Aluminum-Magnesium-Silicon (6000) alloys are characterized by their primary components magnesium and silicon, which form Mg2Si precipitates responsible for age hardening. These alloys often include iron correctors like manganese or chromium, and small amounts of copper or zinc to enhance strength while maintaining corrosion resistance. The proper Mg/Si ratio for Mg2Si formation is 1.73, though practical tolerances result in either magnesium or silicon excess, affecting corrosion resistance and mechanical properties. These alloys demonstrate excellent strength-to-corrosion resistance ratios, good weldability, and are extensively used in architectural applications and electrical conductors due to their favorable combination of mechanical properties and electrical conductivity.
Aluminum-magnesium-silicon alloys, commonly known as 6000 series alloys, represent one of the most versatile and widely used aluminum alloy systems. The fundamental strengthening mechanism in these aluminum-magnesium-silicon alloys relies on the formation of Mg2Si precipitates through controlled heat treatment and aging processes.
The composition range of aluminum-magnesium-silicon alloys currently in commercial use varies significantly based on intended applications. The main alloying elements are magnesium and silicon, which combine to form the strengthening phase Mg2Si. Iron correctors such as manganese or chromium are frequently added to control the detrimental effects of iron impurities.Small amounts of copper or zinc are occasionally incorporated to improve strength without substantial loss of corrosion resistance. Boron additions in conductor applications serve to remove titanium and vanadium through boride formation. Zirconium or titanium additions help control grain size during processing. Lead and bismuth are sometimes added to improve machinability, though they prove less effective in aluminum-magnesium-silicon alloys compared to magnesium-free compositions.
The theoretical optimal ratio for Mg2Si formation is Mg/Si = 1.73, but achieving this precise ratio proves impossible with ordinary operating tolerances. Consequently, most aluminum-magnesium-silicon alloys contain either magnesium or silicon excess. Magnesium excess leads to better corrosion resistance but results in lower strength and reduced formability. Silicon excess produces higher strength without compromising formability and weldability, though it may create some tendency toward intergranular corrosion.
The microstructure of aluminum-magnesium-silicon alloys remains relatively simple compared to other aluminum alloy systems. The main constituent is Mg2Si, which dissolves into solution during heat treatment and subsequently precipitates during artificial aging, providing the characteristic age hardening response.When sufficient copper and silicon are present, the strengthening phase may be replaced, at least partially, by Cu2Mg8Si6Al5, which produces hardening during both natural and artificial aging processes.
Iron may exist in various intermetallic forms including FeAl3, FeAl6, Fe2SiAl8, or FeMg3Si6Al8 in manganese- and chromium-free aluminum-magnesium-silicon alloys. In alloys containing manganese and chromium, iron combines with these elements to form more complex intermetallics. Zinc typically remains in solid solution, while boron, titanium, and zirconium are seldom added in sufficient quantities to produce visible compounds.Spheroidizing of Mg2Si precipitates can be achieved through additions of phosphorus, sulfur, tellurium, iodine, or ferric chloride, which can improve mechanical properties and workability.
The lattice parameter of commercial aluminum-magnesium-silicon alloys is primarily controlled by magnesium and silicon contents. Copper, manganese, chromium, and zinc are usually present in amounts too small to produce measurable effects on lattice parameters. Iron, titanium, and boron have no substantial influence on this property.In most aluminum-magnesium-silicon alloys, elements other than magnesium and silicon comprise less than 1% of the composition, with total alloying additions seldom exceeding 3%. This relatively simple chemistry means many physical properties do not differ substantially from ternary alloys or even pure aluminum.Surface tension, density, thermal expansion, length changes during aging, specific heat, shrinkage during freezing, and superconducting temperature all fall within experimental error of corresponding values for high-purity alloys or pure aluminum. Thermal conductivities are approximately 10-20% lower than pure aluminum.
Electric resistivity in aluminum-magnesium-silicon alloys is slightly higher than in ternary aluminum-magnesium-silicon compositions, typically ranging from 3.0-3.2 x 10-8Ωm (50-55% IACS) for alloys containing 0.4-0.5% Mg with slight silicon excess in the artificially aged condition.Higher magnesium, manganese, and copper contents reduce electrical conductivity, which is also lower in naturally aged tempers but may reach 55-60% IACS in annealed material. Aluminum-magnesium-silicon alloys with lower magnesium and silicon contents exhibit better conductivity but correspondingly lower strengths.Increasing iron or silicon content from 0.2% to 2% decreases conductivity by approximately 10%. Chromium, manganese, and especially titanium, vanadium, and zirconium markedly reduce electrical conductivity. Boron additions, which precipitate these elements as borides, are used to remove them in conductor applications.
