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Transcript of Aleaciones Aeroespaciales
Aluminum production starts with the mineral bauxite, which contains approximately 50% alumina Al2O3.
High strength-to-weight ratio
Group: 14 Symbol: Al
The third and fourth digits designate the specific alloy within the series; there is no special significance to the values of those digits
Melting and Primary Fabrication
During casting of aluminum ingots, it is important to remove as many oxide inclusions and as much hydrogen gas as possible.
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.
For cast alloys the first digit again refers to the major alloying
element, while the second and third digits identify the specific alloy.
The first digit defines the
major alloying class of the series
after a period identifies the alloy as a cast product. If the period is followed
by the number 1, it indicates an ingot composition that would be supplied to
a casting house.
• Heat treatable
• High strength, at room and elevated temperatures
• Typical ultimate tensile strength range: 27–62 ksi
• Usually joined mechanically but some alloys are weldable
• Not as corrosion resistant as other alloys
Wrought non-heat treatable alloys
Aluminum Alloy Designation
Types of joining
Forms a natural alumina Al2O3 oxide on its surface that helps prevent corrosion.
ROLLING PLATE AND SHEET
Density of 0.1 lb/in.3
Face centered cubic (FCC) structure
Melting point at 1215 F.
Adlemi, Sandy, Adrian, Lizzeth & Jahaziel
Low weight material
The attractiveness of aluminum
is that it is a relatively low cost, light weight metal that can be heat treated
to fairly high strength levels
Disadvantages of high strength aluminum alloys include a low modulus of
elasticity, rather low elevated temperature capability, and susceptibility to corrosion.
Since the FCC structure contains multiple slip planes, this crystalline structure greatly contributes to the excellent formability of aluminum alloys. Only a few elements have sufficient solid solubility in aluminum to be major alloying elements.
Commercially pure aluminum series (1XXX)
• Strain hardenable
• Exceptionally high formability, corrosion resistance, and electrical conductivity
• Typical ultimate tensile strength range: 10–27 ksi
• Readily joined by welding, brazing, soldering
Aluminum–manganese series (3XXX)
• High formability and corrosion resistance with medium strength
• Typical ultimate tensile strength range: 16–41 ksi
• Readily joined by all commercial procedures
• Hardened by strain hardening
Aluminum–silicon series (4XXX)
• Some heat treatable
• Good flow characteristics, medium strength
• Typical ultimate tensile strength range: 25–55 ksi
• Easily joined, especially by brazing and soldering
Aluminum–magnesium series (5XXX)
• Strain hardenable
• Excellent corrosion resistance, toughness, weldability, moderate strength
• Building and construction, automotive, cryogenic, marine applications
• Typical ultimate tensile strength range: 18–51 ksi
Wrought heat treatable alloys
Al–Cu Alloys (2XXX)
• Heat treatable
• High corrosion resistance, excellent extrudability; moderate strength
• Typical ultimate tensile strength range: 18–58 ksi
• Readily welded by GMAW and GTAW methods
• Outstanding extrudability
Al–Mg–Si Alloys (6XXX)
• Heat treatable
• Very high strength; special high toughness versions
• Typical ultimate tensile strength range: 32–88 ksi
• Mechanically joined
Al–Zn Alloys (7XXX)
• Heat treatable
• High conductivity, strength, hardness
• Typical ultimate tensile strength range: 17–60 ksi
• Common alloying elements include Fe, Ni and Li
Alloys with Al-Other Elements (8XXX)
Wrought aluminum alloys are designated by a four digit numerical system developed by the Aluminum Association.
The second digit defines variations in the
original basic alloy
The digit is always a zero (0) for the original composition,
a one (1) for the first variation, a two (2) for the second variation, and so forth.
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.
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
6. Cold rolling and annealing for sheet.
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 work piece 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.
Extruded structural sections are produced by hot extrusion in which a heated cylindrical billet is pushed under high pressure through a steel die to produce the desired structural shape.
