Microstructure of Aluminium Alloy 2618a for Recrystallization

Introduction

The usage of metallic materials for different purposes by mankind has started from decades (Ferguson, n.d.). There are different types of metals available and their properties are different. The use of metallic materials is inevitable in any field (Hanawa, 2012). Medical, Electrical, Mechanical and in almost every field the use of metals is enormous. The usages of metals differ with respect to their properties. Improvements in the properties of the materials are achieved by chemical, mechanical and thermal processes (Suryanarayana, n.d.). In this study, we are interested in aluminium alloy. The purest form of aluminium is relatively soft and has a yield strength of only 34.5 N/mm2(5,000 lb/in2) and a tensile strength of 90 N/mm2 (13,000 lb/in2) (P.G. Sheasby, n.d.). Most pure materials have relatively low strength. However, it can be improved by certain processes such as alloying, heat treatment, Coldworking etc. Alloying is a process of adding ingredients into the molten aluminium to improve the desired mechanical or chemical properties. The main alloying materials used to improve the properties of the aluminium are copper, magnesium, silicon, manganese, nickel and zinc (P.G. Sheasby, n.d.). The application of aluminium alloy in aerospace application replaces many materials which are not economical. In this research, we are specifically interested in the 2618a alloy which is a 2000 series alloy. 

 The alloying material use in 2000 series aluminium alloy is copper. The weight percentage of copper usually varies within 1-8 wt% depending upon various applications. They are the main alloy that finds use at elevated temperature. 2000 series aluminium alloys are extensively used in aerospace application due to their properties such as high strength, corrosion resistance, Specific modulus and reasonable ductility. When these materials are used in the aerospace application they are undergone multiple types of loads. In this study, we will be analyzing the microstructure of aluminium alloy 2618a to understand the recrystallization characteristics while undergoing compression loading and how it affects the mechanical properties of the alloy. 

1. Types of Aluminium alloy

 Aluminium alloys are divided into two based on how they are produced, wrought aluminium and cast aluminium. Both are further divided into heat treatable and non-heat treatable alloys. Figure 1 shows the classification on aluminium alloy.

Figure 1 Aluminium alloy classification

The metal which is subjected to mechanical working in the form of rolling, extrusion and forging results in the formation of wrought aluminium. The metal alloys which are prepared using casting is the cast alloys. There is a certain nomenclature procedure to identify the type of heat treatment and temper. The wrought aluminium compositions are generally represented in a four-digit number, whereas the cast composition is represented by a three-digit number followed by a decimal value (Davis, n.d.).

1.1 Properties and composition

Table 1 Wrought aluminium composition and properties. Adapted from (Davis, n.d.)

Cast Aluminium and its application

The alloy nomenclature for cast aluminium composition is represented in a 3-digit number with a decimal value in the end. The decimal value representing .0 in a cast aluminium represents the alloy limits, decimals 0.1 and 0.2 represents ingot compositions. Table 2 below shows the composition and applications of cast aluminium alloys

Table 2 Cast aluminium composition and properties. Adapted from (Davis, n.d.)

Mechanical behavior of aluminium alloy under tensile loading

Y. Chen et.al studied the stress-strain behaviour of aluminium alloys at a wide range of strain rates. (Y. Chen *, 2008) Automotive industries use a wide range of aluminium and its alloys due to their low weight. Understanding the stress-strain behaviour of the aluminium used in the automotive industry is inevitable since there are numerous tests carried out such as crash test. During the crashworthiness tests, there is a high deformation rate in the structural components. The overall mechanical properties of the materials used have a high significance in the load acting on the components and the energy absorption. Experimental analysis is carried out using the sample specimens of extruded AA6060, AA6082, AA7003 and AA77018 aluminium alloys in T6 temper. These samples are subjected to a wide range of strain rates. High rates of strain are induced using the split-Hopkinson tension bar and standard tensile test machine is used for low to medium strain rates. The experiment is carried out in all three directions with respect to the direction of extrusion due to the anisotropic behaviour of the extruded alloy. It is found from the results that the 6000-series alloy sample shows less sensitivity to the strain rate and the 7000 series exhibits a marked sensitivity in the strain rate. Among the recrystallized alloy AA6060-T6 the strength isotropy is found to be small and for the remaining samples of non-recrystallized alloys, the maximum strength is along the direction of extrusion with a minimum strength in the 45° direction.

