Metallic magnesium can be produced by several extractive metallurgy processes; however, the most widely used process involves precipitating magnesium in dolomite [CaMg(CO3)2] and seawater as insoluble magnesium hydroxide [Mg(OH)2], which is then treated with hydrochloric acid to produce magnesium chloride. The MgCl2 is fed into electrolytic cells where electricity is used to convert it to magnesium metal and chlorine gas.3
Wrought magnesium alloys, like other alloys with the HCP structure, are much more formable at elevated temperatures than at room temperature. Wrought alloys are usually formed at elevated temperatures; room temperature forming is used only for mild deformations around generous radii. Minimum bend radii for annealed sheet formed at room temperature are 5-10T, and 10-20T for work hardened sheet, where T is the sheet thickness.
Forming magnesium alloys at elevated temperatures has several advantages: (1) forming operations can usually be conducted in one step without the need for intermediate anneals; (2) parts can be made to closer tolerances with less springback; and (3) hardened steel dies are not necessary for most forming operations. The approximate formability of magnesium alloy sheet is indicated by its ability to withstand bending over a 90° mandrel without cracking. The formability depends on composition and temper, material thickness, and forming temperature. With correct temperatures and forming parameters, all magnesium sheet alloys can be deep drawn to about equal reductions.
Since magnesium is a rather soft metal, both the part and forming tools should be clean and free of scratches, and a forming lubricant, such as colloidal graphite, must be used. In most hot forming operations, both the sheet and the tools are heated. Acceptable heating methods include electric cartridge heaters embedded in dies, radiant heating, infrared, gas, and heat transfer fluids. Lubrication is more important in hot forming than cold forming, because the tendency of galling increases with increasing temperature.
For severe forming operations, the annealed O temper is preferred. Sheet in the partially hardened temper, such as H24, can be formed to a considerable extent but the time at temperature will cause softening and a reduction in properties. It should also be noted that the time at temperature is cumulative if multiple forming operations are involved. AZ31-H24 sheet is normally formed at temperatures less than 325° F to avoid excessive annealing and lower than desired properties.
For deep drawing operations, magnesium alloys can be cold drawn to a maximum reduction of 15-25% in the annealed condition. The cold drawability of AZ31-O is about 20%. Cold drawn parts are stress relieved at 300° F for 1 h after the final draw to eliminate the possibility of cracking from residual stresses. Although both hydraulic and mechanical presses can be used, hydraulic presses are preferred because they are slower and easier to control. Hot drawing has the advantage that the operation can usually be conducted in one step. For example, AZ31-O can be hot drawn up to 68% in a single operation.
Both magnesium sheet and extrusions can be stretch formed. Sheet is usually heated to 325-550° F and slowly stretched to the desired contour. AZ31-O sheet is usually stretch formed at 550° F without a change in mechanical properties, while AZ31-H24 is usually formed at 325° F for times less than 1 h to prevent an appreciable strength loss.
Sand casting is the most economical method of producing low volume castings, which explains why it is often the process of choice for the aerospace industry. The reactivity of magnesium causes reactions between the liquid metal and water in the sand of "green" sand molds or oxygen in dry sand molds. These reactions cause a blackening of the casting skin to an appreciable depth with local porosity and gray oxide powder effects, called burning. To avoid these defects, which adversely affects strength, the sand is mixed with inhibitors such as 0.4-0.8% potassium fluoroborate or sodium silica fluoride. Magnesium casting alloys are normally melted in a low carbon steel crucible. The metal can be either poured from the steel crucible or transferred to a ladle for pouring. Molten magnesium alloys tend to oxidize and burn in air; therefore, molten surfaces must be protected from air. Although there are both flux and flux-less processes, the flux-less process is the most widely used. In the flux-less process, either a protective atmosphere of air/sulfur hexafluoride gas or air/carbon dioxide/sulfur hexafluoride gas mixture is used to eliminate the contamination problems inherent in solid chloride fluxes.
