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6XXX Series Aluminum alloy is an Aluminum–Magnesium–Silicon family (6XXX series) alloy which is the most common and widely used in many sectors such as automobile, aircraft, marine and construction, owing to its low cost, high strength to weight ratio, good formability, weldability, excellent corrosion resistance, and higher thermal conductivity. The principal alloying elements in 6XXX series are Silicon (Si) and Magnesium (Mg) and they are controlling the mechanical properties of the alloy with the precipitation on heat treatment process. Considering all elements used in 6XXX alloys, the main objective for alloy development is to get high productivity with superior mechanical properties which means to create alloy chemistry for having easy flow during extrusion, less quench sensitivity, and high mechanical properties with short aging cycle after extrusion process. In this chapter, Alloy developments for aluminum alloys which is mainly used for automotive (6063 and 6082 alloy will be the main target) will be discussed and industrial practice results will be given. Many Optical microscope and SEM (Scanning Electron Microscope) pictures will be shared coming from the lab and industrial scale works. The characterization method to see the effect of alloy elements will be also one of the focuses on this chapter.
TRI METALURJI A.S., Yedpa Ticaret Merkezi, Atasehir-Istanbul, Turkey
*Address all correspondence to: emrah.ozdogru@trimetalurji.com
1. Introduction
Aluminum is third most abundant elements (the most abundant as a metal) found in the earth crust as bauxite. It has a face centered cubic crystal structure and it is a silvery-white, soft, ductile, and non-magnetic metal. The physical and mechanical of aluminum makes it the most broadly used metal after steel on the market. Aluminum is one of the lightest engineering metals, having a strength to weight ratio higher to steel and similar approaches for conductor cables against the copper.
Aluminum in its pure forms relatively soft and for this reason, the aluminum is alloyed with a range of different elements to achieve required mechanical and physical properties. The most important property of aluminum is its light weight with having density 2.7 g/cm3, which is about one-third of steel.
Aluminum alloys have excellent casting application with its low melting point, high degree of fluidity in molten stage. Structural materials require not only high strength to weight ratio, besides reasonable cost, but also high fatigue resistance. Aluminum alloys have some other attributes like good electrical and thermal conductivities, corrosion resistance, good workability, ductility, and strength which make aluminum alloy widely usable and useful materials. Because of its good formability and workability, it can be easily forged and extruded to any shape and size. Aluminum and its alloys when exposed to atmosphere, combines with oxygen to form a protective oxide coating which blocks further oxidation and protect the surface especially on the DC casting application. The formation of protective oxide coating makes it highly corrosion resistant. If the protective layer of aluminum is scratched, it will instantly reseal itself [1, 2].
Among the aluminum alloys, 6XXX series aluminum alloy which is the Al-Mg-Si family are the most common group and seen on the engineering applications such as construction, automobile, aircraft, marine and railway due to its high mechanical strength, corrosion resistance. 6XXX series commercial aluminum alloys do not promote the mechanical properties like either 2XXX or 7XXX series alloys, but the range is one of the most versatile heat-treatable aluminum and offer good corrosion resistance and formability with medium strength. The ability to improve their mechanical properties is very clear in 6XXX series. The reason behind the use of aluminum alloys in so many different purposes is the element diversity in the chemical composition and the ability to adjust the heat treatments as desired.
The requirements for producing high quality 6XXX series aluminum alloys are inevitably associated with the precision alloy design, innovative production technology and use versatile heat-treatment practices and enhancement of mechanical properties [3, 4, 5].
The principal alloying elements in 6XXX series are Silicon (Si) and Magnesium (Mg) and they are controlling the mechanical properties of the alloy with the precipitation on heat treatment process. The 6XXX series aluminum alloys are the best alloy for the extrusion process and one the most common alloys for extrusion application due to wide range profile production and focus for many applications such as construction, home appliances such as kitchen and furniture, electricity purposes, industrial application and automotive, rail and aerospace.
The focus of this review paper is to put together the latest knowledge available from various sources on alloy designation, industrial processing, development of properties and potential use of AA6XXX wrought alloys.
