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May 8, 2008 - Heat Treatments Applied to Wrought Magnesium Alloys
 In most wrought alloys, maximum mechanical properties are developed through strain hardening, and these alloys generally are used either without subsequent heat treatment or merely aged to a T5 temper. Occasionally, solution treatment, or a combination of solution treatment with strain hardening and artificial aging, will substantially improve mechanical properties. Wrought alloys that can be strengthened by heat treatment are grouped into five general classes according to composition including: Magnesium-aluminum-zinc (e.g., AZ80A) Magnesium-thorium-zirconium (e.g., HK31A) Magnesium-thorium-manganese (e.g., HM21A and HM31A) Magnesium-zinc-zirconium (e.g., ZK60A) Magnesium-zinc-copper (e.g., ZC71A) Source: ASM Handbook, Vol. 4, Heat Treating, ASM International, 1991, p 899.
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May 1, 2008 - Heat Treatments Applied to Cast Magnesium Alloys
 The mechanical properties of most casting alloys can be improved by heat treatment. Casting alloys are grouped into seven general classes of commercial importance on the basis of composition including: Magnesium-aluminum-manganese (e.g., AM100A) Magnesium-aluminum-zinc (e.g., AZ63A and AZ91C) Magnesium-zinc-zirconium (e.g., ZK51A and ZK61A) Magnesium-rare earth metal-zinc-zirconium (e.g., EZ33A and ZE41A) Magnesium-rare earth metal-silver-zirconium, with or without thorium (e.g., QE22A and QH21A) Magnesium-thorium-zirconium, with or without zinc (HK31A and ZH62A) Magnesium-zinc-copper (e.g., ZC63A)
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April 24, 2008 - Heat Treating Magnesium Alloys
 Magnesium alloys usually are heat treated to improve mechanical properties or as a means of conditioning for specific fabricating operations. The type of heat treatment selected depends on alloy composition and product form (cast or wrought) and on anticipated service conditions. The types of heat treatment commonly used for various magnesium alloys, both cast and wrought, include annealing, stress relieving, solution treating and aging, and reheatreating. Basic temper designations are used to indicate the various types of heat treatments, which are the same as those applied to aluminum alloys. For certain magnesium alloys, development of properties depends almost entirely on heat treatment.
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April 17, 2008 - Alpha-Beta Aluminum Bronzes
 Complex alpha-beta aluminum bronzes are those whose normal microstructures contain more than one phase to the extent that beneficial quench and temper treatments are possible. These alloys, with and without iron, are heat treated using procedures somewhat similar to those used to heat treat steel, and have isothermal transformation diagrams that resemble those of carbon steels. For these alloys, the quench-hardening treatment essentially is a high-temperature soak intended to dissolve all of the alphaphase into the beta phase. Quenching results in a hard room-temperature beat-martensite structure, and subsequent tempering reprecipitates fine alpha needles in the structure, forming a tempered beta martensite.
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April 10, 2008 - Aging of Spinodal Hardening Alloys
 The microstructures and consequent heat treatabilitites of aluminum bronzes vary with aluminum content much the same as these characteristics vary with carbon content in steels. Alpha-aluminum bronzes contain less than 9% Al, or less than 8.5% Al with up to 3% Fe, and are essentially single-phase alloys. Effective strengthening of these alloys can be achieved only by cold work, and annealing and/or stress relieving are the only heat treatments of practical use. The most prevalent alloys of this group are C60600, C61000, C61300, and C61400. Annealing of alpha-aluminum bronzes is carried out at temperatures between 540 to 870°C (1000 to 1600°F), with the iron-containing alloys requiring temperatures nearer the high end of the range. Alloys of intermediate composition (containing small amounts of beta phase), such as C61900, are typically annealed at 5495to 650°C (1100 to 1200°F).
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April 3, 2008 - Aging of Spinodal Hardening Alloys
 The copper-nickel-tin (Cu-Ni-Sn) alloys are hardened by treating in the rather narrow temperature range of 350 to 360°C (660 to 680°F). The development of the optimum properties requires the careful control of temperature and time at temperature. The use of hardness alone to evaluate results may not be adequate because high hardness may be maintained where excessive aging causes a decrease in elastic properties. Variations in tensile properties of 70 to 100 MPa (10 to 15 ksi) are possible without a significant hardness change. A combination of cold working and heat treatment (microduplexing) can be used to impart moderately high tensile properties and significantly greater ductility. The process consists of cold working to significant reductions (typically 40 to 60%) and partially solution treating below the single-phase boundary, typically at 725°C (1340°F). The alloy is then aged at the higher spinodal-hardening temperature level of 425°C (800°F) for an extended time.