The term “heat treating” refers to the heating and cooling operations that are performed in order to change the mechanical properties, metallurgical structure or residual stress state of a metal product.
Solution Heat Treating and Aging
the elements or compounds must have an appreciable solubility at high temperatures and only minimal solubility at lower temperatures.
Alloys that are not aged sufficiently to obtain maximum hardness are said to be underaged, while those that are aged past peak hardness are said to be overaged.
1. Heating to the solution heat treating temperature and soaking for long enough to put the elements or compounds into solution.
The precipitation hardening process is conducted in three steps:
2. Quenching to room or some intermediate temperature (e.g., boiling water) to keep the alloying elements or compounds in solution; essentially this creates a supersaturated solid solution.
3. Aging at either room temperature (natural aging) or a moderately elevated temperature (artificial aging) to cause the supersaturated solution to form a very fine precipitate in the aluminum matrix.
Cold working results in an increase in internal energy due to an increase in dis-locations, point defects, and vacancies.
. The tensile and yield strengths increase with cold working, while the ductility and elongation decrease.
Occurs in 3 stages
Annealing treatments are used during complex cold-forming operations to allow further forming without the danger of sheet cracking.
The softest, most ductile and most formable condition for aluminum alloys is produced by full annealing to the O condition.
Strain hardened products normally recrystallize during annealing, while hot worked products may or may not recrystallize, depending upon the amount of cold work present.
Forgings are often preferred for aircraft bulkheads and other highly loaded parts because the forging process allows for thinner cross-section product forms prior to heat treat and quenching, enabling superior properties.
Due to their FCC structure and their relatively slow rate of work hardening, aluminum alloys are highly formable at room temperature.
Blanking and Piercing
Blanking is a process in which a shape is sheared from a larger piece of sheet, while piercing produces a hole in the sheet by punching out a slug of metal.
Clearance is the distance between the mating surfaces of the punch and die, usually expressed as a percentage of sheet thickness.
The sheet is placed over a die and pressed down by a punch that is actuated by the hydraulic ram of a press brake.
The sheet is placed over a die and pressed down by a punch that is actuated by the hydraulic ram of a press brake.
The smallest angle that can be safely bent, called the minimum bend radius, depends on the yield strength and on the design, dimensions, and conditions of the tooling. The most severe bends can be made across the rolling direction.
Punch presses are used for most deep drawing operations. A punch or male die pushes the sheet into the die cavity while it is supported around the periphery by a blankholder.
During blank preparation, excessive stock at the corners must be avoided because it obstructs the uniform flow of metal under the blankholder leading to wrinkles or cracks.
The rate of strain hardening during drawing is greater for the high strength aluminum alloys than for the low to intermediate strength alloys.
The material is stretched over a tool beyond its yield strength to produce the desired shape. Large compound shapes can be formed by stretching the sheet both longitudinally and transversely.
Rubber Pad Forming
In rubber pad forming, a rubber pad is used to exert nearly equal pressure over the part as it is formed down over a form block.
Superplasticity is a property that allows sheet to elongate to quite large strains without localized necking and rupture.
The Ashby and Verrall model for superplasticity, based on grain boundary sliding with diffusional accommodation, is shown in Fig. 2.20 in which grains switch places with their neighbors to facilitate elongation.
Due to their lower properties and higher variability than wrought product forms, aluminum castings are not used for primary structural applications.
Aluminum casting alloys have different compositions than the wrought alloys, i.e. they are tailored to
Increase the fluidity of the molten metal
Be resistant to hot tearing during solidification
Reproduce the details of the mold shape.
Aluminum ingots for casting are usually reheated and melted using one of three types of furnaces
Direct fuel fired furnaces
Indirect fuel fired furnace
Types of casting
Sand casting is perhaps the oldest casting process known. The molten metal is poured into a cavity shaped inside a body of sand and allowed to solidify.
Advantages of sand casting
Low equipment costs
the ability to use a large number of aluminum casting alloys.
