xxx and7xxx alloys, 6xxx series alloys have good machinability, formability, weldability,and corrosion resistance, with medium strength39.Heat treatment to increasestrength of aluminum alloys is a process that have three steps; • Solution heattreatment: dissolution of the soluble phases• Quenching: developmentof the supersaturation• Age hardening:precipitation of solute atoms at room temperature (natural aging) or eitherelevated temperature (precipitation heat treatment or artificial aging)1.1.1 Aluminum heattreatmentAluminum heat treating requiresstringent controls. These controls are put in place to avoid melting thealuminum alloy during solution heat treatment, and also to ensure that a safeand durable product is manufactured and created40. Temperatureuniformity requirements inside furnaces are tight (±5ºF)in order to preventeutectic melting, and also insure uniform properties throughout the workload.The ideal properties ofaluminum are achieved by alloying additions and heat treatments.
This promotesthe creation of small hard precipitates that interfere with the motion ofdislocations and improve its mechanical properties. 7075 aluminum alloy is oneof the most commonly used aluminum alloy for structural applications due to itsattractive comprehensive properties such as high strength, low density,toughness ductility and resistance to fatigue. It has been completely utilizedin aircraft structural parts and also in other highly stressed structuralapplications.
Aluminum alloy 7075 Chemicalcomposition. Element %wt. Zn 5.6 Mg 2.5 Cu 1.6 Al Balance 1.
1.2 Defects that OccurDuring Heat TreatmentDuring the production ofa part, defects may occur. These defects can come from operations before heattreatment, such as midline porosity, inclusions which are formed during castingof the ingot. More defects can form during homogenization of the ingot, such assegregation, the formation of hard intermetallic and second phase particles41. Most thesedefects associated with heat treatment of aluminum can occur either duringsolution heat treatment, or during quenching. Solution heat treating defectsinclude incipient melting, oxidation and under-heating. Defects which occurduring quenching are typically distortion or inadequate properties which is causedby a slow quench, resulting in precipitation during quenching and inadequatesupersaturation.
1.1.3 Oxidation. If part is exposed to temperature for toolong, high temperature oxidation could become a problem41. This term hightemperature oxidation is really a misnomer. The culprit is actually moisture inthe air during the process of solution heat treatment.
This moisture that is asource of hydrogen, which diffuses into base metal. Voids form at theinclusions or other discontinuities. The hydrogen gas accumulates, and thenforms a surface blister on the part. In general, 7XXX alloys is one of the mostsusceptible (particularly 7050), then followed by 2XXX alloys. Extrusions aremostly prone to blistering then followed by forgings.Elimination of moistureminimizes the problem of the surface blistering. This is accomplished by the sequencingof door over quench tanks, and thoroughly drying and then cleaning furnaceloads prior to the solution heat treatment.
It is also important to sure thatthe load racks used for the solution heat treatment are also dry. However, itis not always possible to eliminate the high humidity in air to prevent surfaceblistering. Often the ambient relative humidity is very high, so that the othermeasures may have to be taken42.Use of ammoniumfluoroborate is typically used to prevent blistering on the 7XXX extrusions. Anamount equivalent to 5 g per m3 of workload space is usually used in preventionof surface blistering. This is applied as a powder in the shallow pan hangingfrom furnace load rack.
This material is very corrosive and it requiresoperators to wear the appropriate personal protective safety equipment’s. Thematerial is corrosive at temperature, it is highly recommended that the insidepanels in the furnace be manufactured using stainless steel. This reducescorrosion and maintenance.Anodizing of parts priorto the solution heat treatment is an alternative to ammonium fluoroborate. Thisis generally practical for the larger extrusions and forgings, where cost of anodizingis small compared to cost of the part38.Distortionduring Quenching. Of all possible “defects” occurring duringheat treatment of aluminum, distortion during quenching is the very mostcommon.
It is probably responsible for most of the non-value-added work(straightening) and costs associated with the aluminum heat-treating. This distortionduring quenching is caused by differential thermal strains developed duringquenching, and differential cooling 17. These thermalstrains could be developed surface-to-surface or center-to-surface.
Differentialcooling can be caused by large quench rates, so that center is cooled muchslower than the surface (non-Newtonian cooling) or by non-uniform heat transferacross surface of the part.1.1.
4 Stress ReliefImmediately after the partis quenched, most aluminum alloys are nearly ductile as they are in annealed condition. Consequently, it is oftenadvantageous to stress relieve the parts by working the metal immediately afterquenching process. Numerous attempts have been made to develop a thermaltreatment which will remove, or appreciably reduce these quenching stresses. Thenormal precipitation heat-treating temperatures are generally too low in providingappreciable stress relief. Exposure to some higher temperatures (which stressesare relieved more effectively) results in some lower properties.
However, suchtreatments are sometimes utilized when even the moderate reduction of the residualstress levels is important enough so that some sacrifices in mechanicalproperties can be accepted43.1.1.
5 Mechanical StressRelief. Deformation consists ofstretching (plate, extrusions, and bar) or compressing (forgings) productsufficiently to achieve a small but a controlled amount (1 to 3%) of plasticdeformation. If benefits of mechanicalstress relieving are in need, the user should refrain from the reheat treating44.Effect of thePrecipitation Heat Treating on Residual Stress. The stresses that developedduring quenching from solution heat treatment are reduced during the subsequentprecipitation heat treatment. Degree of relaxation of stresses is highlydependent upon the time and temperature of precipitation treatment and alloy composition.In general, the precipitation treatments that is used to obtain the T6 tempersprovide only modest reduction in stresses, ranging from around 10 to 35%. Toachieve a substantial lowering of a quenching stresses by a thermal stressrelaxation, higher-temperature treatments of T7 type are required.
Thesetreatments are used when the lower strengths resulting from the averaging areacceptable.1.1.6 Other thermalstress-relief treatments,They are known as subzerotreatment and cold stabilization, involve cycling of the parts above and belowroom temperature. Temperatures chosen are those that can be readily obtainedwith boiling water and mixtures of a dry ice and alcohol–namely, 100 and -73°C 1212 and -100 °F)–and the number of thecycles ranges from one to five. The maximum reduction in residual stresses thatcan be effected by some these techniques is about 25%. The maximum effect that canbe obtained only if the subzero step is performed first, and immediately after thequenching from solution- treating temperature while yield strength is low.
Nobenefit that is gained from more than one cycle. A 25% reduction in residualstresses is sometimes sufficient to permit fabrication of parts that can not bemade without this reduction45. However, incasea general reduction is needed, as much as 83% relief of residual stress ispossible by increasing the severity of uphill quench–that is, more closelyapproximating the reverse of cooling rate differential during the originalquench46.
1.1.7 Dimensional ChangesduringHeat Treatment In additionto completely reversible changes in dimension which are simple functions oftemperature change and caused by thermal expansion and contraction, dimensionalchanges of a more permanent character are also encountered during heattreatment. These changes are of different types, some are of mechanical originand others are caused by changes in metallurgical structure.
Changes of themechanical origin include those arising from stresses developed by thegravitational or other applied forces, from thermally induced stresses or from therelaxation of the residual stresses. Dimensional changes also accompany the recrystallization,solution, and the precipitation of alloying elements.