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March 27, 2008 - Heat Treatment that Causes Spinodal Decomposition
 The heat treatment that causes the spinodal decomposition of a spinodal alloy is to homogenize at a temperature above the miscibility gap so that only statistical variation in composition exist within the specimen followed by rapid cooling to a temperature within the spinodal region, and holding at that temperature, or continuously cooling from the solution temperature to room temperature. It is important to maintain control within the specified solution treatment temperature range for a particular alloy to obtain the proper heat treating response in the subsequent spinodal aging treatment. Exceeding the upper limit can result in brittle material that does not respond to spinodal hardening. Solution treating below the minimum temperature results in incomplete solution and failure of the material to harden fully during the spinodal aging treatment.
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March 20, 2008 - Spinodal-Hardening Copper Alloys
 Spinodal structures are composed of a fine, homogeneous mixture of two phases that form by the growth of composition waves in a solid solution during a suitable heat treatment. The phases of the spinodal product differ in composition from each other and from the parent phase, but have the same crystal structure as the parent phase. The fineness of the spinodal structure is characterized by the distance between regions of identical composition, which is of the order of 50 to 1,000 angstroms. To form a spinodal structure, the particular alloy system must have a miscibility gap (either stable or metastable), and the atoms of the two component metals must possess sufficient mobility at the heat treating temperature. If an alloy decomposes within the spinodal region by a diffusional process that allows composition variations to increase in magnitude, it is said to decompose spinodally.
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March 13, 2008 - Quality Control of Aged Wrought Copper-Beryllium Alloys
 The close control of temperature is critical in the conventional aging of copper-beryllium alloys. A change in temperature affects the time required to develop maximum properties. Also, the higher temperatures can result in lower property values. In most instances, the completeness of aging can be verified by harness testing. Exceptions are tensile testing of specimens taken from large parts and simulated service testing to determine elastic performance. Hardness measurements always should be made using the method and load most suitable for the thickness of the material and the normal level of hardness expected. Hardness test methods typically suggested for testing various thicknesses of hardened beryllium-copper alloy include diamond pyramid, Rockwell superficial 15N, and Rockwell B or C.
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March 7, 2008 - Precipitation Hardening Wrought Copper-Beryllium Alloys
 The cold working of solution-treated copper-beryllium alloys influences the strength attainable through subsequent aging. The greatest response to aging occurs in material in the cold-rolled hard temper condition. In general, work hardening offers no advantage beyond the hard temper condition because formability is poor and control of the precipitation-hardening treatment for maximum strength is critical. However, in some applications wire is drawn to higher levels of cold work prior to precipitation hardening. Special combinations of properties can be obtained by varying either the aging time or the aging temperatures. As tensile strength increases, elongation decreases and does not recover substantially with overaging.
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February 29, 2008 - Solution Treating Wrought Copper-Beryllium Alloys
 Solution-treating temperature limits for wrought copper-beryllium alloy mill products must be adhered to if optimum properties are to be obtained from the precipitation-hardening treatment. Solution treating below the specified minimum temperature results in insufficient solution of the beryllium-rich phase. This results in lower hardness after precipitation hardening. Also, solution treating must be carefully controlled to produce the desired grain size, dimensional tolerances, and mechanical properties and to prevent oxidation. Exceeding the upper temperature limit causes grain coarsening in wrought and cast materials. A coarse grain size impairs formability; overheating results in a brittle material that does not fully respond to precipitation hardening. To minimize grain growth, it is recommended that wrought alloys be held at temperature 1 hour for each inch or fraction of an inch of section thickness. The optimum time for a specific application must be determined by mechanical testing and microscopic examination of the alloy.
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February 22, 2008 - Wrought Copper-Beryllium Alloy Mill Products
 Wrought copper-beryllium alloy mill products generally are supplied solution treated or solution treated and cold worked. Material in these conditions can be fabricated without further heat treatment. Thus, solution treating typically is not a part of the fabricating process unless it is necessitated by a special requirement such as softening of the material for additional forming, or is used as a salvage operation for parts that have been incorrectly heated for precipitation hardening. Cu-Be alloys in the quenched condition are easily fabricated using standard production methods. Even though fully solution-annealed material is the softest form available, better age hardening properties can be obtained if the material is cold worked after the final solution anneal. The selection of a proper cold-worked temper for a particular application is based on the severity of cold forming and the mechanical property requirements.