It is often used for the economical production of small lot sizes and is capable of producing fairly intricate designs.
Steps involved in sand casting are:
1. Fabricate a pattern
2. Place the bottom half of the pattern
3. Apply a release coating to the pattern
4. Turn the drag half of the mold over and place the top half of the flask on top of it.
5. Risers and a sprue are then installed in the cope half of the flask.
6. The cope half is then packed with sand and rammed.
7. The two halves are separated and the patterns are removed.
8. The two halves are reassembled and clamped or bolted shut for casting.
Is basically the same as sand casting except gypsum plasters replace the sand in this process.
Plaster and Shell Molding
Very smooth surfaces
Good dimensional tolerances
Uniformity due to slow uniform cooling.
1. A fine silica sand coated with a phenolic resin is placed in a dump box that can be rotated.
2. A metal pattern is heated to 400–500F, mold released and placed in the dump box.
3. The pattern and sand are inverted allowing the sand to coat the heated pattern
4. The dump box is turned right side up, the pattern with the shell crust is removed and cured in an oven at 650–750F.
5. The same process is repeated for the other half of the mold.
6. The two mold halves are clamped together and placed in a flask supported with either sand or metal shot.
Permanent Mold Casting
In permanent mold casting, liquid metal is poured into a metal mold and allowed to solidify.
Compared to sand castings, permanent mold castings are more uniform and have better dimensional tolerances, superior surfaces finishes 275–500in. are typical), and better mechanical properties due to the faster solidification rates
Die casting is a permanent mold casting process in which the liquid metal is injected into a metal die under high pressure.
Extremely good surface finishes and the ability to hold tight dimensions; however, die castings should not be specified where high mechanical properties are important because of the inherently high porosity level.
Aluminum exhibits extremely good machinability. Its high thermal conductivity readily conducts heat away from the cutting zone allowing high cutting speeds, usually expressed in surface feet per minute (SFM).
Step cutting uses the workpiece to help provide rigidity during the machining process.
All cutters have a maximum depth of cut before they chatter; therefore, taking small depths of cut with the step cutting technique helps to prevent chatter.
The comparison between conventional and high speed machining in Table 2.12 illustrates the small depths of cuts used in high speed machining.
Vibration is a natural concern when machining at high speeds.
Two types of vibration can occur
Appears as a series of uniform continuous series of marks on the workpiece surface & occurs when the impact frequency of the cutter begins to vibrate near its natural frequency.
Causes deeper marks that are more randomly distributed & occurs in two slightly different ways depending on whether a web or floor is being cut or a rib or flange is being machined.
What is an alloy?
Aluminum alloys can be joined by a variety of commercial methods including:
"Ability to produce a weld free of discontinuities and defects that results in a joint with acceptable mechanical properties, either in the as-welded condition or after a post-weld heat treatment."
Although aluminum has a low melting point, it can be rather difficult to weld for several reasons:
(1)The stable surface oxide must be removed by either chemical methods or more typically by thoroughly wire brushing the joint area.
(2) The high coefficient of thermal expansion of aluminum can result in residual stresses leading to weld cracking or distortion.
(3) The high thermal conductivity of aluminum requires high heat input during welding further leading to the possibility of distortion or cracking.
(4) Aluminum’s high solidification shrinkage with a wide solidification range also contributes to cracking
(5) Aluminum’s high solubility for hydrogen when in the molten state leads to weld porosity.
The ability to fusion weld aluminum is often defined as the ability to make sound welds without weld cracking
Two types of weld cracking can be experienced:
Solidification cracking or hot tearing
Occurs due to the combined influence of high levels of thermal stresses and solidification shrinkage. Also occurs in fusion zone, normally.
Solidification cracking can be reduced by minimal heat input and by proper filler metal selection.
Occurs in the grain boundaries next to the heat affected zone (HAZ).
It can be minimized by lower heat input and proper filler wire selection.
Gas metal arc welding is an arc welding process that creates the heat for welding by an electric arc that is established between a consumable electrode wire and the workpiece.
