In this article we will discuss about:- 1. Introduction to Aluminium Alloys 2. Microstructural Features of Aluminium Alloys 3. Designation System 4. Heat Treatment Specifications 5. Heat Treatment Processes.
Contents:
- Introduction to Aluminium Alloys
- Microstructural Features of Aluminium Alloys
- Aluminium Alloy Designation System
- Heat Treatment Specifications of Aluminium Alloys
- Heat Treatment Processes for Aluminium Alloys
1. Introduction to Aluminium Alloys:
Aluminium became a common structural material because of the following properties:
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1. Light Weight:
Aluminium weighs roughly one-third as much as most of the common metals, but is one and a half times as heavy as magnesium. It finds applications to reduce the weight of components and structures, particularly connected with transport, specially with aerospace.
High strength-to- weight ratios can be achieved in certain alloys, which show marked response to age-hardening. High strength-to-weight ratio saves a lot commercially, when the ‘dead weight, is decreased, and ‘pay load’ of transport vehicles is increased. This ratio is of particular significance in engineering designs, where stiffness is involved.
For example, stiffness for equal weights of similar beams are in ratio 1:2.9:8.2:18.9 for steel, titanium, aluminium and magnesium respectively. Mg-Li (relative density 1.35) is the lightest structural alloy available commercially.
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2. Ease of Fabrication and Machinability:
It can be easily cast, rolled to any desired thickness (aluminium foils are so common), stamped, drawn, spun, forged, and extruded to all shapes.
3. High Resistance to Atmospheric Corrosion due to thin, impermeable aluminum oxide layer on the surface. Titanium too has very good resistance to corrosion.
4. Good Thermal Conductivity (as well as electrical).
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5. High Metallic Lustre:
Aluminium is used for photographic-reflectors.
6. Non-Magnetic and Non-Sparking:
In annealed state, commercially pure aluminium (+99.0% Al) is very ductile to be drawn to greater depths than copper, or brass. The strength of aluminium can be markedly increased by cold working with simultaneous loss of ductility. Annealed commercial aluminium has tensile strength of 89.7 MN/m2.
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Commercially pure aluminium, AISI 1100 alloy has been used extensively for applications where high strength is not prime a requirement but excellent formability and corrosion resistance are. For example for cooking utensils, food and chemical handling and storage tanks, etc.
Alloying Elements:
The most common alloying elements added to commercial wrought aluminium alloys, singly, or in combinations are copper, silicon, manganese, magnesium, zinc, chromium and nickel.
Copper is added up to 5.5% to improve dynamic fatigue, elastic properties, strength and hardness but impairs ductility. It improves castability and machinability but decreases corrosion-resistance. The resulting alloys are age-hardenable to obtain optimum physical properties.
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Silicon is added up to 1% along with copper and magnesium to enhance the response to age-hardening heat treatment.
Manganese is added up to 1.5% to improve hardness, strength, even at elevated temperatures, and machinability without impairing corrosion-resistance, but decreases ductility.
Magnesium is added up to 3% to increase strength, hardness, machinability. It improves corrosion resistance to salt-sprays and mild alkaline solutions but lowers in dilute acids.
Zinc is added up to 6% but commonly alongwith copper, or magnesium, to increase strength and hardness with little loss of ductility.
Nickel is added up to 2.5% always alongwith copper to improve hardness and strength, particularly at high temperatures, but lowers ductility and corrosion-resistance.
Chromium is added in minute amounts up to 0.3% to act as grain-refiner, to improve corrosion-resistance, and physical properties at elevated temperatures.
2. Microstructural Features of Aluminium Alloys:
(i) Coarse intermetallic compounds (0.5-10 µm), obtained during solidification, or while processing, include virtually insoluble compounds, Al7Cu2Fe, α-Al (Fe, Mn)Si, (Mn, Fe)Al6, FeAl3, and relatively soluble compounds CuAl2, Mg2Si and Al2CuMg. These are seen as mechanical fibers due to elongated stringers in the direction of metal flow.
(ii) Fine, submicron particles (0.05-0.5 µm) of intermetallic compounds (which contain transition metals) such as Al20Cu2Mn3, Al12Mg2Cr and ZrAl3. These particles retard recrystalisation and grain growth in alloys.
(iii) Fine precipitates (up to 0.01 µm) are formed during age-hardening heat treatment cycle and are responsible for strengthening and hardening.
(iv) Grain size and shape of the grains.
(v) Dislocation substructure, particularly obtained during cold working.