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February 15, 2008 - Copper-beryllium Alloys
 Copper-beryllium alloys are precipitation hardenable because the solid solubility of beryllium in an alpha-copper matrix decreases with decreasing temperature. Heat treatment typically consists of solution annealing followed by precipitation hardening. Optimum mechanical and physical properties for specific applications can be achieved by varying the typical recommended heat treating schedules. Better age hardening characteristics can be obtained if the material is cold worked after the solution anneal. In addition to the wrought Cu-Be alloys, there is a wide variety of copper-base casting alloys (C81300 through C82800) that contain beryllium. Appropriate solution treating and aging schedules for these alloys are dictated by the levels of beryllium and other additives.
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February 8, 2008 - Aging and Stress-Relieving Treatment of Cu Alloys
 Because of the necessity for close temperature control, forced convection (recirculating air) and salt-bath furnaces are commonly used for aging and stress relieving. Salt baths can reduce total furnace time by up to 30% compared with that required using atmosphere furnaces. Salt baths are particularly valuable when the age hardening time is of short duration and when precise control of time at the aging temperature is required. Commercially available nitrate-nitrite salt mixtures (40 to 50% sodium nitrate, remainder sodium or potassium nitrite) that melt at 143°C (290°F) are used for aging and stress relieving. All material to be heated in salt should be properly cleaned and dried before being immersed in the molten salt; any organic substance (such as oil or grease) will react violently with the nitrate-nitrite salt.
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February 1, 2008 - Heat Treating Equipment for Cu Alloys
 Batch-type atmosphere furnaces can be heated electrically and by using oil or gas. Furnaces heated using oil or gas that have a protective atmosphere sometimes use a muffle to contain the atmosphere and prevent air infiltration by maintaining positive pressure when explosive atmospheres, such as hydrogen, are used. Direct natural gas-fired furnaces can be used if some surface oxidation and discoloration can be tolerated. Parts annealed in reducing atmospheres require cleaning to restore luster. Continuous atmosphere furnaces have a vestibule that provides a seal for the atmosphere, a heating chamber long enough to ensure complete solution treating, and a cooling or quenching chamber that also serves as an atmosphere seal. Salt baths consisting of molten neutral salts also are used for annealing, stress relieving, solution heat treating, and aging of copper alloys.
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January 25, 2008 - Precipitation-Hardening Cu Alloys
 Alloys that harden during low to intermediate-temperature treatments following solution quenching include precipitation-hardening, spinodal-hardening, and order-hardening types. Most Cu alloys of the precipitation hardening type are used in electrical and heat-conduction applications. Thus, the heat treatment must impart the necessary mechanical strength and electrical conductivity. Copper alloys harden via elevated-temperature treatment rather than ambient-temperature (natural) aging. Hardness increases, reaches a peak, and then decreases with time. Electrical conductivity increases continuously with time until some maximum is reached, typically in the fully precipitated condition. The optimum condition generally preferred results from treating at a temperature and time just beyond those corresponding to the hardness aging peak. Cold working prior to precipitation aging tends to improve heat treated hardness.
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January 18, 2008 - Hardening Cu Alloys
 Two general types of copper alloys that are hardened by heat treatment are: those that are softened by high-temperature quenching and hardened by lower temperature precipitation heat treatments, and those that are hardened by quenching from high temperatures through martensitic-type reactions. Alloys that harden during low to intermediate-temperature treatments following solution quenching include precipitation-hardening, spinodal-hardening, and order-hardening types. Quench-hardening alloys comprise aluminum bronzes, nickel-aluminum bronzes, and a few special copper-zinc alloys. Usually quench-hardened alloys are tempered to improve toughness and ductility and reduce hardness in a manner similar to that used for alloy steels.
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January 11, 2008 - Stress Relieving Cu Alloys
 Stress relieving is a process intended to relieve internal stress in materials or parts without appreciably affecting their properties. Stress-relief treatments are carried out at temperatures below those normally used for annealing. From a practical standpoint, higher-temperature/shorter time treatments are preferable. However, to guarantee the preservation of mechanical properties, lower temperatures and longer times are sometimes necessary. The optimum cycle produces adequate stress relief without adversely affecting properties. Thermal stress relief reduces residual stress by eliminating part of the residual elastic strain. Some alloys may undergo slight increases in property values during stress relief heat treatment.