2. Chemistry and Its effect on phase formation in 6XXX aluminum alloys
The pure aluminum is very soft and easy to form. However, it is not possible to use in any engineering application. Therefore, aluminum is alloyed in order to increase strength. On 6XXX series aluminum alloys, the Mg and Si are the major alloying elements. The main strengthening mechanisms are precipitation or age hardening, work hardening and grain boundary strengthening. The purpose of adding Mg and Si is to from MgxSiy compound (stochiometric relationship will be detailed on further pages) inside the grain. The other popular alloying elements are Mn, Cr, Cu, Zr, Sc and Ti (mostly as a grain refiner). Fe is a trace element and always coming from the mineral and production of the pure aluminum. The final purpose of the alloy design is to establish a perfect microstructure to achieve best strength, ductility and toughness along with other properties like corrosion and fatigue. Figure 1 shows the summary of alloying elements in some common 6XXX series aluminum alloys.
By increasing the Mg and Si content on the alloy system, the mechanical strength improves and but also, the other applications such as process response, aging, and surface treatment will be affected. The well-known 6XXX series alloy grade according to Mg/Si ratio is given in Figure 2.
Mg and Si are major alloying elements and easily dissolute on the aluminum solid solution during solution heat treatment process [6]. Magnesium (Mg) and silicon (Si) which combine to form the Mg2Si precipitates. These precipitates occur in several forms which may be divided into the following three categories.
β” (beta double prime) Mg2Si, the smallest type of Mg2Si precipitate that is rod- shaped and contributes most to mechanical properties when densely dispersed.
β’ (beta prime) Mg2Si, a larger version of rod-shaped precipitate that grows from the β” category. The β’ precipitates have a negligible contribution to mechanical properties.
β (beta) Mg2Si, the largest Mg2Si precipitate that is cube-like in shape and due to its size Its not affecting to mechanical properties. Figure 3 shows typical view of MgxSiy precipitation in 6082 aluminum alloys.
The proper ratio for Mg2Si is Mg/Si = 1.73, but this is impossible to achieve with ordinary operating tolerances; thus, most alloys have either magnesium or silicon excess. Magnesium excess leads to better corrosion resistance but lower strength and formability; silicon excess produces higher strength without loss of formability and weldability, but there is some tendency to intergranular corrosion.
The formation of Mg2Si precipitates give rise to the simple eutectic system with aluminum. At elevated temperature, the solute element dissolves in the solid solution but because of the decrease in solubility at lower temperature forms age-hardenable Mg2Si precipitates. Figure 4 shows the Al-Mg2Si phase diagrams. According to the phase diagram, with increasing amount of Mg and Silicon, peritectic point which means Mg2Si precipitation formation starts to decrease the eutectic reaction starts earlier than the expectation.
Figure 4.
Al-Mg2Si pseudo binary phase diagram [7].
On the practical application such as casting, homogenization and extrusion process, the temperature control is becoming vital parameters to achieve mechanical strength and better surface properties of the aluminum material.
In other words, looking at the alloy simulation program such as JMAT-PRO®, It’s also inevitable seen that the evaluation of Mg2Si formation itself is not enough, since the formation of excess silicon or excess magnesium condition. By increasing the Mg and Si content on the alloy system, the peritectic point shifts to the aluminum solid solution side and peritectic points (temperature) also reduced with some point. Figure 5 shows two different Mg, Si content aluminum alloy peritectic point evaluation (on the phase diagram, The Iron content was not taking into account).
Figure 5.
Two main 6XXX series aluminum alloys peritectic reaction (a) Mg: 0.35% and Si: 0.50% (typical 6060 Al Alloys) (b) Mg: 0.7% and Si: 09% (typical 6082 Al alloys).
Figure 5 shows clearly that why EN AW 6082 aluminum alloy is more sensitive for production and why it should be extruded in low speed. The low eutectic point phases restrict the extrusion speed and not allowed to go up higher production speed. Precipitation of the alloying elements as coherent GP zones and coherent β′′ and hexagonal coherent β′ (Mg2Si), during aging treatment, provides strengthening due to the presence of coherency strain field around the precipitates, which interacts with the moving dislocations. Formation of nonequilibrium coherent hexagonal precipitation gives the highest precipitation strengthening in AA6XXX alloys. The typical phase transformation analysis by using DSC for standard EN AW 6082 aluminum alloys during again is given in Figure 6.