3 Strengthening of Metals.

The pure form of metals usually possesses less strength. The strength of the metals are incorporated by mixing with other metals to tailor-made the mechanical properties and this process is called alloying. The ductility of the metal is sacrificed during the strengthening mechanism of the alloy. Numerous hardening techniques are used for improving the mechanical properties and the selection of the alloying metals depends on the application. The strength is incorporated by plastic deformation of the metals due to dislocation. (William D. Callister, n.d.) says “All strengthening techniques rely on the simple principle of restricting or hindering dislocation motion renders a material harder and stronger”.

3.1 Strain Hardening

Strain hardening, also known as work hardening is a strengthening mechanism which will improve the mechanical properties of the material. During strain hardening, the strength of the metals increases when it is plastically deformed. Straining of the materials occurs during cold working at the temperature below the absolute melting point (R. E. Smallman, 1999, p. 226). Metals are having a crystalline structure which has atoms arranged in a closely packed manner in three dimensions. When the metal is loaded the dislocation moves towards the grain boundary. These dislocations are stacked in the grain boundaries results in increasing the dislocation density. As the dislocation density increases, it marks the end of further dislocation and much more external forces are required. This process increases the mechanical properties of the metals such as strength and hardness, but the ductility reduces. When the external forces applied is so high to overcome this the metal will undergo cracking and due to this annealing is done for further working.

3.2 Grain size Hardening

The strengthening mechanism in which the grain size is altered to improve the strength of the metal is known as grain size hardening or hall-petch strengthening. In a polycrystalline metal, the role of grain size or the diameter of the grain is crucial to control the mechanical properties of the metals. When the size of the grain reduced the strength and toughness of the metals are improved. When a metal is plastically deformed tailed by a heat treatment the grain size can be reduced. The rate of solidification from the liquid phase also influence the reduction of grain size. (William D. Callister, n.d.)

3.3 Solid solution Strengthening

 Mixing of copper and tin will result in the formation of a strong metal has been discovered by human five thousand years ago. That was the first alloy created in the world and the process was solid solution strengthening. When a metal is in liquid form (solvent) during the casting process is mixed with a different metal in a liquid state (solute), the strength and hardness of the metal improves (gedeon, 2010). For a unalloyed metal, it is easy for the dislocation to move. When a metal is alloyed, it obstructs the movement of dislocation which required high stress or temperature for it to move. This results in increasing the strength and toughness of the alloy. There are two types of solid solution, Substitutional solution and interstitial solution.

3.3.1 Substitutional solution

In this type of solid solution, the solvent atom in the crystalline structure is replaced with the solute atom. The atoms of the different metals are of a different size which results in the irregularity in the crystal lattice. The movement of the dislocation is interrupted due to this irregularity in the crystal lattice thus improves the strength of the alloy. An example of such process is tin in bronze.

3.3.2 Interstitial solution

Consider the case of carbon in steel, the process is like that of the substitutional solution. In this case, the atoms in solute acquire the interstitial spaces between the solvent atom. When the solute atoms are fits in between the solvent atom in the crystal lattice, the movement of dislocation is restricted. This results in higher stress or temperature for the dislocation to move further.

3.4 Precipitation Hardening

Precipitation hardening is the widely used technique for the strengthening of the metal alloys. They are also called as age hardening since ageing is the main process involved in the precipitation hardening. Majority of the high strength metal alloys are produced by age hardening (skrócie, 2010). Dr Alfred Whilm, German metallurgist discovered age hardening by accident (1903-1909). Commercial alloy named duralumin was the first alloy discovered using this method of precipitation hardening. During the process of age hardening, small precipitates are formed in the second phase particles. The precipitates formed inside obstructs the dislocation to move through the crystalline structure thereby improve the strength of the metal alloys. Steps involved in the precipitation hardening is discussed below.

3.4.1 Solution treatment

The first step involved in precipitation hardening is solution treatment. The alloy is heated to a suitable temperature and socked until a homogeneous solid solution (α) is formed.

3.4.2 Quenching

The process of rapid cooling a metal from the elevated temperature into the room temperature is known as quenching.

3.4.3 Ageing

The material after quenching is subjected to ageing. In this process the material is allowed to precipitate at certain temperature for a fixed time. Aging of material at the room temperature is termed as natural ageing (Fransson, 2009). Ageing at an elevated temperature for the material to precipitate by heating is called artificial aging. The material properties depend on the ageing temperature and time. During ageing, strength and hardness increases to obtain a maximum value and further ageing results in decreasing the strength which is called as over ageing.