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January 4, 2008 - Annealing Cu Alloys to Specific Properties
 Although specific properties are most frequently produced by the controlled cold working of annealed material, there are occasions in which annealing to temper is necessary or advantageous. An anneal is used to alter hardness and tensile properties to levels between those of hard and fully annealed tempers, with reasonably predictable results. For most copper alloys, the rapid drops in tensile properties and hardness occur with an increase in temperature in the annealing range, with special precautions taken to avoid any overheating. Tensile strengths and hardness levels similar to those of 1/8, 1/4, and 1/2 hard cold-worked tempers can be produced by annealing cold-worked brasses, nickel silvers, and phosphor bronzes.
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December 28, 2007 - Annealing Copper Alloys
 Annealing is a heat treatment intended to soften and to increase the ductility and/or toughness of metals and alloys. Annealing is applied to wrought products, during and after mill processing, and to castings. Annealing of cold-worked metal is accomplished by heating to a temperature that produces recrystallization and, if desirable, by heating beyond the recrystalliztion temperature to initiate grain growth. Annealing primarily is a function of metal temperature and time at temperature. The source and application of heat, furnace design, furnace atmosphere, and shape of the workpiece have a significant influence on part finish, cost of annealing, and uniformity of results obtained.
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December 21, 2007 - Heat Treating of Copper Alloys
 Heat treating processes applied to copper and copper alloys include homogenizing, annealing, stress relieving, solution treating, precipitation (age) hardening, and quench hardening and tempering. Homogenizing involves prolonged high-temperature soaking to reduce chemical or metallurgical segregation commonly known as coring, which occurs as a natural result of solidification in some alloys. The process is applied to copper alloys to improve hot and cold ductility of cast billets for mill processing, and occasionally is applied to castings to meet specified hardness, ductility, or toughness requirements. Typical soak times vary from 3 to over 10 h. Temperatures typically are above the upper annealing range, to within 50°C (90°F) of the solidus temperature. Homogenization slowly decreases tensile and yield strengths and hardness, and significantly increases elongation.
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December 14, 2007 - Effects of Cryogenically Treated Steel
 The absence of a clear-cut understanding of the mechanism(s) by which cryogenic treatment improves performance has hindered its widespread acceptance in the industry. Nevertheless, studies have been conducted to determine the effects of cryogenic treatment. Theories about the reasons for the effects of cryogenic treatment include a more nearly complete transformation of retained austenite into martensite; precipitation of submicroscopic carbides; and a reduction in internal stresses in martensite that occurs when the submicroscopic carbide precipitation occurs. A reduction in microcracking tendencies resulting from reduced internal stresses is suggested as a reason for improved properties.
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December 7, 2007 - Cold and Cryogenic Treating of Steel
 Cold treatment of steel consists of exposing the ferrous material to subzero temperatures to either impart or enhance specific conditions or properties of the material. Unlike heat treating, which requires that temperature be precisely controlled to avoid reversal, successful transformation through cold treating depends only on the attainment of the minimum low temperature (-84°C, or -120°F) and is not affected by lower temperatures. Typical cryogenic treatment consists of a slow cool-down (~2.5°C/min, or 4.5°F/min) from ambient temperature to liquid nitrogen temperature. When the material reaches approximately 80K (-315°F), it is soaked for an appropriate time (generally 24 h). At the end of the soak period, the material is removed from the liquid nitrogen and allowed to warm to room temperature in ambient air. Temperature can be controlled accurately and thermal shock to the material is avoided by conducting the cool-down cycle in gaseous nitrogen.
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November 30, 2007 - Annealing Ductile Cast Iron
 Ductile iron castings generally are given a full ferritizing anneal when maximum ductility and good machinability are required and high strength is not required. The microstructure is converted to ferrite, and the excess carbon is deposited on the existing nodules. This treatment produces ASTM grade 60-40-18. Amounts of manganese, phosphorus, and alloying elements such as chromium and molybdenum should be as low as possible if superior machinability is desired, because these elements retard the annealing process. Three types of annealing treatment are full anneal for unalloyed 2-3% Si iron having no eutectic carbide, full anneal with carbides present, and subcritical anneal to convert pearlite to ferrite.