Mechanical properties of commercial aluminum-magnesium-silicon alloys depend on the content of Mg, Si, Cu, and other alloying elements, treatment conditions (cold or hot working), and heat treatment parameters. Commercial alloys, particularly those containing manganese or chromium, may exhibit strengths approximately 10% higher than base compositions.High strain rates lead to somewhat improved properties in aluminum-magnesium-silicon alloys. Fully hardened alloys show some tendency toward intergranular fractures during tension testing, but manganese additions reduce this tendency. Silicon precipitates, forming as platelets, may be responsible for this brittleness.Compressive strength equals tensile strength even at elevated temperatures. Shear strength approximates 70% of tensile strength and remains unaffected by subzero temperatures or nuclear radiation. The modulus of elasticity is approximately 65 GPa, and notch sensitivity is very low.
Fatigue resistance values of 80-120 MPa are reported for wrought aluminum-magnesium-silicon alloys, though these values are strongly affected by testing conditions, corrosion, notches, and surface finish. Small amounts of cold work have no effect on fatigue resistance, while larger deformations reduce it.At low temperatures, strength increases dramatically, reaching values up to 80% higher at 70K compared to room temperature, without appreciable loss of ductility or fracture toughness. Higher temperatures decrease strength and increase ductility, though the decline is less rapid than in most alloys and approaches that of pure aluminum.
One important feature of aluminum-magnesium-silicon alloys, particularly valuable for electrical cable applications, is their ability to be under-aged without loss of corrosion resistance. This makes them relatively insensitive to long exposure to temperatures slightly above normal or short exposures to temperatures in the 350-600K range. Electric cables can therefore be overloaded and iced cables can be de-iced using current surges without permanent strength loss.
Creep resistance in aluminum-magnesium-silicon alloys is slightly below the yield strength for zero creep at room temperature, decreasing proportionally with increasing temperature. High creep resistance with conductivities above 50% IACS are claimed for alloys containing approximately 1% Mg2Si and small amounts of copper, nickel, manganese, beryllium, and zirconium.Iron provides mild strengthening effects up to 0.8-0.9% but with severe ductility loss, especially in castings. When corrected with manganese, some toughness improvement may result. Manganese, chromium, titanium, and zirconium all slightly harden aluminum-magnesium-silicon alloys, while copper appreciably increases strength. Nickel does not improve mechanical properties.
The corrosion resistance of commercial aluminum-magnesium-silicon alloys is excellent for their strength level, although somewhat inferior to pure aluminum or aluminum-magnesium alloys. Alloys containing more than 0.3-0.4% Cu or silicon excess may occasionally be susceptible to intergranular and stress corrosion, especially in artificially aged tempers. However, up to 0.3-0.4% Cu reduces pitting depth with only slight increases in weight loss.
Iron and nickel reduce corrosion resistance by increasing pitting susceptibility. Manganese and chromium absorb iron and reduce its detrimental effects while also combining with excess silicon to decrease its negative impact. Zinc, like magnesium, tends to make the matrix more electronegative, but in amounts up to 0.5% has little effect on corrosion resistance.Aluminum-magnesium-silicon alloy rivets used with aluminum-copper sheet are rapidly destroyed in corrosive conditions due to galvanic coupling effects.
The few aluminum-magnesium-silicon alloys used for casting contain approximately 2% Si, less than 1% Mg, and occasionally manganese. Castability is poor due to low eutectic content, which provides insufficient feeding during solidification. Best castability occurs at 2% Si content.Sensitivity to casting conditions is low in aluminum-magnesium-silicon alloys. Overheating of the melt has little effect on properties, but coarse interdendritic spacing may reduce heat treatment response.Hot shortness is highest along the Al-Mg2Si composition line and decreases with either magnesium or silicon excess, reaching maximum at approximately 0.5% Mg2Si content.
Homogenization of billets improves hot workability in aluminum-magnesium-silicon alloys, especially when followed by slow cooling to precipitate and spheroidize Mg2Si. Material with coarse Mg2Si precipitate particles exhibits better hot deformability than material with fine precipitate distributions that hinder dislocation movement.Two-step homogenization with the second step as high as 850K produces maximum homogeneity. Grain refinement leads to improved extrusion surface quality but lower formability, while small interdendritic spacing improves formability.
Hot workability is primarily a function of magnesium content in aluminum-magnesium-silicon alloys. Silicon, copper, and iron have little effect, while manganese, chromium, and lead reduce it slightly. Chromium, manganese, and especially zirconium tend to enhance the press effect and produce higher strengths.In aluminum-magnesium-silicon alloys containing Mg2Si, hot rolling tends to produce spheroidizing and coagulation of Mg2Si particles. The number of particles decreases as rolling temperature increases.