The consumable electrode wire is fed through a welding gun that forms an arc between the electrode and the workpiece. The gun controls the wire feed, the current, and the shielding gas. In GMAW, the power supply is direct current with a positive electrode. The positive electrode is hotter than the negative weld joint ensuring complete fusion of the wire in the weld joint.
Gas tungsten arc welding (GTAW) uses a non-consumable tungsten electrode to develop an arc between the electrode and the workpiece.
A schematic of the GTAW process is shown in Fig. 2.38. Although it has lower metal deposition rates than GMAW, it is capable of higher quality welds. However, when the joint thickness exceeds 0.375in. GMAW is probably a more cost-effective method
Plasma Arc Welding
Automated variable polarity plasma arc (VPPA) welding is often used to weld large fuel tank structures. Plasma arc welding, shown in Fig. 2.40, is a shielded arc welding process in which heat is created between a tungsten electrode and the workpiece. The arc is constricted by an orifice in the nozzle to form a highly collimated arc column with the plasma formed through the ionization of a portion of the argon shielding gas.
The keyhole process allows deep penetration and high welding speeds while minimizing the number of weld passes required. VPPA welding can be used for thicknesses up to 0.50in. with square grooved butt joints and even thicker material with edge beveling. While VPPA welding produces high integrity joints, the automated equipment used for this process is expensive and maintenance intense.
Resistance welding can produce excellent joint strengths in the high strength heat treatable aluminum alloys. Resistance welding is normally used for fairly thin sheets where joints are produced with no loss of strength in the base metal and without the need for filler metals.
In resistance welding, the faying surfaces are joined by heat generated by the resistance to the flow of current through workpieces held together by the force of water-cooled copper electrodes. A fused nugget of weld metal is produced by a short pulse of low voltage, high amperage current. The electrode force is maintained while the liquid metal rapidly cools and solidifies.
Friction Stir Welding
A new welding process which has the potential to revolutionize aluminum joining has been developed by The Welding Institute in Cambridge,
The welds are created by the combined action of frictional heating and plastic deformation due to the rotating tool. A tool with a knurled probe of hardened steel or carbide is plunged into the workpiece creating frictional heating that heats a cylindrical column of metal around the probe, as well as a small region of metal underneath the probe
A number of different tool geometries have been developed, which can significantly affect the quality of the weld joint. The threads on the probe cause a downward component to the material flow, inducing a counterflow extrusion toward the top of the weld, or an essentially circumferential flow around. The rotation of the probe tool stirs the material into a plastic
(1) the ability to weld butt, lap and T joints,
(2) minimal or no joint preparation.
(3) the ability to weld the difficult to fusion weld 2XXX and 7XXX alloys.
(4) the ability to join dissimilar alloys
, (5) the elimination of cracking in the fusion and HAZs
(6) lack of weld porosity.
(7) lack of required filler metals.
8) in the case of aluminum, no requirement for shielding gases.
Al2O3 Forms a natural alumina Al2O3 oxide on its surface that helps prevent corrosion.
2 types of coatings
Chemical conversion coatings
Anodizing conversion coatings
Produce a porous and absorptive oxide (0.002–0.003in. thick) that is very uniform and morphologically tailored to bond well with paint primers.
Anodizing is an electrolytic process that produces thicker (0.002–0.005in.) and more durable oxides than those produced by conversion coatings
During the forming operation, the metal sheet is being reduced uniformly in thickness; however,
wherever the sheet makes contact with the die it sticks and no longer thins-out. This results in a
part with non-uniform thickness.
Two other forming methods, were developed to reduce thickness non-uniformity during forming.
However, both of these methods require moving rams within the pressure chamber which
increases capital equipment costs.
Gas pressure is an effective pressure medium for SPF for several reasons:
(1) it permits the application of a controlled uniform pressure
(2) it avoids the local stress concentrations that are inevitable in conventional forming where a tool contacts the sheet
(3) it requires relatively low pressures