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November 16, 2007 - Austenitizing Ductile Cast Iron
 The objective of austenitizing is to produce an austenitic matrix having as uniform a carbon content as possible prior to thermal processing. For a typical hypereutectic ductile cast iron, an upper critical temperature must be exceeded so the austenitizing temperature is in the two-phase (austenite-graphite) field; this temperature varies with alloy content. The “equilibrium” austenite carbon content in equilibrium with graphite increases with increasing austenitizing temperature. The ability to select (within limits) the matrix austenite carbon content makes austenitizing temperature control important in processes that depend on carbon in the matrix to drive a reaction. This is particularly true in structures to be austempered, in which the hardenability (or austemperability) depends to a significant degree on matrix carbon content. Austenitizing temperatures in the range of 900 to 940°C (1650 to 1750°F) typically are used with times ranging from 1 to 3 h.
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November 9, 2007 - Properties of Austempered Ductile Iron
 ADI is a unique cast iron material, having tensile properties attributable to ?H, but with the fine dispersion of ferrite. Austempering is accomplished by heating the casting to a temperature in the austenite-phase range (usually 815 to 925°C, or 1500 to 1700°F), holding for the time required to saturate the austenite with carbon, cooling to a temperature above the Ms temperature at a rate sufficient to avoid the formation of pearlite or other mixed structures, and them holding at that austenitizing temperature for the time required to produce the optimum structure of acicular ferrite and carbon-enriched austenite. The properties of ADI can be varied by changing the austempering temperature.
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November 2, 2007 - Austempering of Ductile Irons
 To produce austempered ductile iron (ADI), austenitizing is followed by rapid quenching (usually in molten salt) to an intermediate temperature range for a time that allows the unique metastable carbon-rich (~2%C) austenitic matrix (gamma subscript H) to evolve simultaneously with nucleation and growth of a plate-like ferrite (alpha) or ferrite plus carbide, depending on the austempering temperature and time at temperature. The austempering reaction progresses to a point at which the entire matrix has been transformed to the metastable product (stage I), and that product is "frozen in" by cooling to room temperature before the true banitic ferrite plus carbide phases can appear (stage II). The presence of 2-3%C prevents the rapid formation of iron carbide (Fe3C), and, thus, the carbon rejected during ferrite formation in stage I enters the matrix austentite, enriching it and thermally stabilizing it to prevent martensite formation upon subsequent cooling.
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October 26, 2007 - Heat Treating of Ductile Irons
 Ductile cast iron (also known as nodular or spheroidal graphite iron) is heat treated primarily to create matrix microstructures and associated mechanical properties not readily obtained in the as-cast condition. The microstructures achievable can be depicted using a continuous cooling transformation (CCT) diagram and cooling curves for furnace cooling, air cooling, and quenching. Slow furnace cooling results in a ferritic matrix (the desired product of annealing); whereas air cooling, or normalizing, results in a pearlitic matrix; and quenching produces a matrix microstructure consisting mostly of martensite with some retained austenite. Tempering softens the normalized and quenched conditions, resulting in microstructures consisting of the matrix ferrite with small particles of iron carbide (or secondary graphite). Actual annealing cycles usually involve more than just furnace cooling, depending on alloy content and prior structure.
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October 19, 2007 - Quench Severity for Aluminum Alloys
 Quench severity is commonly expressed in terms of an H-value (or Grossmann number), where the H-value is related to the thermal conductivity (k) of the parts(s) and the coefficient of heat transfer (C) between the quenchant and the part, related by the equation H = C/2k. Water can achieve cooling rates up to about 200°C/s (360°F/s) at the midplane of 25-mm (1 in.) thick plate. Lower cooling rates are achieved by immersion in heated water, reducing the velocity of quenchant around the part, lowering surface tension, and increasing the stability of the vapor film around the part. Polymer quenchants retard cooling rates by the formation of films around the part.
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October 12, 2007 - Spray Quenching of Aluminum Alloys
 The quench rate for spray quenching is controlled by both the velocity and volume of water per unit area per unit time of impingement of the water on the workpiece; rate of travel of the workpiece through the sprays is an important variable. Local increases in temperature that occur within the first few seconds of quenching, caused by a phenomenon such as plugged spray nozzles, are particularly deleterious. The remaining "internal heat" could be sufficient to reheat the surface region, which causes a large loss in strength at the previously quenched surface. The loss of strength in the affected area of a heavy part is more severe than that caused by an inadequate quenching rate alone.