Iron (Fe) is also present in the alloys and combines with silicon (Si) and aluminum (Al) to form AlFeSi dispersoids. These intermetallic do not contribute to the strength of the alloy but, if they are not correctly processed, they will have a detrimental effect on the extrudability of the alloy. Accurate control of Fe contents in 6000 series alloys is important for surface finishing applications. Different levels of Fe will cause variations in glossiness response during anodizing. Fe is also known to reduce conductivity. As Its well known, the homogenization process is applied to DC casting billet in order to provide phase transformation for Fe based dispersoid. During homogenization, the needle shape Kβ-AlFeSi dispersoid transform into cubic shape. Figure 7 shows before and after homogenization microstructure of EN AW 6063 aluminum alloys.
DC casting is the most common and well-known method for 6XXX series. On the metallurgy of 6XXX series DC casting, the two main metallurgical issues which are MgxSiy precipitation and AlFeSi phase transformation, are being controlled in order to match required mechanical properties and surface effect, respectively.
To understand better of the effect of these two phases, the thermal history of the billet and extrusion process should be analyzed in detail. Figure 8 shows the thermal history of the 6XXX series aluminum alloys for billet casting and extrusion.
After the casting, the needle shape AlFeSi(Mn,Cr) based dispersoid and very low number of MgxSiy phase are formed. During the high temperature heat treatment, which is called solution heat treatment or homogenization process, all alloying elements starts to diffuse to the aluminum solid solution phases. However, due to the low diffusion rate of some elements such as Fe, Mn, and Cr, the dispersoids are formed but different morphology. Table 1 shows the diffusion rate (diffusion coefficient rate) of the elements in aluminum at 600°C.
Elements
D/cm2/s at 600°C
Elements
D/cm2/s at 600°C
Cu
4,8.10−9
Fe
3,8.10−10
Mg
2,1.10−8
Mn
2,5.10−10
Zn
1,6.10−8
Co
8,4.10−10
Si
1,6.10−8
Ni
8,1.10−10
Cr
1,3.10−12
Zr
2,4.10−12
Table 1.
Diffusion coefficient of elements in Al at 600°C.
The more important points regarding the diffusion coefficient are that during the solidification of the liquid aluminum alloy, the segregation zone is appearing on the surface of the DC cast billet. The solidification rate on the surface of the billet is so fast that elements cannot move on time to travel to the aluminum solute solution and remains on the dendrite arms and create the inverse segregation zone (ISZ) on the surface. Figure 9 shows typical ISZ microstructure after homogeneous.
The Inverse segregation zone is a superficial phenomenon and behaves different depending on casting parameters and alloy chemistry. Figure 10 shows SEM images of the different surface intermetallic formation on the surface of the billet.
As a result of uneven solidification on the surface, the segregation zone is occurred and, on this zone, the chemical composition is also uneven and different than the bulk material chemical composition. Figure 11 also shows the uneven element composition (weight%) on the inverse segregation zone area on typical EN AW 6060 aluminum alloys.
Figure 11.
The element compositional deviation on the inverse segregation zone area [8].
During homogenization heat treatment, The needle shape of AlFeSi(Mn,Cr) transforms to rod/round shape AlFeSi(Mn,Cr) phases as shown in Figure 7. This transformation is crucial for the following thermo-mechanic process such as extrusion. The needle shape AlFeSi(Mn,Cr) is very brittle and low eutectic temperature behavior and during the extrusion and open to create a surface defect such as pick-up. Besides the surface problem, the productivity of the extrusion (or extrusion speed) will also decrease, and the extrusion die life will be also affected by low phase transformation ratio. On the contrary, the higher amount of transformed AlFeSi (saying as cubic alpha-αc) will provide higher extrusion speed, better surface condition (or better esthetic) and increase die life. The aim for transformation ratio from needle shape (Kβ) to round shape (αc) should be minimum %95. However, the industrial practices are showing us the average ratio is between %85–95 depending on the furnaces type (batch or continuous type homogeneous furnace).