4.Optical Microscopy

The microstructure of the aluminium alloy is not visible with the naked eye. The study of aluminium alloy microstructure is necessary to understand the effect of heat treatments and manufacturing in the grain size, grain boundaries etc (Hatch, n.d.). The microstructure can be analyzed by optical microscopy. Optical microscope uses light to get an image of the sample of interest. The method is only limited to the surface analysis, it cannot be used for analyzing the internal microstructure.

4.1 Microstructure and mechanical properties

Kun Yu et.al conducted a research on “mechanical properties and microstructure of aluminium alloy 2618 with Al₃(Sc, Zr) phases” (Kun Yu∗, 2003). Scandium and zirconium were added to understand the mechanical properties due to the formation of Al₃(Sc, Zr) phases. They compared two alloys, Alloy A with the composition (mass %) Al-2.23, Cu-1.21, Mg-0.93, Fe-1.09, Ni-0.30, Sc-0.30Zr and alloy B which has the same composition as that of 2618AA. The cast billet was hot rolled at 723k and it is cold rolled into sheets of 2mm thickness.The cold process was reduced about 50 and 75% respectively. Some of the specimens were annealed and the remaining were solution treated after the cold rolling. Finally, they were quenched in water and immediately aged at 473 and 573k respectively. The tensile properties of the alloy were measured at 293, 473, 523 and 573K to study how the temperature influences the alloy. The microstructure of the alloy was observed using the optical microscopy, Scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The grain size of Alloy A (about 20–30µm) was found to be smaller than that of Alloy B. Hardness of Alloy B decreased significantly in the temperature range from 473 to 573 K due to the recrystallization during the annealing. The recrystallization temperature of Alloy A is found to be about 200K higher than that of AA2618. The grains of the Alloy A containing scandium and zirconium were refined. The thermo-stable Al₃(Sc, Zr) particles impeded the nucleation of recrystallization grains and retarded their growth. The ambient and elevated temperature strengths of Alloy A were greater than those of Alloy B. The yield strength and ultimate strength of Alloy A increased by about 80 MPa at ambient temperature and 40 MPa at 573 K with almost no ductility changes. Alloy A had many double-layer of Al₃ fully coherent with the Aluminum matrix. It was found that high coherent strain energy existed in the matrix around the Al₃(Sc, Zr) particles and produced a coherent strengthening. Also, the size of the precipitates of Al₃(Sc, Zr) were small and dispersed, providing large force to retard the movement of dislocations and increase the alloy strength. Thus, strength of the alloy increased without a change in ductility.

4.2 Grain refinement

Rosmamuhamadani Ramli et.al conducted a research on “Microstructure and mechanical properties of Al-Si cast alloy grain refined with Ti-B-Sr-Sc-Mg” (Arawi, n.d.). The microstructure is an important factor which defines the mechanical properties of an alloy. Grain refiners are added to improve the microstructure of the alloy. The coarser grain size results in deteriorate the mechanical properties. Silicon is used as an alloying element because of its excellent casting characteristics. Permanent mold casting method is adopted for the cast aluminum alloy preparation. The method is suitable for producing material with the good surface finish and dimensional tolerance. The molten material is cooled in the cavity of the mold with a controlled cooling rate for avoiding any inherent defects during the manufacturing process. Alloys of four sample with weight percentage varying from 0.02 to 0.08 of Sc is used and the hardness test result of the alloy samples shows that hardness increases as the weight percentage of Sc increases. Tensile stress and hardness of 399.4 Mpa and 95.8 respectively are found for the sample contains 0.08 wt% of Sc.