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October 5, 2007 - Water-Immersion Quenching of Aluminum Alloys
 Water-immersion quenching typically is controlled in practice by stipulating maximum quench-delay time and maximum water temperature. The first requirement controls the cooling rate during transfer and, for high-strength alloys, often is based on the criterion of complete immersion before the metal cools below 415°C (775°F). This specified temperature is based on a critical temperature for alloy 7075, which has one of the more severe C-curves. Therefore, the criterion for complete immersion of other alloys might be based on a temperature lower than the 415°C specification, depending on the characteristics of the particular C-curve.
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September 28, 2007 - Effects of Quench Delay of Solution Treated Aluminum Alloys
 Whether the transfer of parts from the furnace to the quench is performed manually or mechanically, it must be completed in less than the specified maximum time. The maximum allowable transfer time or "quench delay" varies with the temperature and velocity of the ambient air and the mass and emissivity of the parts. Maximum quench delays can be determined from cooling curves that will ensure complete immersion before the parts cool below 400°C (750°F).
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September 21, 2007 - Quench Rate vs. Properties of Solution Treated Aluminum Alloys.
 Average quench rates are useful to compare experimental results from various quench methods. However, average quench rates only compare results in a "critical" temperature range, where precipitation is most likely to occur. This method is not entirely accurate because significant precipitation can also occur outside the specified critical temperature range of average quench rates. In addition, for high-strength alloys, toughness and corrosion resistance could be impaired without significant loss of tensile strength. Therefore, the more sophisticated comparison called quench-factor analysis is required for quantitative property prediction or property optimization.
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September 14, 2007 - Quenching Solution Treated Aluminum Alloys
 The objective of quenching solution treated components is to preserve the solid solution formed at the solution heat treating temperature by rapidly cooling to some lower temperature, usually near room temperature. This applies not only to retaining solute atoms in solution, but also to maintaining a certain minimum number of vacant lattice sites to assist in promoting the low-temperature diffusion required for zone formation. As a broad generalization, the highest strengths achievable and the best combination of strength and toughness are those associated with the most rapid quench rates. The effect of quench rate on mechanical properties also can depend on the desired temper. For example, in the underaged condition, a slow quench rate is more detrimental on ductility and fracture toughness. Strength would be more affected after near-to-peak aging. The relative effects of quench methods can be compared in terms of average quench rates.
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September 7, 2007 - Solution Treating Time for Aluminum Alloys
 Heat treatment to increase the strength of aluminum alloys involves solution heat treatment (dissolution of soluble phases), quenching (development of supersaturation), and age hardening (precipitation of solute atoms). The time at the nominal solution heat treating temperature (soak time) required to effect a satisfactory degree of solution of undissolved or precipitated soluble phase constituents and to achieve good homogeneity of the solid solution is a function of microstructure before heat treatment. The time can vary from less than a minute for thin sheet product to as much as 20 h for large sand or plaster-mold castings. The time required to heat the load to the treatment temperature in furnace heat treatment also increases with section thickness and furnace loading, and, thus, total cycle time increases with these factors.
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August 31, 2007 - GP Zones in Precipitation Hardening Aluminum Alloys
 The exact size, shape, and distribution of Guinier-Preston (GP) zones depend on the alloy in which they form and on the thermal and mechanical history of the specimen. GP zones essentially are distorted regions on the matrix lattice, rather than discrete particles of a new phase having a different lattice. Thus, they are completely coherent with the matrix, imposing local but often large strains on it. These mechanical strains, as well as the presence of a locally solute-rich, sometimes ordered lattice, can account for large changes in mechanical properties of the alloy before any long-range microstructural changes occur. GP zones are metastable, and, thus, dissolve in the presence of a more stable precipitate. This dissolution causes a precipitate-free, visibly denuded region to form around the stable precipitate particles.
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August 24, 2007 - Precipitation from Solid Solution in Aluminum Alloys
 An essential feature of a precipitation-hardening alloy system is a temperature-dependent equilibrium solid solubility characterized by increasing solubility with increasing temperature. The major aluminum alloy systems with precipitation hardening include:
- Aluminum-copper systems with strengthening from CuAl2
- Aluminum-copper-magnesium systems (magnesium intensifies precipitation)
- Aluminum-magnesium-silicon systems with strengthening from Mg2Si
- Aluminum-zinc-magnesium systems with strengthening from MgZn2
- Aluminum-zinc-magnesium-copper systems
The general requirement for precipitation strengthening of supersaturated solid solution involves the formation of finely dispersed precipitates during aging treatments. Aging must be accomplished not only below the equilibrium solvus temperature, but also below the metastable miscibility gap called the Guinier-Preston (GP) zone solvus line.