3. Aluminium Alloy Designation System:
Most of the countries have adopted the classification adopted by International Alloy Development System (IADS). The basis of designation of wrought aluminium alloys is the four-digit system as illustrated in table 14.1. The first digit is assigned on the basis of main alloying element(s).
The second digit indicates modification of the original alloy, or impurity limits- Zero is used for original alloy, and integers 1 through 9 indicate alloy modifications. The third and fourth digits are important in the 1XXX series, and not in others alloys, i.e., in other alloys, these digits mean nothing more than a serial number. In 1XXX series, the minimum purity is denoted by these digits. For example, 1145 has a minimum purity of 99.45%, whereas as 1200 has a minimum purity of 99.00%.
In U.K. three principal types of specifications are traditionally used for aluminium and its alloys:
(i) BS (British Standard)-for general engineering use
(ii) BS for aeronautical use (designated as the L series)
(iii) DTD (Directorate of Technical Development) specifications are for special aeronautical applications.
4. Heat Treatment Specifications of Aluminium Alloys:
The state of the alloy in which the mechanical properties have been attained is designated by a system of temper nomenclature. It is in the form of assigning letters and digits as suffixes to the alloy number.
There are four basic tempers such as:
Except for the annealed, and as fabricated tempers, the ‘digit’, following the letter, indicates the specific temper.
The system of designation deals separately for the non-heat treatable, strain hardening alloys on the one hand, and the heat treatable alloys on the other, such as:
H 1:
Strain – hardened only, as is indicated by the first suffix digit 1. The second digit indicates the amount of cold work (or strain hardening). The numeral ‘8’ in H 18 indicates fully hard condition (≈ 75% reduction in area). Thus, ‘6’ in H 16 indicates three-quarter hard; H 14 is half-hard; H 12 is quarter-hard condition. Sometimes, extra hard tempers are indicated by the numeral 9.
H 2:
Strain – hardened and partially annealed (annealing is done to reduce the harder-temper obtained by cold working to induce the strength of desired level).
H 3:
Strain – hardened and stabilised. This temper applies only to Al-Mg alloys, which have a tendency to soften with time at room temperature after cold working. Thus, to avoid this, alloys are heated (to ensure the softening process) for a short time at an elevated temperature (120° – 175°C) to stabilise the properties. A series of H temper designations having three digits have been assigned to wrought products. For example, – Hill applies to products which have been strain-hardened less than the amount required for a controlled H 11 temper.
The heat treatable alloys are given a different nomenclature. – T applies to heat treated alloys with, or without strain hardening. The letter – T has suffixes from 2 to 10 as described below. Many times, a second digit if used indicates the amount of cold work given in between solutionised state and artificial ageing to improve the strength. For example – T 85 indicates 5% of cold work.
(i) T 2 – Annealed (for castings only)
(ii) T 3 – Solution heat-treated, and then cold-worked
(iii) T 4 – Solution heat-treated, and naturally age-hardened
(iv) T 5 – Artificially aged only
(v) T 6 – Solution heat-treated, and then artificially age-hardened
(vi) T 7 – Solution heat-treated and stabilised (by slight overaging)
(vii) T 8 – Solution heat-treated, cold-worked, and then artificially-aged
(viii) T 9 – Solution heat-treated, artificially-aged, and then cold-worked
(ix) T 10 – Artificially-aged, and then cold-worked.
5. Heat Treatment Processes for Aluminium Alloys:
It is essential to produce an aluminium alloy ingot of fine and uniform grain structure to obtain wrought-aluminium-alloys products with uniform and consistent properties. Even the heat treatment cycles become simpler of such alloys. Normally, the ingot is produced by direct-chill (DC), semi-continuous process.
A faster rate of solidification of the ingot results in finer dendritic arm spacings (which, thus, reduces micro-segregation in the inter-dendritic spaces), the grain size is finer and uniform, the intermetallic compounds precipitated are fine-sized. The grain size is made still smaller, if an alloy of Al-Ti, or Al- Ti-B is added to the melt before casting.
Depending on the requirement and stage of processing of aluminium alloys, the main heat treatment processes used for aluminium alloys are:
(a) Homogenizing
(b) Annealing:
(i) Stress-relieving annealing
(ii) Full annealing
(c) Precipitation hardening
(a) Homogenizing Heat Treatment:
Homogenizing heat treatment is a common first heat treatment given to ingots, to:
(a) Reduce segregation by the process of diffusion
(b) Remove low melting eutectics by the process of diffusion. Hot working can now be done without liquation, or overheating.