The second part of the homogenization process is cooling. During soaking period of homogenization process, all Mg and Si atoms dissolve in alpha aluminum phase due their high solubility and create super solute solution structure. The aim of the cooling process is to keep them in solute solution condition. However, it never happens even in high cooling rate. There is always some low percentage coherent β-Mg2Si present on the system.
The 6XXX series aluminum alloys are a quench sensitive group and that’s why, the quenching system should be well adjusted in billet homogenization and extrusion process as well. Figure 12 presents the quench effect vs. precipitation of the MgxSiy particles.
As Its clearly seen that on Figure 12, the faster cooling provides the more β” phase which is finer and gives better/reliable mechanical properties and faster extrusion speed.
By reducing the cooling rate, especially on EN AW 6082 aluminum alloy, creating the problem on mechanical problems and extrudability of the alloy. The soft cooling practices creates plate like (coarse) Mg2Si phases (shown in Figure 3b) and increase the precipitate free zone. On automotive purposes application such as crash box, the fast cooling is a must to get better folding characteristics. Figure 13 shows different folding characteristics based on different cooling application after extrusion process.
Copper has appreciable solubility and strengthening effect. The addition of Cu, in variable concentrations, produces substantial solid solution and precipitation strengthening. In presence of magnesium and silicon, copper produces age-hardening effects at room temperature. The weldability and corrosion resistance are decreased, and the weight of the alloy is increased with the copper addition. On 6043 alloys, copper gives more mirror-like surface effect after anodizing process. On the contrary, Increasing the iron level gives more dull surface effect. Copper addition to the 6XXX series aluminum alloys is more popular in automotive purposes profile production such as crash box and some side support profiles which is applied high deformation energy on the car. In case of copper addition to the alloy, the two new phases are forming on the billet microstructure. The first one is Al2Cu-ϴ and the second one Al5Cu2Mg8Si6-Q phase. Theta phase behaves like β (Mg2Si) phases and depending on the temperature incoherent and coherent phases precipitate and present different behavior. And Q phase is thermodynamically stable phases and enhance the response for long term thermal stability test for crash application. The other important alloying elements are Mn and Cr which give good homogenization response on the 6063 and 6060 aluminum side. And as for 6082 and 6005 A, The Mn and Cr control the grain growth during extrusion and refers to anti-recrystallization elements like other Zr and Sc elements. Figure 14 shows grain structure micrograph about the grain size control during the extrusion.
Figure 14 shows the effect of Cr element after the extrusion process in grain size. Cr addition creates AlCr based dispersoid and due to grain boundary pinning effect, the final grain size and its structure changed completely from abnormal grain size to equiaxed and fine grain size.
The planned additions of trace elements like Cr, Mn, Zr, and Sc can restrict the softening mechanisms during elevated temperature deformation. The fine grain structure ensures strengthening by grain refinement and work hardening. The trace elements show significant role in altering the physical, mechanical, and corrosion behavior of Al and its alloys.
Alloying elements B, Ti in some AA6XXX alloys cause the control of grain size and produce grain boundary strengthening of those alloys. The machinability can be higher by the addition of Bi and Pb in AA6XXX alloys. The solid solution strengthening to some extent can be caused by lattice parameter control in commercial alloys due to presence of Mg and Si in solid solution.
Among the aluminum alloys AA6XXX series is the most used alloy group for all engineering applications. Understanding of alloy chemistry and obtaining optimized production parameters and correct usage of materials generates high efficiency with reduced production costs. Even small changes of Si and Mg in alloy chemistry could create differences on the mechanical properties of alloy and combining this alloy chemistry with process parameters of heat treatment shows major progress on the alloy properties and usage.
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Written By
Emrah Fahri Ozdogru
Submitted: 06 December 2022Reviewed: 25 January 2023Published: 07 March 2023
Open access peer-reviewed chapter