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August 17, 2007 - Effect of Precipitation on Aluminum Alloy Properties
 Formability is the extent to which a material can be deformed in a particular process before the onset of failure. Aluminum sheet and aluminum shapes usually fail by localized necking or by ductile fracture. Precipitation-strengthened aluminum alloys usually are formed in the naturally aged (T4) condition, or in the annealed (O) condition, but only very rarely in the peak-strength (T6) condition where both the necking and fracture limits are low. Curves can be plotted for most of the precipitation-strengthened alloys in the 2xxx and 6xxx series showing the effect of a wide range of precipitation structures on some of the forming properties.
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August 10, 2007 - Artificial (Over)Aging Aluminum Alloys
 Artificial aging includes exposure to temperatures above room temperature to produce the transitional (metastable) forms of the equilibrium precipitate of a particular alloy system, which remain coherent with the solid-solution matrix, thus contributing to precipitation strengthening. Further heating at the temperatures that cause strengthening, or at higher temperatures causes the precipitates to grow, but even more importantly, to convert to equilibrium phases, which generally are not coherent. These changes soften the material, and if carried further, produce the softest or annealed condition.
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August 3, 2007 - Artificial Aging in Aluminum Alloys
 The rates and amounts of the changes in the strength and hardness of Al-Cu alloys can be increased by holding the alloys at moderately elevated temperatures (for alloys of all types, the useful range is about 120 to 230°C, or 250 to 450°F). This treatment is called artificial aging or precipitation heat treating. In the Al-Cu system, alloys having as little as 1% Cu, slowly quenched, start to harden after about 20 days at a temperature of 150°C (300°F). The alloys of this system having less than about 3% Cu show little or no natural aging after low cooling-rate quenching, which introduces little stress.
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July 27, 2007 - Natural Aging in Aluminum Alloys
 Natural aging refers to spontaneous formation of a Guinier-Preston (G-P) zone structure during exposure at room temperature. Solute atoms either cluster or segregate to selected atomic lattice planes, depending on the alloy system, to form the G-P zones, which are more resistant to movement of dislocations through the lattice, and, therefore, are stronger. Of the binary alloys, aluminum-copper alloys undergo natural aging after being solution heat treated and quenched. The amounts by which strength and hardness increase become larger with time of natural aging and with the copper content of the alloy, from about 3% to the limit of solid solubility (i.e., 5.67%).
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July 20, 2007 - Heat-Treatable Aluminum Alloys
 Heat-treatable (precipitation-hardening) aluminum alloys for wrought and cast products contain elements that decrease in solubility with decreasing temperature, and in concentrations that exceed their equilibrium solid solubility at room temperature and moderately higher temperatures. Heat treatment for precipitation strengthening includes a solution heat treatment at a high temperature to maximize solubility, followed by rapid cooling or quenching to a low temperature to obtain a solid solution supersaturated with both solute elements and vacancies. The heat treatment is designed to maximize the solubility of elements that precipitate in subsequent aging treatments, which may include either natural aging or artificial aging.
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July 13, 2007 - Strain Hardening of Aluminum Alloys
 Strain hardening by cold rolling, drawing, or stretching is a highly effective way to increase the strength of non-heat treatable aluminum alloys. The increases in strength that accompany increasing reduction by cold rolling are obtained at the expense of ductility, as measured by percent elongation in a tensile test and by reducing formability in operations such as bending and drawing. All mill products can be supplied in the strain-hardened condition, although there are limitations on the amount of strain hardening that can be applied to products.
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July 6, 2007 - Second-Phase Strengthening of Aluminum Alloys
 Elements and combinations that form predominantly second-phase constituents with relatively low solid solubility include Fe, Ni, Ti, Mn, and Cr, and combinations thereof. The presence of increasing volume fractions of intermetallic-compound phases formed by these elements and the elemental Si constituent formed by Si during solidification or by precipitation in the solid state during post-solidification heating also increase strength and hardness. These irregularly shaped particles form during solidification and occur mostly along grain boundaries and between dendrite arms. For alloys that consist of both solid solution and second-phase constituents and/or dispersoid precipitates, all of these microstructural components contribute to strength in a roughly additive manner.