(c) Improve mechanical properties.
(d) Improve hot and cold workability.
Homogenising heat treatment is done at a required temperature (depending on the alloy), but normally in the range of 420°C to 520°C. The time at this temperature of homogenizing, though depends on the alloying elements in the alloy ingot, but is, in general related as inversely to the square of the dendritic arm spacing (in the ingot cast structure). However, the time could be 20-30 hours.
Homogenizing treatment is of greater importance for higher strength aluminium alloys. As it not only homogenizes the composition, but precipitates and redistributes the submicron intermetallic compounds of transition metals (MnAl6, ZnAl3, AI12Mg2Cr), which were supersaturated dissolved state due to faster cooling of the ingot during solidification. The uniform and finely dispersed form of these precipitates, not only controls the grain size (during heating), but also effects the mechanical properties obtained, particularly after age-hardening.
Thus, while homogenising ingots of different alloys, particularly when above mentioned precipitation occurs, the important steps to be chosen for each alloy are:
(a) Rate of heating to homogenising temperature
(b) Temperature of homogenising
(c) Time of homogenising.
Slow heating rate (75°C/hour) helps to promote nucleation and growth of fine and uniform dispersion of the compounds. These submicron sized compounds can be further modified by controlling the rate of cooling after homogenising annealing.
(b) Annealing:
Annealing heat treatment is given to-the aluminium alloys to induce softness and ductility.
There are two types of annealing treatments given to aluminium alloys:
(i) Stress-relieving annealing
(ii) Full annealing
(i) Stress-Relieving Annealing:
It is the type of annealing in which the internal stresses developed by cold working are relieved. The main purpose of this process is to completely soften the alloy after cold working. The process consists of heating to 315 to 415°C for a time of few minutes to 3 hours (depending on alloy and thickness). The rate of heating is not important except in alloy 3003, where rapid heating is done to prevent grain growth. It is normally advisable not to raise the temperature higher than 415°C to avoid grain growth and oxidation.
Even intermittent annealing is also done in this range of temperature for continuing further cold work. In this process, recovery and recrystallisation occur of the cold worked alloy. The directional grains recrystallize into equiaxed grains having isotropic properties. Precautions should be taken if the alloy has been only critically deformed, because the recrystallisation results in coarse grained structure.
A fine grained equiaxed structure in semi-finished wrought products can be obtained, if recrystallisation process is interrupted just after nucleation process, or fast rate of heating is done just like heating in a salt bath (nitrate), or recrystallisation is carried out at maximum possible temperature and in minimum time.
(ii) Full Annealing:
The basic aim of full annealing is to increase the softness and ductility of the aluminium alloys. This heat treatment is meant if the hardening effects of the precipitation hardening heat treatment, or of cooling from the hot working temperature, are to be removed. The full annealing heat treatment increases the softness and ductility by producing coarse and widely spaced precipitate panicles. The coarser and fewer are the intermetallic compound particles, more ductile is the alloy.
The process of full annealing consists of heating and then soaking at about 415°-440°C for 2 hours, and then cooling slowly at 10°C/hour up to at least 260°C. This slow cooling also helps in growth and coalescence of the precipitating particles, so that it results in minimum hardness. In this state, the alloy can undergo high degree of deformation during cold working, as the dislocations can easily move, or cross slip, or even by pass these coarse precipitate particles.
(c) Precipitation Hardening:
Precipitation hardening is done to increase the strength and hardness of heat treatable aluminium alloys to desired values required for consistent service performance. The consistency of properties is only possible if care is taken to have a uniform microstructure in the earlier processing of the part.
The actual precipitation hardening heat treatment can be divided into three stages:
Solution Treatment:
It consists of soaking the alloy at a temperature sufficiently high to achieve a homogeneous solid solution, i.e., to obtain the complete solution of all the alloying elements at a temperature within the single phase equilibrium solid solution range for the given alloy. Overheating or under-heating should be avoided.
The alloy should not be heated above solidus temperature (or initial eutectic melting temperature), as the liquation occurs of the compound at the grain boundary regions, renders the alloy brittle, and thus, adversely affects the ductility and other mechanical properties. The lower limiting temperature should be above the temperature at which complete solution occurs.
