外文翻译-镁合金的腐蚀与应用.doc

Translation Corrosion Resistance of Aluminum and Magnesium Alloys Part Three Performance and Corrosion Forms of Magnesium and Its Alloys Edward GhaliCopyright 2010 by John Wiley & Sons, Inc. All rights reservedPublished by John Wiley & Sons, Inc., Hoboken, New JerseyPublished simultaneously in CanadaGhali, Edward. Printed in the United States of America Translator: yang chunyan Properties, Use, and Performanceof Magnesium and Its AlloysOverview Magnesium crystallizes in the hexagonal close packed structure and is therefore not amenable to cold forming. Cast and wrought magnesium alloys, powder metallurgy (P/M) prepared alloys, and metal matrix composites (MMCs) are described. Currently, the majority of large components of magnesium alloys are produced by high-pressure hot and cold chamber die casting, while an extensive number of ultra thin components are cast by thixomolding, gravity sand, low-pressure sand and metal die molding,and semisolid and squeeze casting. An example of sheet and plate alloys is AZ31 (Mg3Al1Zn0.3Mn), which is the most widely used. Magnesium composites containing boron, silicon carbide, and graphite are of increasing interest, particularly in the aerospace industry. The silicon containing-AS- based alloys show good creep properties at elevated temperatures and the Mg-Y-RE-Zr alloys show the highest creep performance. Corrosion due to poor design, flux inclusions, surface contamination, galvanic couples, and incorrectly applied or inadequate surface protection schemes is avoidable. Magnesium alloys are used in the automotive, aerospace, electronics, and guided weapons industries because of their light weight and high strength-to-weight ratio. The use of magnesium alloys in structural applications is the most active area. Wrought magnesium products, such as sheet and hydro-formed extrusions, and a variety of other magnesium products, whether semi solid or forged, are being developed for automotive and other applications at an increasing rate. Composites based on aluminum and magnesium matrices are of great interest to the automotive and aerospace industries.Corrosion resistance of alloys to natural and industrial atmospheres, and to aqueous solutions at different pH values and containing salts or organic compounds, is discussed. Performance in some dry gases or organic compound media at different temperatures is summarized. The effect of increasing the temperature is to increase the severity of attack. Mg is rapidly attacked by all mineral acids and alkalis except hydrofluoric and chromic acids. Severe corrosion may occur in neutral solutions of salts of heavy metals, such as copper, iron, and nickel. Moistened foreign materials on the surface can promote corrosion and pitting of some alloys unless the metal is protected by properly applied coatings. The presence of BF3 or SF6 in the ambient atmosphere is particularly effective in suppressing high-temperature oxidation up to and including the temperature at which the alloy normally ignites.A. PROPERTIES OF MAGNESIUM ALLOYS9.1 PHYSICAL AND GENERAL PROPERTIES OF MAGNESIUM The perception of magnesium as a rapidly corroding material has been a major obstacle to its growth in structural applications despite its other obviously desirable physical properties. In fact, under normal environmental conditions, the corrosion resistance of magnesium alloys is comparable to or better than that of mild steel. It has been the uneducated use of magnesium in wet, salt-laden environments that has given rise to its poor corrosion reputation. Corrosion due to poor design, flux inclusions, surface contamination, galvanic couples, and incorrectly applied or inadequate surface protection schemes is avoidable not only for magnesium but for many other metals as well. Progressively, designers and engineers in the magnesium industry are establishing the correct use of magnesium in corrosive environments and are developing methods to improve the corrosion resistance of magnesium alloys by modifying alloy chemistry and improving surface protection technologies 1,2. Discovered in 1774, magnesium is the sixth most abundant element, constituting 2% of the total mass of the Earths crust. The most important