Grain refinement is an important aspect of magnesium alloy sand castings. The Mg-Al and Mg-Al-Zn alloys are usually grain refined by carbon inoculation with hexachloroethane or hexachlorobenzene compressed tablets. Grain refinement is attained due to the formation of aluminum carbide (Al4C3), which provides heterogeneous nucleation sites. The release of chlorine from the tablets also helps to remove hydrogen gas from the melt. Zirconium is added to the non-aluminum containing magnesium castings alloys to refine grain size. Zirconium cannot be used for grain refinement in the aluminum-containing magnesium alloys because it forms a brittle intermetallic alloy with aluminum. Along with grain refiners, manganese chloride (MnCl2) is added to the melt to precipitate out iron impurities. After the alloy melt is stirred, the molten metal is allowed to stand for about 15 min to allow the Al-Mn-Fe intermetallic compounds to form and settle to the bottom of the melting pot. After pouring, the precipitated sludge of Al-Mn-Fe compounds is removed from the bottom of the melting pot. Grain size during casting is often checked by pouring a small bar along with the casting and then fracturing it and comparing the fracture appearance to a known set of samples that have different grain sizes. For aerospace castings, tensile test bars are also poured during casting. In addition, a complete destructive analysis of castings may be required on a sampling basis.
Gravity pouring is normally used for magnesium sand castings. The metal flows down a sprue and into the runner system. The sprues are tapered to help keep air from entering the casting. Gating practices are important because turbulence during pouring can result in surface oxides and dross being folded into the flowing metal causing inclusions or surface pitting. Screens or filters are frequently used to remove oxide films and dross. Advancements in sand composition and core manufacturing now allow quite complex magnesium alloy castings to be cast. They are frequently used for complex gear box housings that contain integral small diameter oil cooling holes surrounded by thin wall compartments. A typical cast gearbox housing is shown in Fig. 3.6.
Magnesium alloys are cast in permanent molds when the number of parts justifies the higher cost of the tooling. The mechanical properties of sand and permanent mold castings are comparable, but tighter dimensional control and better surface finishes result from permanent mold casting. As a result of the rather slow solidification rates in both sand and permanent mold castings, a heat treatment is usually required to obtain acceptable properties.
Manufacturing Technology for Aerospace Structural Materials 3.3.3 Magnesium Heat Treating6
Wrought magnesium alloys can be annealed by heating to 550-850° F for 1 to 4h to produce the maximum anneal practical. Because most forming operations are done at elevated temperatures, the need for full annealing is less than with many other metals.
Stress relieving is used to remove or reduce residual stresses in wrought magnesium alloys produced by cold and hot working, shaping and forming, straightening, and welding. Stress relieving is generally conducted at 300-800° F for times ranging from 15 to 180 min. Castings are also stress relieved for a variety of reasons: (1) to prevent stress corrosion cracking for magnesium castings containing more than 1.5% aluminum, especially if the casting has been weld repaired; (2) to allow precision machining of castings to close dimensional tolerances; and (3) to avoid warpage and distortion in service. Although magnesium castings do not normally contain high residual stresses, even moderate residual stresses can cause large elastic strains due to magnesium's low modulus of elasticity. Residual stresses can result from non-uniform contraction during solidification, non-uniform cooling during heat treatment, machining operations, and weld repair. Welded Mg-Al-Zn castings that do not require solution heat treatment after welding should be stress relieved 1 h at 500° F to eliminate the possibility of stress corrosion cracking. Likewise, Mg-Al-Zn wrought alloys require stress relieving after cold forming to prevent stress corrosion cracking.
Although magnesium alloys do not attain the high strengths that aluminum alloys experience during precipitation hardening, there is some strength benefit to heat treatment for a number of the casting alloys. The solution heat treatment helps to reduce or eliminate the brittle interdendritic networks in the as-cast structure. Thus, solution-treated castings have better ductility than as-cast alloys with some increase in strength. The most prevalent precipitation hardening treatments for cast magnesium alloys are solution treating and naturally aging (T4), naturally aging only after casting (T5), and solution treating and artificially aging (T6).