5. Microalloying

Purnendu Kumar Mandal et.al conducted a study to understand the effect of addition of Ag(silver) on the hot deformation behavior of 2219 Al-alloy containing 0.1 wt % Ag prepared by casting technique. Aluminum-1100 alloy (99% pure) ingot, International Annealed Copper Standard grade copper rod, and sterling silver (92.5% pure) were used to prepare Al-42 wt % Cu and Al-5.8 wt % Ag master alloys by melting followed by solidification in metal ingots (Mandal, 2018). The required quantities of the Al ingot and master alloys were then heated in a clay graphite crucible using a resistance heated melting furnace. The molten metal was subsequently poured at 700˚C into sand molds to obtain cylindrical rods of the two alloy compositions, i.e., 2219 Al alloy (alloy-A) and 2219 Al+0.1 wt % Ag alloy (alloy-B). Cylindrical compression specimens were then machined from these rods. Composition analysis of both the alloys was determined by atomic absorption spectrophotometer. The samples were heated up to test temperature inside a resistance heated split furnace and uniaxial compression tests were carried out at different temperatures and constant true strain rates, using a dynamic 100 kN capacity universal testing machine. From the stress versus strain plots, the peak flow stresses for each test conditions were determined. The predicted and experimental values of peak flow stresses were well in agreement. Plastic deformation was accompanied by rapid dislocation multiplication resulting in work hardening leading to a rapid rise in the flow stress. As dislocation density increased with further deformation, dynamic softening occurred affecting work hardening. Due to this, the flow stress increased at a decreasing rate until it attained a peak value. With further increase in strain, the flow stress almost remained constant indicating a balance between work hardening rates and softening rates. The value of flow stress was less in alloy B. Samples after hot compression test were water quenched, sectioned parallel to the load axis using a precision saw, polished using variable speed grinder-polisher and were etched by dipping in freshly prepared Keller’s reagent for 8–15 s. The etched specimens were observed under an upright optical microscope. Investigation of the flow curves and microstructure revealed that dynamic recrystallization is responsible for the continuous flow softening behavior at high temperature and low strain rate. It can be taken from this work that, at low temperature and high strain rate, deformation is accompanied by deformation band formation. It was also observed that the flow stress for hot deformation of 2219 Al alloy decreased with the addition of 0.1 wt % silver. Silver addition resulted in flow softening of the 2219 Al alloy at elevated temperature. Addition of 0.1 wt % silver in 2219 alloy resulted in decrease in the activation energy (Q) for deformation in the range of strain rates and temperatures.

6. Recrystallization

Liangming Yan et.al, conducted a research on the “Dynamic recrystallization of 7055 aluminium during hot deformation” (Liangming Yan1, 2010). They used an ingot of 7055 alloy of dimension Փ10mm X 15mm for hot compression loading of strain rate 1.0 X 10^-2 and 1.0 X 10^-1 s^-1 and a maximum strain of 0.7. Hot compression was conducted at a temperature ranging from 350-450 degree Celsius. The deformed material was evaluated using the Electron backscatter diffraction (EBSD) which uses the principle of electron diffraction to understand the microstructure of the specimen. At temperature of 400 °C, the grain boundaries extensively became serrated and bulging, along which a few small dynamically recrystallized grains had already been developed. At a nominal strain of 0.7, the amount of new recrystallized grains had further increased, and the size of new recrystallized grains had also increased. At temperature of 450 °C, bulging was hardly observed under OIM micrographs at a strain of 0.3. The original grains were elongated evidently and sub grain boundaries increased with increasing strain. At a strain of 0.7, bulging was observed but dynamic recrystallization grains did not appear. At high Z value, the grain boundaries extensively became serrated and bulging, along which some small dynamically recrystallized grains had developed. At lnZ value of 23.2, the low angle boundaries decreased, and diameter of dynamically recrystallized grains increased. The amount of dynamically recrystallized grains was observed to be less than that at the high Z value. With increase in Z, the amount of dynamically recrystallized grains had increased. The initiation of DRX began with continuous dynamic recrystallization (CDRX) which involved the transformation of low angle boundaries into high angle boundaries. It is observed that the DRX nucleation of 7055 aluminum alloys can be operated by bulging of the original grain boundaries, which is assisted by sub grain rotation. Also, deformation condition (Z value) has a great influence on the nucleation mechanisms of DRX in 7055 aluminum alloy. Discontinuous dynamic recrystallization (DDRX) happened at lower deformation temperature leading to nucleation of DRX in the alloy. During deformation at lower temperature, dislocation movement became difficult. The second phase particles, including the un-dissolved particles during homogenization and the precipitate on the hot deformation prevented the movement of dislocation. It was observed that DRX happened when distortion energy formed by dislocations stress field reach the energy of developing dynamic recrystallization. The microstructure of the deformed material showed that the fraction of the new grain increased with the Z value. The stress-strain curve obtained from the compression test show curves, where it is found that the true stress increases with decreases in temperature. The curve of 1.0×10^-2s^-1 of strain rate shows the stress increases as with respect to increasing in strain and after a point, it shows steady region. From this analysis, they found that the deformation condition has great influence on the nucleation mechanism of the dynamic recrystallization in 7055 aluminum alloy.

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