Heat Treatment Of Aluminum Part 1 (1945)

the value of this aluminum alloy part when installed in place on an airplane rests on its physical and chemical characteristics through heat treatment these characteristics can be improved to produce a part which although it has the same appearance as before is stronger and harder and better able to meet the demands of an airplane in flight it can stand up to the forces tending to fracture it by compression it can resist forces acting in tension upon it tending to fracture it by stretching it can repel the forces tending to bend and break it and it can more successfully resist corrosive attack by the elements heat treated and non heat treated parts look no different from each other since changes in physical and chemical properties do not strike the naked eye they are measured indirectly by testing instruments and studied by examination of alloy specimens under the microscope the changes in the metals properties our reflections of changes in the metals internal structure an understanding of the aluminum alloys crystal grain structure shown in this photo micrograph is essential to an understanding of the heat treatment process and its accompanying property changes crystallization occurs in the initial production or casting of aluminium and aluminium alloys and is the physical root of the metals characteristics as the molten metal is cool too it's freezing point groups of atoms join together at various points throughout the liquid to form small individual lattices or unit cells of solid material the single lattice or unit cell is conceived to be cube shaped with atoms of the metal arranged in regular order one at each corner and one in the center of each face of the cube aluminium unit cells are classified as the face centered cubic type the axes of these unit cells are oriented or aimed at random in many different directions each crystal formation expands and becomes a larger unit known as the grain in growing adjacent grains make contact interfering with the free growth of the outermost cells these cells sheared and broken during growth give the grain its jagged and irregular shape although the body of the grain is regular and crystalline it is the grains in their boundaries that constitute the network structure shown in the photo micrograph of an aluminum alloy specimen within the lattice type pattern produced by the combination of unit cells atoms of the metal exist in families of parallel planes with definite spacings between planes it is along certain planes of weakness call slip planes that gliding or slipping may occur when the lattice structure is subjected to a load the plane along which slip is most likely to occur is generally the one with the greatest atomic population and the largest spacing between it and its parallel neighbor these are important slip planes along which movement may take place in the aluminum crystal the process of slip or deformation of the metal begins with application of a load or force and the distortion of the lattice structure under that load when the load exceeds the elastic limit of the metal blocks of the crystal lattice begin to slide over each other upon removal of the force the elastic stresses and the individual blocks are relieved but the lattice structure remains deformed continuance of the force beyond the metals ultimate strength would result in rupture or failure of the metal to strengthen the metal alloying elements are added further strength may be gained through heat treatment of the alloys thus formed the aluminum alloy consists of two main types of metallic combinations the first is the solid solution of the alloying element in aluminum the atoms of the alloying element are dispersed throughout the aluminum displacing some of the atoms in the aluminum crystal lattice the second is the formation of an inter metallic compound between the alloying element and the aluminum in the case of alloy 24s the compound formed by copper with aluminum is copper aluminide or CUA l2 as temperature is increased the alloying element becomes increasingly soluble in the solid aluminum the solubility of copper in solid aluminum increases from 5/10 percent at 392 degrees Fahrenheit to 6% at ten hundred and forty degrees Fahrenheit in actual heat treatment practice care must be taken to stay well below the melting point of the alloy or any of its constituents prior to heat treatment the alloy twenty-four s contains relatively large undissolved particles of the copper aluminium compound CU AL - scattered on evenly throughout the mass of the aluminum in a solution stage of the heat treatment process temperature is increased and the Cu al 2 part decrease in size as the copper gradually goes into solid solution with the aluminum subjection of the alloy to the proper temperature for the right length of time it brings about the disappearance of the CUA l2 particles the copper has now been largely dissolved in the aluminum the solid solution is now retained or captured by a quick quench which provides rapid cooling of the alloy however at room temperature following the quench fine sub microscopic particles of Cu al to begin to precipitate out of solution and to disperse evenly throughout the metal these particles represent the copper brought into solution at higher temperatures and which the aluminum is unable to retain in solution at room temperature many of these precipitated particles are distributed along the slip planes or planes of weakness in these locations the particles serve as keys or props which secure blocks of unit cells in position and interfere with slippage this is the most generally accepted explanation of the increase in the metals hardness and strength as a result of heat treatment the extent of change in