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The microstructural and mechanical characteristics of rapid radiation heating and convection gas-fired heating on AA2618 forged samples have been studied. The rapid heating obtains refinement in grain size. The mechanical properties are greater in the AA2618 samples of rapid radiation heating.
Forging is a metal forming process used to produce large quantities of identical parts, as in the manufacture of automobiles, and to improve the mechanical properties of the metal being forged, as in aerospace parts or military equipment. The process is typically performed by preheating the metal to a desired temperature before it is worked (1). Aluminium alloy 2618 (AA2618) is a heat treatable Al-Cu-Mg-Fe-Ni forging alloy developed for high temperature applications especially in the manufacture of aircraft engine components (2). The presence of stable intermetallic particles (such as aluminide particles of the phase Al9FeNi) helps to control grain size and impede dislocation movement (3).
Currently in the domestic industry, aluminum alloys are heated for forging with a wide variety of equipment: electric furnaces, muffled gas furnaces, oil furnaces, induction heating furnaces, fluidized bed furnaces, and resistance heating furnaces.
The objective of this paper was to investigate the application of an infrared heating technique to aluminum 2618 alloy forgings and subsequent heat treatment operations. The mechanical and metallurgical characteristics were studied and compared with that of forgings heated in electrical furnace.
II. EXPERIMENTAL PROCEDURE
AA2618 alloy billets of 63.5 mm diameter and 150 mm long were rapid heated in an infrared furnace of capacity. The infra red furnace consists of two halves: an upper half carrying an array of tungsten halogen quartz lamps and a lower half with a drop-down arrangement used to load specimens into the furnace. The upper and lower halves are made of water-cooled stainless steel plates with firebrick lining. Thermocouples were placed on the surfaces of AA2618 alloy billets. Power was increased in 10% increments, and the time and temperature profiles were recorded with a multi-channel data acquisition system. To determine the effects of infra red heating on the metallurgy of this alloy, a comparative study was carried out conventional electric furnace. The chemical composition of Aluminium 2618 alloy billets is given in Table-1.
The flowchart of various operations carried out in this work is shown in figure 1. The forging and solution heat treatment temperatures were in accordance with AMS 2772 specifications. The times required for the billets to reach the desired temperatures by the two heating methods were given Table-2. The rapid solution heat treatment is shown in figure 2.
Table-1: Chemical composition of AA2618
Table-2: Heating practice
Forging temperature, 4250C
Solution heat treatment temperature, 5300C
Electric furnace heating
Figure 1: Flowchart of various operations
Figure 2: Rapid solution heat treatment
Figure 3: Shape and dimensions of tensile specimen
The forging dies were heated to 3500C using infrared insert heater prior to forging. Forging of preheated billets at 4250C was carried out using a hydraulic press with an upset ratio of 2:1 and a strain rate of 0.5 s-1. Temperatures during infra red heating of the billets prior to forging and during solution heat treatment were monitored.
The microstructure of characterization of AA2618 before forging and after forging were examined using Madras metallurgical optical microscope. The Rockwell hardness tester was used to measure the hardness of the forged specimens. The Rockwell hardness test method was carried out with a hardened steel ball having diameter 1.6 mm. a minor load of 10 kgf and major load of 100 kgf. The forged samples were machined to get specimens for tensile test. The shape and dimensions of the tensile specimen are shown in figure 3. The computer-interfaced UTM (Universal Testing Machine) was used for the tensile test. The specimens were loaded hydraulically at a crosshead speed of 0.02 mm/s. The loads at which the specimen has reached the yield point and broken were noted down. The extensometer was used to measure the elongation.
III. RESULTS AND DISCUSSION
Each experiment was repeated on three samples to find mechanical properties. The average of three readings is reported.
A. Effect of rapid preheating on AA 2618 samples before forging
Optical micrographs on transverse sections of AA2618 samples are shown in figure 4. The specimens were etched with a mixture of 2 g of NaOH, 4 g of Na2CO3 and 94 ml of water. Both specimens exhibit similar microstructures, characterized by intermetallic particles distributed in a recovered polycrystalline Î± matrix. The presence of stable intermetallic particles (such as aluminide particles of the phase Al9FeNi) was found Energy-dispersive X-ray spectroscopy (EDS) analysis in both samples (figure 5). The specimens which were heated rapidly to the preheating temperature of forging, appear to reveal a slightly higher volume fraction of intermetallic particles as compared to conventionally heated samples. There are instances where the magnesium-rich samples in the conventionally preheated AA2618 samples (figure 6). The conventionally preheated AA2618 samples had more time to form magnesium rich intermetallic particles. The intermetallic particles during conventional heating undergo spherodization and partially dissolve.
Figure 4: Micrographs of AA2618 samples under rapid (a) and conventionally (b) heating
Figure 5: Energy-dispersive X-ray spectroscopy analysis for identification of intermetallic aluminide particles
Figure 6: Energy-dispersive X-ray spectroscopy analysis for identification of magnesium-rich particles
Figure 7: Micrographs of AA 2618 samples after forging
B. Effect of rapid preheating on AA 2618 samples after forging
The preheated AA2618 samples were forged at 4250C and water quenched. Due to deformation, the intermetallic particles have become round and uniformly distributed in the Î±-matrix. The rapidly preheated samples appeared to have more intermetallic particles than the conventionally preheated samples (figure 7).
C. Effect of rapid solution heat treatment after forging
Barkerâ€™s reagent was used to reveal the grain boundaries. The specimens of rapid heating and conventional heating exhibited equiaxed spheroidal grains indicative of full recrystallization during the solution heat treatment (figure 8). The specimen processed rapid heating exhibits much smaller grains than the conventionally heated specimen. This resulted because the shorter exposure to high temperature of rapid heated specimen left more intermetallic particles with less particle coarsening in the Î±-matrix.
D. Effect of rapid heating on the mechanical properties
The mechanical properties of the specimens are given in Table-3. Hardness tests were conducted at the centre of transverse cross-section of the forgings. Tensile test specimens were cut longitudinally from the centre of the forging samples. The mechanical properties rapidly heated specimens are greater than those of conventionally heated specimens. The improvement in mechanical properties was due to grain refinement and due to the retention of larger amounts of solute elements in the matrix during the short heat treatment time.
Figure 8: Micrographs of AA2618 after solution heat treatment
Table-3: Mechanical properties of forgings
Yield strength, MPa
Ultimate tensile strength, MPa
The rapid heating obtains refinement in grain size. The mechanical properties are improved. The rapid heating results in more productive in terms of time than the convection gas-fired furnace heating.