For solution heat treatment, the parts are usually placed in a preheated furnace (500° F) and slowly heated to 735-980° F. Solution heat treating furnaces are usually electrically heated or gas-fired controlled to ±10° F and are equipped with fans to maximize circulation. To prevent excessive surface oxidation during solution heat treating, protective atmospheres of sulfur hexafluoride, sulfur dioxide, or carbon dioxide are used. The furnaces are also equipped to handle a fire in case the furnace malfunctions and overheating occurs. In the event of a fire, boron trifluoride gas can be pumped into the furnace. Although there are exceptions, slow heating to the solution treating temperature is recommended to avoid melting of eutectic compounds with the subsequent formation of grain boundary voids. The parts are held at the solution heat treating temperature for times in the range of 16-24 h. These hold times are long because the solution treatment also serves the purpose of homogenizing the cast structure. Castings often require support fixtures during solution heat treating to prevent them from sagging under their own weight. Some magnesium alloys are subject to excessive grain growth during solution heat treating; however, there are special heat treatments available to minimize grain growth.
Magnesium is normally quenched in air following the solution treatment. Still air is usually sufficient but forced air cooling is recommended for dense loads or parts that have thick sections. Hot water quenching is used for the alloys QE22 and QH21 to develop the best mechanical properties. Glycol quenchants can also be used to help prevent distortion. Artificial aging consists of reheating to 335-450° F and holding for 5-25 h. Hardness cannot be used for verification of heat treatment. For cast products, tensile test specimens can be either cut from a portion of the casting or cast as separate tensile test bars.
Magnesium is extremely easy to machine at high speeds using greater depths of cuts and higher feed rates than other structural metals. Dimensional tolerances of a few thousandths of an inch are possible with surfaces finishes as fine as 3-5 ^in. Machining is usually conducted dry; however, cutting fluids can be used to reduce the chances of distortion and minimize the danger of fire when chips are fine. Fine finishing cuts are a greater fire hazard than heavier roughing cuts. When magnesium chips ignite, they burn with a brilliant white light. To reduce the fire hazard when machining magnesium: (1) use only sharp tools; (2) use heavy feeds to produce thick chips; (3) use mineral oil coolants, especially during fine finishing cuts; (4) remove chips frequently from the work area and store in clean covered metal containers; and (5) keep an adequate supply of recommended magnesium fire extinguishers at all work areas.
Magnesium alloys can be welded by gas shielded arc welding and by resistance spot welding. The GTAW process uses a tungsten electrode, magnesium alloy filler wire, and an inert gas, such as argon or helium, for shielding. In the GMAW process, a continuously fed magnesium alloy wire acts as the electrode for maintaining the arc while an argon gas shield prevents oxidation of the weld puddle. No flux is required and welding operations are similar to those for aluminum alloys. Welds in magnesium alloys are characterized by a fine grain size, averaging less than 0.01 in. Welding problems due to residual stresses and the tendency for certain alloys to crack can be minimized by preheating, post-weld heating, and stress relieving. In the Mg-Al-Zn alloys (AZ31, AZ61, AZ63, AZ80, AZ81, AZ91 and AZ92), aluminum contents of up to 10% aids weldability by refining the grain structure, while zinc contents of more than 1% increases hot shortness which can cause weld cracking. Weld joints in the Mg-Al-Zn alloys and alloys containing more than 1% aluminum require stress relieving, because they are subject to stress corrosion cracking if not stress relieved. Stress relief is usually conducted by heating to 300-800° F for times ranging from 15 to 120 min. Alloys with high zinc contents, such as ZH62, ZK51, ZK60, and ZK61, are very susceptible to weld cracking and have poor weldability. Weld repaired castings are normally heat treated after welding to either the T4, T5, or T6 tempers. If the casting is not heat treated after weld repair, it is usually stress relieved.
Although magnesium sheet can be spot welded, it is usually not because the fatigue strength of spot welded joints is lower than that for either riveted or adhesive bonded joints. Thus, spot welding should not be used for joints in fatigue or vibration environments. In riveted joints, only galvanically compatible rivets, such as 5056 aluminum, should be used. Quarter-hard 5056-H32 aluminum rivets are satisfactory for normal riveting.
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