physical properties may be judged by testing the aluminum alloy before heat treatment in the s condition and after heat treatment in the esti condition one of these is a test for hardness generally conducted on a rockwell hardness tester a penetrator is applied to the specimen with a fixed load and the depth of penetration is taken as an indication of hardness another test is for resistance to corrosion the solution potential test provides voltage readings which are a measure of this property a third is a test for tensile strength this measures the force required to fracture the metal in tension the first stage and heat treatment procedure consists of a heating operation followed by a quick quench of the alloy part two different types of heating and crunching equipment may be employed in this process the alloy part may be heated in an air furnace or an assault bath furnace rapid cooling of the part may be accomplished by means of a spray quench booth or a quench tank farts to be heat-treated must be properly racked to minimize their tendency to sag at elevated temperatures and to reduce warpage during the quench sufficient space should be allowed between parts to promote free circulation of the heating medium and later of the quenching water the loaded rack is now ready to be placed in the furnace operation of the furnace is guided by automatic temperature and time controls a thermocouple unit keeps the temperature within the range prescribed for a given alloy the proper heat treating range for aluminum alloy 24 s lies between 910 and 930 degrees Fahrenheit the automatic timer is set to cover the proper heating time or soaking period this will vary with a thickness of the alloy in its composition for example an alloy 24 s part 1/8 to 1/4 inch thick should be kept at heating temperature for a minimum of 60 minutes in an air furnace after the furnace has been heated up to heat reading temperature it is ready for loading the furnace door is raised pneumatically or hydraulically the rack containing the alloy parts is rolled into the furnace and the furnace door is lowered as quickly as possible the two essential characteristics of an air furnace are rapid circulation of air and controlled heat these are achieved by forcing heated air among the parts and maintaining nearly uniform temperature by controlling the heating elements during the soaking period the microstructure of the metal is altered the Cu Al two particles growing smaller as the copper goes into solid solution with the aluminum when the soaking period is completed the operators prepare to remove the load from the furnace the water is now turned on in the quench booth the rear door of the furnace and the adjoining door the quench booth are raised simultaneously the rack is pulled out of the furnace and into the spray quench booth the transfer from furnace to booth should be as rapid as possible taking no more than a few seconds delay in transferring the load reduces the corrosion resistance of alloy parts the load of alloy parts is quenched by the cold spray for a period of several minutes the water accumulates at the bottom of the booth and is carried away through drainage pits to be used over again in quenching the microstructure of the metal now shows most of the copper dissolved in the aluminum the quick quench captures or holds the solid solution in this condition after a load is removed from the furnace an inspector checks the parametric control record to make sure that the load has been within heat-treating temperature limits throughout the soaking period use of a salt bath as well as the air furnace requires correct racking of parts and setting of controls after this has been done the load is heated in the salt bath at the end of the soaking period the alloy parts are rapidly removed from the bath transferred to the quench tank and immersed in the cold water the parts are then rinsed in the rinse tank where the salt is washed away both quench tank and spray booth operations may cause distortion of the alloy parts resulting from the swift drop in temperature and hammering of the parts may be employed at this point to remove much of the warping caused by the quick quench in aging some of the copper precipitates out of solution in uniformly distributed particles of fine submicroscopic CU AL - in the case of alloy 24s precipitation occurs at room temperature about 70 degrees Fahrenheit and is practically complete in one day certain alloys aid very slowly at room temperature to speed this process they may be reheated in an air furnace for example alloys 53 s and 61's are aged at a temperature of from 315 to 325 degrees Fahrenheit for a period of from 12 to 18 hours through refrigeration age hardening may be in order to keep part soft enough for forming operations for example by refrigerating 24s rivets their driving characteristics may be retained for several days if we take sample tests of the properties of a typical aluminum alloy 24s part in the st or heat treated an aged condition we will obtain a measure of the improvement in the metals physical characteristics as a result of heat treatment we might find for instance that the tensile strength of the aluminum alloy part has risen from 30 thousand pounds per square inch to sixty thousand pounds per square inch we may find that the rockwell hardness number of the part has increased from h 82 h 112 and we would also find that the alloys resistance to corrosion has increased as shown by a change in solution potential test reading from minus point seven 100 millivolt to minus point 6 7 all millivolt the solution heat treatment has affected a desirable change in the physical properties of an aluminum alloy by controlling the size and distribution of the particles of the alloy constituents