magnesium sources are magnesite MgCO3 (27%Mg), dolomite MgCO3-CaCO3 (13%Mg), and carnallite KCl-MgCl2-6H2O (8% Mg), as well as seawater, which contains 0.13% Mg or 1.1 kg Mg m3 (third most abundant among the dissolved minerals in seawater)3. Magnesium and its alloys are different and have some unique and inherent corrosion behaviors, phenomena, and kinetics as compared to other metals. Under normal environmental conditions, the corrosion resistance of magnesium alloys is generally comparable to or better than that of mild steel. Magnesium is silvery white in appearance. It is a divalent metal. The atomic mass is 24.32 and the specific gravity of the pure metal 1.738 at 20. The structure is hexagonal close packed. The lattice structure of magnesium has c/a1.624, and its atomic diameter (0.320 nm) is such that it enjoys favorable size factors with a diverse range of solute elements that have 15% difference in atomic size 2,4. Magnesium crystallizes in the hexagonal close packed structure and is therefore not amenable to cold forming. Below 225, only 00011120basal plane slipping is possible, along with pyramidal 1012 1011 twinning. Pure magnesium and conventionally cast alloys show a tendency for brittleness due to intercrystalline failure and local trans-crystalline fracture at twin zones or 0001 basal planes with big grains. Above 225, new 1011 basal planes are formed and magnesium suddenly shows good strength 3. Table 9.1shows the physical data and the properties of magnesium. (See also aluminum properties as compared to magnesium, Chapter 4.)Table 9.1 Physical Data and Properties of MagnesiumProperty MagnesiumReferencesAtomic mass 24.30506 g Chapter 4 18Tensile strength No dataMaximum oxidation number2+48Minimum oxidation number 0 Chapter 4 1Crystal structure hexagonal close packed48Spectroscopy (1s)2, (2s)2, (2p)6, (3s)2 48Ionization potential I (7.64 CV), II(15.0 CV) Chapter 4 1Potential standard -2.37V Chapter 4 13Pauling electronegativity 1.2Chapter 4 1Density at 201.741 g/cm 48Melting point649.548Boiling point 110748Specific heat at 201.030 KJ/kg4Thermal conductivity at 20 157.5W/m4Van der Waals radius0.16nmChapter 4 1Ionic radius0.065nmChapter 4 1Number of common isotope 3 (isotopes 24, 25,26) 48 The most widely used magnesium die casting alloy is AZ91 because of its super castability even for the most complex and thin-walled parts. Currently, the majority of large components of magnesium alloys are produced by high-pressure hot and cold chamber die casting, while an extensive number of ultra thin components are cast by thixomolding, gravity sand, low-pressure sand and metal die molding, and semisolid and squeeze casting. There are also processes that will be used more extensively in the future with Mg alloys, and processing parameters required to produce sound components. Magnesium has an important and growing future in the metalworking technologies of rolling, stamping, extrusion, and forging. The mechanical properties of cast and wrought magnesium components at ambient and elevated temperatures are greatly affected by their processing,and can differ considerably from test bar specimens. There are major efforts to improve the high-temperature mechanical properties of cast and worked magnesium alloys and extensive efforts to produce sheet, stampings and extrusions 5.A number of methods are available for the production of novel or nonequilibrium magnesium alloys with substantially improved corrosion resistance. Included in these methods are new rheocasting processes, rapid solidification processes, ion implantation, and vapor deposition. These processing methods typically enhance corrosion resistance by producing a more homogeneous microstructure, by increasing the solubility limits of alloying additions, or by some combination of both 6.9.2. PROPERTIES OF CAST MAGNESIUM ALLOYSCast magnesium alloys have always predominated over wrought alloys, particularly in Europe, where, traditionally, cast alloys have comprised 8590% of all magnesium products. The choice of a casting method for a particular part depends on factors such as the configuration of the proposed design, the application, the properties required, the total number of castings required, and the