1.1 HEAT TREATment of aluminum alloyHeattreatment in its broadest sense, refers to any of the heating and cooling methodsthat are performed for the purpose of changing the mechanical properties andthe metallurgical structure, or the residual stress state of a metal product38.Themarketable heat-treatable aluminum alloys are, with few exceptions, based onternary or quaternary systems with regard to the solutes involved in establishingstrength by 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.Heattreatment to increase strength of aluminum alloys is a process that have threesteps; •Solution heat treatment: dissolution of the soluble phases•Quenching: development of the supersaturation•Age hardening: precipitation of solute atoms at room temperature (natural aging)or either elevated temperature (precipitation heat treatment or artificialaging)1.1.1 Aluminum heattreatmentAluminumheat treating requires stringent controls. These controls are put in place toavoid melting the aluminum alloy during solution heat treatment, and also toensure that a safe and durable product is manufactured and created40. Temperature uniformity requirementsinside furnaces are tight (±5ºF)in order to prevent eutectic melting, and alsoinsure uniform properties throughout the workload.Theideal properties of aluminum are achieved by alloying additions and heattreatments.
This promotes the creation of small hard precipitates thatinterfere with the motion of dislocations and improve its mechanicalproperties. 7075 aluminum alloy is one of the most commonly used aluminum alloyfor structural applications due to its attractive comprehensive properties suchas high strength, low density, toughness ductility and resistance to fatigue.It has been completely utilized in aircraft structural parts and also in otherhighly stressed structural applications.Aluminumalloy 7075 Chemical composition. Element %wt. Zn 5.6 Mg 2.5 Cu 1.
6 Al Balance 1.1.2 Defects that OccurDuring Heat TreatmentDuringthe production of a part, defects may occur. These defects can come fromoperations before heat treatment, such as midline porosity, inclusions whichare formed during casting of the ingot. More defects can form duringhomogenization of the ingot, such as segregation, the formation of hardintermetallic and second phase particles41. Most these defects associated withheat treatment of aluminum can occur either during solution heat treatment, orduring quenching. Solution heat treating defects include incipient melting, oxidationand under-heating. Defects which occur during quenching are typicallydistortion or inadequate properties which is caused by a slow quench, resultingin precipitation during quenching and inadequate supersaturation.
1.1.3 Oxidation. If part is exposed to temperature for toolong, high temperature oxidation could become a problem41. This term high temperatureoxidation is really a misnomer. The culprit is actually moisture in the airduring the process of solution heat treatment. This moisture that is a sourceof hydrogen, which diffuses into base metal. Voids form at the inclusions orother discontinuities.
The hydrogen gas accumulates, and then forms a surfaceblister on the part. In general, 7XXX alloys is one of the most susceptible(particularly 7050), then followed by 2XXX alloys. Extrusions are mostly proneto blistering then followed by forgings.Eliminationof moisture minimizes the problem of the surface blistering.
This isaccomplished by the sequencing of door over quench tanks, and thoroughly dryingand then cleaning furnace loads prior to the solution heat treatment. It isalso important to sure that the load racks used for the solution heat treatmentare also dry. However, it is not always possible to eliminate the high humidityin air to prevent surface blistering. Often the ambient relative humidity is veryhigh, so that the other measures may have to be taken42.Useof ammonium fluoroborate is typically used to prevent blistering on the 7XXXextrusions.
An amount equivalent to 5 g per m3 of workload space is usuallyused in prevention of surface blistering. This is applied as a powder in theshallow pan hanging from furnace load rack. This material is very corrosive andit requires operators to wear the appropriate personal protective safety equipment’s.
The material is corrosive at temperature, it is highly recommended that theinside panels in the furnace be manufactured using stainless steel. This reducescorrosion and maintenance.Anodizingof parts prior to the solution heat treatment is an alternative to ammoniumfluoroborate. This is generally practical for the larger extrusions andforgings, where cost of anodizing is small compared to cost of the part38.Distortion during Quenching.
Of all possible “defects” occurringduring heat 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 thermal strains could bedeveloped surface-to-surface or center-to-surface. Differential cooling can becaused by large quench rates, so that center is cooled much slower than thesurface (non-Newtonian cooling) or by non-uniform heat transfer across surfaceof the part.1.1.
4 Stress ReliefImmediatelyafter the part is quenched, most aluminum alloys are nearly ductile as they arein annealed condition. Consequently, itis often advantageous to stress relieve the parts by working the metalimmediately after quenching process. Numerous attempts have been made todevelop a thermal treatment which will remove, or appreciably reduce thesequenching stresses. The normal precipitation heat-treating temperatures aregenerally too low in providing appreciable stress relief. Exposure to somehigher temperatures (which stresses are relieved more effectively) results in somelower properties. However, such treatments are sometimes utilized when even themoderate reduction of the residual stress levels is important enough so thatsome sacrifices in mechanical properties can be accepted43.1.1.
5 Mechanical StressRelief.Deformationconsists of stretching (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.Effectof the Precipitation Heat Treating on Residual Stress. The stresses thatdeveloped during quenching from solution heat treatment are reduced during thesubsequent precipitation heat treatment. Degree of relaxation of stresses ishighly dependent 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,Theyare known as subzero treatment and cold stabilization, involve cycling of theparts above and below room temperature.
Temperatures chosen are those that canbe readily obtained with boiling water and mixtures of a dry ice andalcohol–namely, 100 and -73 °C 1212 and -100 °F)–and the number of the cycles ranges from one to five. Themaximum reduction in residual stresses that can be effected by some thesetechniques is about 25%. The maximum effect that can be obtained only if thesubzero step is performed first, and immediately after the quenching fromsolution- treating temperature while yield strength is low.
No benefit that isgained from more than one cycle. A 25% reduction in residual stresses issometimes sufficient to permit fabrication of parts that can not be madewithout this reduction45. However, incase a general reductionis needed, as much as 83% relief of residual stress is possible by increasingthe severity of uphill quench–that is, more closely approximating the reverseof cooling rate differential during the original quench46.1.
1.7 Dimensional ChangesduringHeatTreatment In addition to completely reversible changes in dimension which aresimple functions of temperature change and caused by thermal expansion andcontraction, dimensional changes of a more permanent character are alsoencountered during heat treatment. These changes are of different types, some areof mechanical origin and others are caused by changes in metallurgicalstructure. Changes of the mechanical origin include those arising from stressesdeveloped by the gravitational or other applied forces, from thermally inducedstresses or from the relaxation of the residual stresses. Dimensional changesalso accompany the recrystallization, solution, and the precipitation ofalloying elements.