The most extensively used aluminum-magnesium-silicon alloys, especially for extrusions where cost is the primary consideration, have low magnesium and silicon contents and usually contain no manganese or chromium. Optimal iron content is approximately 0.15-0.20%; lower iron leads to coarse grain structure, while higher iron deteriorates reflectivity and surface appearance.
Annealing for recrystallization in aluminum-magnesium-silicon alloys is performed in the 600-700K range. Generally, higher temperatures require shorter times. However, at higher temperatures, magnesium and silicon tend to dissolve, yielding less soft material.Grain size is controlled more by nucleation than growth; fast heating proves more effective than large amounts of deformation for achieving fine grain structure. Recrystallization temperature and resulting softening are controlled by previous thermal history. Material solution treated and quenched before cold working and annealing softens more slowly and less completely than material previously annealed in the recovery range.
Heat treatment of aluminum-magnesium-silicon alloys is not overly critical. In many alloys, the temperature at which all soluble constituents dissolve is well below the incipient melting point. Heat treatment temperatures range from 720 to 850K. Solution treatment of wrought products requires very short times, reportedly on the order of seconds. High-temperature deterioration may result in dimensional growth.Repeated heat treatments have no substantial effects on properties, although magnesium loss may occur with prolonged heat treatment. Aluminum-magnesium-silicon alloys with low Mg2Si content are not overly sensitive to quench rates, but sensitivity increases with higher magnesium, silicon, copper, manganese, and chromium contents.
Natural aging is very slow in aluminum-magnesium-silicon alloys and produces only limited hardening, especially in copper-free compositions. Artificial aging in the 400-500K range for periods from 1 to 4 hours produces maximum hardening.Cold working after aging can increase strength but with significant ductility loss. Increases up to 40% in ultimate tensile strength and up to 60% in yield strength can be obtained by 75% deformation, but elongation may decrease by 80%. Copper reduces the effect of delayed aging, while iron and zinc have no appreciable effect. Manganese and chromium reduce grain boundary precipitation, thus reducing embrittlement and susceptibility to intergranular corrosion.
When proper technique and filler materials are used, aluminum-magnesium-silicon alloys are easily welded by both fusion and pressure methods. Strength of the fusion weld zone may be 80-90% of annealed strength unless weldments are heat treated afterward, in which case strength and fatigue resistance very close to maximum values can be achieved.Porosity, cracks, and flux inclusions substantially reduce weld strength, as does repeated welding. Fast welding produces better properties, while pressure welds achieve the same strength as base metal. Fatigue strength of butt welds is very low.
Corrosion resistance of welds closely matches that of base metal in aluminum-magnesium-silicon alloys. Precipitation at grain boundaries due to welding heat may cause brittleness near the weld zone. High silicon content in the weld, obtained by using aluminum-silicon filler material, substantially reduces cracking. Dilution of magnesium in the weld zone leads to cracking.The cracking tendency is minimized in aluminum-magnesium-silicon alloys with 1% Si, 1% Mg, and 0.2-0.6% Fe.
Machinability of aluminum-magnesium-silicon alloys is good, especially in aged tempers. Iron and manganese in normal amounts have little effect on machinability, while excess silicon reduces it slightly. Additions of cadmium, bismuth, and lead do not improve machinability as effectively as in magnesium-free alloys. Magnesium combines with these elements to produce hard particles that act as chip breakers but increase tool wear.
Aluminum-magnesium-silicon alloys are used extensively for architectural purposes, where bright etching or electropolishing and anodizing are often employed. Reflectivity and surface finish are therefore important considerations. Reflectivities of 75-80% can be obtained in commercial alloys that are electropolished and anodized. Use of high-purity material may raise reflectivity to 85%.Coring, segregation, uneven cooling, undissolved constituents, and high iron content lead to uneven brightness, streaks, and other surface defects in aluminum-magnesium-silicon alloys.
Aluminum-magnesium-silicon alloys represent a versatile and economical choice for applications requiring moderate strength combined with good corrosion resistance, weldability, and formability. Their age hardening characteristics, electrical conductivity, and surface finishing capabilities make them particularly suitable for architectural applications, electrical conductors, and general structural uses where the combination of properties and cost-effectiveness is paramount.
Total Materia Horizon contains property information for 30,000+ alumiums: composition, mechanical, physical and electrical properties, nonlinear properties and much more.
Get a FREE test account at Total Materia Horizon and join a community of over 500,000 users from more than 120 countries.