properties of the alloy 2.9.2.1. Designation of Cast Magnesium AlloysAn international code for designating magnesium alloys does not exist, although there has been a tendency toward adopting the method used by the ASTMB275-94. In this system, the first two letters indicate the principal alloying elements according to the following code: A, aluminum; B, bismuth; C, copper; D, cadmium; E, rare earths; F, iron; H, thorium; K, zirconium; L, lithium; M, manganese; N, nickel; P, lead; Q, silver; R, chromium; S, silicon; T, tin; W, yttrium; Y, antimony; and Z, zinc. The letter corresponding to the element present in greater quantity in the alloy is used first; if they are equal in quantity, the letters are listed alphabetically. Letters are followed by numbers that represent the nominal compositions of these principal alloying elements in weight percent, rounded off to the nearest whole number; for example, AZ91 indicates the alloy Mg-9Al-lZn, the actual composition ranges being 8.39.7% Al and 0.41.0% Zn. Suffix letters A, B, C are chronologically assigned and usually refer to purity improvement. X is reserved for experimental alloys. For heat-treated or work-hardened conditions, the designations are specified by the same system as that used for aluminum alloys 7. The two major systems of alloys, magnesium-aluminum and magnesium-zirconium are examined 2. As an example, the designation AZ91B-F indicates the following: AZ The two principal alloying elements B This is the second alloy developed with the above aluminum and zinc compositions, being principally used in die casting. In this case, the “B”indicates that a higher residual copper level (0.35%) is permitted. F The alloy is used in its as-cast condition. The wrought alloys may also be divided into two groups according to whether or not they contain zirconium.Specific alloys have been developed that are suitable for wrought products,most of which fall into the same categories as the casting alloys. The wrought alloys can be obtained in a number of tempers. The commonly used tempers are: T4, solution heat treatment only for 16 hours at 415 to homogenize the solution; T5, alloys artificially aged after casting; T6, alloys solution treated, quenched and artificially aged; T7, alloys solution treated and stabilized 2, 8.9.2.2. Alloying ElementsThe earliest commercially used alloying elements were aluminum, zinc and manganese and the Mg-Al-Zn system remains the most widely used for castings. Aluminum, zinc, cerium, yttrium, silver, thorium, and zirconium are examples of widely differing metals that may be present in commercial magnesium alloys. Apart from magnesium and cadmium,which form a continuous series of solid solutions, the magnesium-rich sections of binary-phase diagrams show peritectic or, more commonly, eutectic systems. Solubility data for binary magnesium alloys are given in Table 9.2; the first ten elements are those used in commercially available alloys 2.Table 9.2 Solubility Data for Binary Magnesium AlloysSolid solubilityElement Atomic % Weight %SystemLithium 17.00 5.50EutecticAluminum11.8012.70EutecticSilver3.80 15.00EutecticYttrium 3.7512.50EutecticZinc2.40 6.20 EutecticNeodymium 1.003.00EutecticZirconium1.00 3.80 PeritecticManganese1.002.20PeritecticThorium0.52 4.75EutecticCerium0.10 0.50EutecticCadmium 100.00 100.00Complete solid solubility Indium19.40 53.20 PeritecticThallium15.4060.50EutecticScandium 15.00 24.50Peritectic Lead7.7541.90EutecticThulium6.3031.80EutecticTerbium 4.60 24.00 EutecticTin 3.3514.50 EutecticGallium3.108.40EutecticYtterbium1.208.00 EutecticBismuth1.108.90 EutecticCalcium0.82 1.35 EutecticSamarium 1.006.40EutecticGold 0.10 0.80EutecticTitanium0.100.20PeritecticSources: References 10 and 11. Although early Mg-Al-Zn castings suffered severe corrosion in wet or moist conditions, the corrosion performance was significantly improved as a result of the discovery, in 1925, that small additions (0.2%) of manganese gave increased resistance. With this element, iron and certain other heavy metal impurities formed relatively harmless intermetallic compounds,some of which separate out during melting. In this regard, the classic work by Hanawalt et al. 9 showed that the corrosion rate increased