Aluminum production starts with the mineral bauxite, which contains approximately 50% alumina (Al2O3). In the Bayer process, pure alumina is extracted from bauxite using a sodium hydroxide solution to precipitate aluminum hydroxide which is then subjected to calcination to form alumina. The Hall-Heroult process is then used to reduce the alumina to pure aluminum. This is an electrolytic process in which alumina, dissolved in a bath of cryolite, is reduced to pure aluminum by high electrical currents.
During casting of aluminum ingots, it is important to remove as many oxide inclusions and as much hydrogen gas as possible. Oxides originate primarily from moisture on the furnace charge being melted; therefore, every effort is made to insure that the materials are dry and free of moisture. Hydrogen gas can cause surface blistering in sheets and is a primary cause of porosity in castings. Fluxing with chlorine, inert gases, and salts is used to remove the oxides and hydrogen from the melt before casting.
Semi-continuous direct chill casting is the primary method for producing ingots for high performance aluminum alloys. In this process (Fig. 2.7), the molten metal is extracted through the bottom of a water-cooled mold producing fine grained ingots with a minimum amount of segregation. Low melting point alloying elements, such as magnesium, copper, and zinc, are added to the molten charge as pure elements, while high melting elements (e.g., titanium, chromium, zirconium, and manganese) are added in the form of master alloys. To prevent hot cracking and refine the grain size, inoculants such as Ti and Ti-B are added to the melt.
The molten aluminum is poured into a shallow water-cooled cross-sectional shape of the ingot desired. When the metal begins to freeze in the mold, the false
bottom of the mold is slowly lowered and water is sprayed on the surface of the freshly solidified metal as it comes out of the mold. The temperature and flow rate of the water are controlled so that it will wet the surfaces and then cascade down the surfaces. Typical casting speeds are in the range of 1-5 in./min.
Because the liquid metal freezing front is almost horizontal and the metal freezes from the bottom to the top of the ingot, the direct chill casting process produces fine grained ingots with a minimum of segregation. It can also produce fairly large ingots at slow speeds, a necessary requirement for the high strength alloys to prevent cracking. Also, metal can be transferred to the mold slowly, uniformly, and at relatively low temperatures. Low temperatures, only about 50° F above the liquidus temperature, are used to minimize hydrogen pickup and oxide formation. Also, if the temperature of the aluminum is too high, coarse grained structures will result.
Typical problems encountered in the direct-chill process can include ingot cracking or splitting, segregation, liquation, bleeding, and cold shutting. The higher strength aluminum alloys, and the 7XXX series in particular, are susceptible to splitting during or after casting. The first metal to freeze during casting forms a solid outer shell filled with liquid metal. After the outer shell has contracted from freezing, the inner metal tries to contract as it freezes, setting up large internal tensile stresses in the ingot that can cause immediate cracking or delayed cracking up to several weeks later. Slow casting speeds help minimize this problem since they minimize thermal gradients during freezing. Due to their relatively high solidification rates, direct-chill ingots are prone to inverse segregation. In inverse segregation, a high concentration of lower melting constituents is found at the surfaces of the casting rather than at the center due to the large shrinkage of aluminum during freezing. Lower melting constituents which are squeezed to the surface of the cast ingots due to liquation must be removed by scalping before hot rolling. Bleeding occurs when the hot liquid metal in the center of the ingot melts the already frozen outer shell and runs along the outer surface. Slower casting speeds are used to eliminate surface bleeding. However, slow casting speeds can cause cold shutting in which deep wrinkles form on the surface of the ingot as the frozen surface layer contracts toward the center of the ingot. Therefore, to successfully cast an ingot using the direct-chill process, the ingot must be cast slow enough to eliminate splitting, bleeding, and liquation, while fast enough to eliminate cold shutting. Nevertheless, the use of the direct-chill process results in higher mechanical properties than the previous tilt mold process.9
Hot rolling is conducted at temperatures above the recrystallization temperature to create a finer grain size and less grain directionality. The upper temperature is determined by the lowest melting point eutectic in the alloy, while the lower temperature is determined by the lowest temperature that can safely be passed through the rolling mill without cracking. Sheet is defined as material that is 0.006-0.249 in. thick, while plate is material that is over 0.250 in. thick. For aircraft applications, typical sheet and plate thicknesses are 0.04-0.40 in. for fuselage skins and stringers, 1 to 2 in. for wing skins and up to 8 in. for bulkheads and wing spars. Extrusions of various angles and shapes (L, J, T, and H) are commonly used for wing stringers and fuselage frames. Hot rolling of as-cast ingots consists of:
1. Scalping of ingots
2. Homogenizing the ingots
3. Reheating the ingots to the hot rolling temperature, if necessary
4. Hot rolling to form a slab
5. Intermediate annealing and
Scalping of the ingot, in which approximately 0.25-0.38 in. of material is removed from each surface, is conducted so that surface defects will not be rolled into the finished sheet and plate. Homogenizing of the ingots is conducted to remove any residual stresses in the ingots and improve the homogeneity of the as-cast structure by reducing the coring experienced during casting. Good temperature control is required during homogenization because the ingots are heated to within 20-40° F of the lowest melting eutectic in the alloy. The 2XXX alloys are usually soaked for 4-12 h at 900-950° F, while the 7XXX alloys are soaked for 8-24h at 850-875° F. Since homogenization is a diffusion controlled process, the long times are necessary to allow time for the alloying elements to diffuse from the grain boundaries and other solute-rich regions to the grain centers. An important function of homogenization is to remove non-equilibrium low melting point eutectics that could cause ingot cracking during subsequent hot working operations. Electric heaters and fans are used to circulate the air to insure temperature uniformity and produce maximum heat transfer. If the alloy is going to be Alclad for corrosion protection, the scalping operation is usually done after homogenization to remove the heavy oxide layer that builds up during the rather long homogenization soaks, making it easier to obtain a good bond between the Alclad and the core during hot rolling.
The ingots can be hot rolled right after removing them from the soaking pits, or if cooled to room temperature after homogenization, they must be reheated for hot rolling. Ingots as large as 20 ft long by 6 ft wide by 2 ft thick weighing over 20 tons are initially hot rolled back and forth through the rolling mill into plate between 0.250 and 8.0 in. thick. Modern rolling mill facilities can heat the plate, roll it to the desired thickness, spray quench it to harden it, and then stretch it to relieve stresses. The 2XXX alloys are hot rolled in the temperature range of 750-850° F, while the 7XXX alloys are rolled at 750-825° F. Hot rolling helps to break up the as-cast structure and to provide a more uniform grain size and a better distribution and size of constituent particles. During hot rolling, the grain structure becomes elongated in the rolling direction, as shown in Fig. 2.8. This grain directionally can have a substantial effect on some of the mechanical properties, especially fracture toughness and corrosion resistance, in which the properties are lowest in the through-the-thickness or short transverse direction.
Initial rolling is done in a four-high reversing mill to breakdown the ingot. As shown in Fig. 2.9, a four-high mill uses four rolls in which the two smaller center rolls contact the workpiece and the two larger outer rolls provide support for the inner rolls. As the ingot is run back and forth through the mill, it rapidly becomes longer as the thickness is reduced. If wide plate or sheet is required, the ingot is removed after the first few passes and rotated 90° and then cross-rolled. After the slab has been reduced in thickness, it is removed from the mill, given an intermediate anneal, and then placed in a five stand four-high mill to roll to thinner plate or sheet with successive reductions at each station.
Short Transverse x Direction
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