Table 14.6 gives the effect of the solutionising temperature on the strength attained after age hardening of alloy 2024-T 4 state:
Table 14.7 gives solutionising temperature of some wrought aluminium alloys, and Table 14.8 gives solutionising temperature of cast aluminium alloys. As the soaking temperature is raised, the rate of dissolution of alloying elements into solid solution increases. The soaking time should be just enough to dissolve the solutes completely. The soaking time should be just minimum in thin clad sheets to avoid the diffusion of solutes from the alloy to the aluminium outside sheet.
Salt baths take less time than air-furnaces. H2 gas dissolves by the reaction of alloy with water vapour in atmosphere of furnace to cause surface blisters. The blistering is normally due to overheating of the alloy. Overheating also causes grain growth to develop inferior properties. The surface reaction with water vapours can be decreased by adding fluoride salt in the furnace.
The soaking time, apart from the thickness and shape of the component, depends on conditions under which the alloy is cast. It is basically dependent on the coarseness, or fineness of the microstructure. Sand cast parts have coarser structure than permanent mould cast parts, and thus, former need longer soaking time (12 hours) to dissolve phases than latter parts (8 hours). The earlier thermal history of alloy also effects the dissolution of the alloying elements during solution-heat treatment.
For example, the rate of dissolution increases if the component has been given repeated solution heat treatment. Thus, the soaking time is decreased. Full-annealed components, having coarse precipitate particles, show decreased rate of dissolution of the alloying element (precipitates), thus, demand more soaking time. A microstructure having fine and uniform dispersion of precipitates is easy and quick to be solutionised in solution heat treatment.
Quenching:
To avoid precipitation detrimental to mechanical properties, or corrosion resistance, the solution treated aluminium alloy is quenched rapidly to obtain supersaturated solid solution at room temperature, by immersing in cold water particularly the thick parts. The fastest precipitation occurs in range 200° to 400°C. Thus, the part should get completely submerged in quenchant before it attains a temperature of 410°C during cooling, to avoid precipitation during cooling.
There should be enough coolant, without raising its temperature during cooling, so that no precipitation occurs in this critical range of 200°C to 400°C. The quench-delay time, i.e., the maximum allowable transfer time (from furnace to quench bath) should be determined to fix cooling rate, the temperature of coolant, etc. A cooling rate of about 315°C/sec in the critical range, or more is needed to avoid precipitation, though it depends on the alloy, size of the part, coolant, etc.
A cooling rate faster than that required to avoid precipitation, enhances unnecessary chances of distortion of the parts. Milder coolants like hot water (60- 80°C), boiling water, water sprays, and air blasts can be used to reduce distortion.
Al-Zn-Mg alloys are quenched in salt bath at temperature, 180°C, soaked there for some time, and then cooled to room temperature. Now-a-days organic quenchants are also being used. If an alloy is quenched slowly, the quenched-in vacancies are small, and thus, precipitation kinetics may be different. The precipitation may require more time.
Straightening, if required, of the aluminium alloy part may be clone in the as-quenched state. The precipitation characteristics at room temperature vary from alloy to alloy. Thus, in such cases, the parts may be refrigerated after solution treatment, and before the ageing is done.
Ageing:
The choice of time-temperature cycles for precipitation should be carefully thought. The objective is to select a cycle that produces optimum precipitate size and distribution.
Thus, depending on the alloy, the ageing is classified as:
(a) Natural ageing – ageing at room temperature.
(b) Artificial ageing – ageing at temperatures higher than room temperature but commonly between 110° to 190°C for 5 to 48 hours.
Most alloys require heating for a time interval (ageing) at one, or more elevated temperatures. When single ageing is to be done, a temperature is selected for which ageing time to develop high strengths properties is of convenient duration. At room temperature, alloy 2024-T 4 takes 4 days, but alloy 6061 T 4 takes a month. When multiple ageing is done, then the treatment may require more days at room temperature, followed by one, or two periods at elevated temperatures.
Precipitation hardening of castings should be based on preliminary tests. The foundry practice effects lots of properties. The principles and procedures for wrought and cast products are similar. Soaking times for solution treatment of castings are much longer as castings have very coarse structures. Castings have to be quenched more slowly, and thus, boiling water or milder quench may be used.
Some aluminium alloys yield high strength properties, if they are cold worked after quenching but before ageing. Thermo-mechanical processing has been developed when cold working is done at some stage during precipitation reaction. By this process, a uniform distribution of dislocation is made and this substructure is made stable by precipitating the compounds there. Al-Zn-Mg-Cu show high strength, toughness and fatigue strength due to precipitation and substructure hardening.