abruptly once tolerance limits were exceeded; these tolerance limits are 5,170, and 1300 ppm for nickel, iron and copper, respectively. The corrosion rate of pure magnesium as a function of iron content is shown in Figure 9.1, which clearly illustrates the tolerance limit for iron 2, 9. Figure 9.1 Effect of iron on corrosion of pure magnesium following on alternate immersion test in3% NaCl 9. Another problem with earlier magnesium alloy castings was that grain size tended to be large and variable, often resulting in poor mechanical properties, microporosity, and in wrought products, excessive directionality of properties. Values of proof stress also tended to be low relative to tensile strength 2, 9.9.2.3. Cast Magnesium Alloys Series Pure magnesium ingots produced by the Pidgeon process or electrolytic process were utilized to produce ZM1 (Mg-4.5Zn-0.7Zr), ZM5 (Mg-8.2Al-0.5Zn-0.3Mn), and ZM6(Mg-2.4Nd-0.7Zr-0.4Zn). The alloys ZM5-A and ZM6-A, which were prepared using the pure magnesium ingots produced by the Pidgeon process (i.e., distilled magnesium), showed better corrosion resistance than ZM5-B and ZM6-B, which were prepared using the pure magnesium ingots produced by the electrolytic process (i.e., electrolytic magnesium) 12. For the same purity grade, the distilled magnesium may possess less impurity elements(Fe, Ni, Cu, and Cl) than electrolytic magnesium. The impurities, (e.g., Fe, Ni, Cu, chlorides)could accelerate the corrosion of these magnesium alloys, especially for ZM5, while alloying elements Zr and/or Nd could increase corrosion resistance of ZM1 and ZM6 12. The potential fields for magnesium alloys in the automotive industry are the segments of high-temperature applications. In the last 50 years, the main goal of alloy development has been to increase the high-temperature strength and creep resistance. Additions of silicon and rare earths as alloying elements improve the creep strength due to formation of high-temperature stable precipitates during solidification.Silicon containing AS-alloy, which had been used for gearbox housing in the Volkswagen Beetleand AE-alloys, which show even better creep properties but are more costly, were the state of the art for high temperature creep-resistant magnesium alloys at the end of the last century. Advanced magnesium alloys such as QE-alloys and WE-alloys containing silver (Q) and yttrium (W) in combination with rare earths (E) show even better creep properties at temperatures, that are suitable for applications in the power train or transmission housing of automobiles. Unfortunately, these alloys are not suitable for high-pressure die casting (HPDC). Magnesium producers and automobile companies developed a number of requirements for magnesium alloys to extend their applicability 13: Room temperature properties as good as AZ91 Elevated-temperature (120) properties better than AZ91 Castability comparable to AZ91 Corrosion properties comparable to AZ91E Creep properties better than AE42 Maximum increase in costs compared to AZ91In order to reach these objectives, first tests with modifications of the commonly used magnesium alloys AZ91 and AM50, using additions of silicon, rare earths, tin, calcium, or strontium, have been done. Alloys were developed and patented but most of them were never brought to commercial application. Close to series production are some alloy developed by Volkswagen in cooperation with the Magnesium Research Institute. Mechanical properties at room and elevated temperatures of some common and newly developed alloys are listed in Table 9.3 13.Table 9.3 Mechanical Properties of Common and Newly Developed AlloysPropertyAZ91 AE42ACM522MRI153MMRI230DAJ62x Ultimate tensile strength (UTS) at room temperature (RT) (MPa)260240 200 250 235 240Yield Strength (YS) at RT (MPa)160135158170 180143Elongation at RT (%)612 46 57UTS at 150(MPa)160 160175 190205 166YS at 150 (MPa) 105 100138135 150 116Elongation at 150 (%) 1822 17 1627Compressive YS at RT (MPa) 160 115 170 180 Compressive YS at 150 (MPa) 10585135150 Impact strength (J) 812 8 6Fatigue strength (MPa)10080 120110Corrosion rate (mg/cm2day)0.110.12 0.090.10 0.11Source: Reference 13. The silicon containing AS-alloys show good creep properties at