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June 29, 2007 - Solid-Solution Strengthening of Aluminum Alloys
 The predominant objective in the design of aluminum alloys is to increase strength, hardness, and resistance to wear, creep, stress relaxation, and fatigue. Strengthening in non-heat treatable alloys is achieved through solid-solution formation, second-phase microstructural constituents, dispersoid precipitates, and/or strain hardening. The principal alloys strengthened by alloying elements in solid solution are those in the Al-Mg (5xxx) series, ranging from 0.5 to 6 wt% Mg. These alloys often contain small additions of transition elements, such as Cr and Mn, and less frequently Zr, to control the grain or subgrain structure, and Fe and Si impurities, which usually are present in the form of intermetallic particles.
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June 22, 2007 - Strengthening Aluminum Alloys
Two most common methods to increase the strength of aluminum alloys are to:
- Disperse second-phase constituents or elements in solid solution and cold work the alloy (non heat treatable alloys)
- Dissolve alloying elements into solid solution and precipitate them as coherent submicroscopic particles (heat treatable or precipitation-hardening alloys)
 Elements most commonly present in commercial aluminum alloys to provide increased strength, particularly when coupled with strain hardening by cold working or with heat treatment (or both) are copper, magnesium, manganese, silicon, and zinc. These elements all have significant solid solubility in aluminum, and in all cases, the solubility increases with increasing temperature. Of all the elements, zinc has the highest solid solubility in aluminum (a maximum of 66.4 at.%). The maximum solid solubility in aluminum alloys occurs at the eutectic, peritectic, or monotectic temperature.
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June 15, 2007 - Principal Commercial Aluminum Alloys
Aluminum alloys encompass more than 300 commonly recognized alloy compositions and many additional variations developed in supplier/customer relationships. The principal types of alloys are age-hardening alloys, casting alloys, and work-hardening alloys. All commercial aluminum alloys contain some iron and silicon, as well as two or more elements intentionally added to enhance properties. The principal types of aluminum alloys achieve strengthening through the alloying effects of cooper, magnesium, manganese, silicon, and zinc, in conjunction with strain hardening, heat treatment, or both.
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June 8, 2007 - Laser Hardening Gray Cast Iron
Laser surface heat treatment (or laser transformation hardening) is used to harden localized areas of gray iron machine components. The heat generated by the absorption of the laser beam is controlled to prevent melting, and, therefore, is used in the selective austenitization of local surface regions, which transform to martensite as a result of rapid cooling (self-quenching) by the conduction of heat into the bulk material of the workpiece. No change in chemical composition is produced by laser transformation hardening, and the process, as with induction and flame hardening, provides an effective technique to harden ferrous materials selectively.
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June 1, 2007 - Induction Hardening Gray Cast Iron
Gray cast iron can be surface hardened using the induction heating method, but there may be considerable variation in the hardness due to a variation in the combined carbon content. A minimum combined carbon content of 0.40 to 0.50% C (as pearlite) is recommended. The recommended minimum induction hardening temperature for gray iron is 870 and 925°C (1600 and 1700°F). The surface hardness achieved is influenced by the carbon equivalent (%C + 1/3% Si) when hardness is measured using conventional Rockwell tests. The more graphite present in the microstructure, the lower the surface hardness will appear to be after hardening.
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May 25, 2007 - Flame Hardening Gray Cast Iron
Flame hardening is a method of surface hardening commonly applied to gray iron, resulting in a hard, wear-resistant outer layer of martensite and a core of softer gray iron. Both unalloyed and alloyed gray irons can be flame hardened. One of the most important aspects of chemical composition is the combined carbon content, which should be in the range of 0.50 to 0.70%. Gray iron castings to be flame hardened should be as free as possible from porosity, sand, and slag, which can produce a rough surface or result in cracking after hardening. The surface of flame-hardened gray iron typically has a slightly lower hardness than the metal immediately below the surface, possibly caused by the retention of relatively soft austenite at the surface. The choice of quenching medium is influenced by the flame hardening method used.
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May 18, 2007 - Effect of Austempering Temperature on Properties
Gray iron usually is quenched in salt or oil at a temperature between 230 and 425°C (840 and 900°F) for austempering. The quench bath usually is held at a temperature between 230 and 290°C (450 and 550°F) when high hardness and wear resistance are the ultimate aim of the treatment. The required holding time to achieve maximum transformation is determined by the temperature of the quenching bath and the chemical composition of the iron. The effect of chemical composition on the holding time can be considerable, and alloy additions such as Ni, Cr, and Mo increase the time required for transformation. Casting shape and section thickness determine the amounts of the added hardenability agents (Ni, Cu, Mo), because cooling must be fast enough to prevent any transformation of austenite until the casting reaches the temperature of the bath.
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