CHAPTER INTRODUCTION

Transcription

1 1 CHAPTER INTRODUCTION Contents 1.0 Introduction Aluminium alloys Aluminium alloy classification Aluminium alloys (Wrought) Heat treatable alloys (Wrought) Aluminum alloys (Cast) Aluminium alloy designations Wrought aluminium alloy designation system Alloying elements and their effects Aluminium alloy processing Hot rolling Cold rolling Heat treatment of aluminium alloys Heat treatment for strength Solution heat treatment Quenching Process of age hardening Grain growth Phase diagrams Aluminium zinc system Al Zn-Mg alloys 11

2 2 CHAPTER INTRODUCTION 1.1: ALUMINIUM ALLOYS Aluminium is one of the important engineering application materials available in earth and of the earth's solid surface it occupies around 9% by weight. The one of the important source is bauxite ore. The special properties of Al are low density and corrosion resistance. Due to this reason it finds applications in aerospace industry and automotive industry. The AA (Aluminum Association) alloys finds its application extensively in the following fields: aerospace, automobile parts, railway machine parts and other equipments due to its better mechanical, electrical, thermal and physical properties along with high strength to weight ratio and more importantly it has good corrosion resistance. Because of these properties aluminium alloys are more useful in the transportation and construction equipment. Use of these alloys reduce the weight of the equipment to a greater extent and hence in turn improve efficiency. 1.2 ALUMINIUM ALLOY CLASSIFICATION In these alloys, aluminium is the prominent metal along with combination of copper, magnesium, silicon, manganese, along with zinc as alloying elements. They can be classified as 1. CAST ALLOYS 2. WROUGHT ALLOYS

3 3 Further they are classified based on heat treatment that can be used as heat treatable and non-heat treatable. More than 80% of aluminium is used in wrought form for foils, extrusions and also in rolled plates. More than 80% is the wrought form used in practice for rolled plates, aero-foils along with for extrusions. The other one cast aluminium possess low melting point and is cost effective. Upon heat treatment alluminium alloy with zinc as the major alloying element enhances corrosion resistance, hence Al-Zn system is one of the important cast aluminium alloys with zinc content of 4 to 13%. In dry environment aluminium alloys possess good surface finish, but because of the formation of aluminum oxide as protective layer and in wet environment condition galvanic corrosion occurs Aluminium alloys (Wrought) "Wrought aluminum" are of as-formed alloys of aluminium that have been worked mechanically to obtain better grain structure as well as better physical properties. They are extruded and forged products. Aluminum classification numbering system has been established by ANSI and Aluminum Association. This system of classification makes use of codes to give information about heat treatment condition and alloying elements of the material involved. These are designated by a four digit code for alloy groups. The strength of aluminium can be increased either with the addition of alloying elements or by heat treatment and/or cold working Heat treatable aluminium alloys (Wrought). 2xxx series (Al-Cu and Al-Cu-Mg) alloys strength are increased by hardening (precipitation). At ambient temperature as well as at higher temperature they exhibit

4 4 better strength. 6xxx series (Al-Mg-Si) alloys also are heat treatable and possess good weldability and better resistance to corrosion. The 7xxx series are heat treatable alloys, with high strength and good toughness Aluminum alloys (Cast) Aluminum and its alloys are castable and the quality factor (production of sound castings without any defects) is important in selecting the casting process. The various casting methods that can be used are sand casting and die casting,investment casting etc.. For the mass production of relatively small sized parts die casting is the one which is best suited. Aluminum casting alloys are also classified based on the same system of alloys similar to that of the wrought ones. Strength is enhanced with the same mechanisms, other than strain hardening and is classified in the same manner as that of either heat treatable or non-heat treatable ones. 1.3 ALLUMINIUM ALLOY DESIGNATIONS Wrought or cast aluminium alloys are designated with 4 number digits and out of which the first digit represents the major alloying element present Wrought aluminium alloy designation system In the four digit alloy designation a suffix indicates the heat treatment type or straining process. Minimum aluminum content in the alloy group is represented by last two digits.

5 5 1.4 ALLOYING ELEMENTS AND THEIR EFFECTS All the elements listed are not major alloying ingredients in terms of an intended alloys uses and some of the major elements of one of the alloy and another alloy need not be same. Certain elements, such as Sr for instance, can play a vital role for control of microstructure and mechanical properties Alloying elements are further classified as major (Si, Cu, Mg) alloying elements and minor (Fe, Ti, Cr) alloying elements. A very small amount of chromium is present compare to the major alloying element, due to which the electrical resistivity of aluminium gets affected. Chromium possess slow diffusion rate and in wrought products it will be in the form of dispersed phases. This improves nucleation and grain growth and which in turn improves toughness and is added up to 0.4%. COPPER In addition to some amount of chromium and manganese when added to copper improves strength in aluminium alloys. With increase of degree of super saturation copper also increases the ageing rate along with quench sensitivity under heat treatment. Usually, copper has better influence in improving stress corrosion resistance when compared to regular corrosion. IRON This does not have much effect on aluminium, if it is more than 0.049%, then that remaining iron content with aluminium and other elements will be in the form of second phase inter metallic. MANGANESE Manganese decreases resistivity and increases strength and it is an impurity element.

6 6 MAGNESIUM Without affecting the ductility magnesium increases the strength. The magnesium produces MgZn 2, in 7075 aluminium alloy. Tensile and yield strength increases to some extent with increase in MgZn 2 concentration. SILICON: After iron, silicon is the major impurity present in aluminium. MgSi 2 is formed when silicon reacts with magnesium and a compound is formed, which results in the increase of the fluidity of aluminium. ZINC In 7075 aluminium alloy, zinc is the major alloying element of all the other alloying elements present. Aluminium becomes heat treatable with the addition of zinc along with some other elements (Ex: magnesium and copper), which increases its strength to a greater extent. Precipitation hardening (MgZn 2 ) enhances mainly the strength of 7xxx series alloy to a greater extent. The wrought aluminium-zinc alloys are more prone to stress corrosion cracking (due to decrease in corrosion resistance), hence its usage is curtailed. TITANIUM This acts as a grain refiner and its effect is more prominent in the presence of boron. TiB 2 is produced because of the reaction of boron with titanium. This provides better nucleation site for grains and makes grains more and finer. 1.5 ALUMINIUM ALLOY PROCESSING. Rolling is one of the most widely used metal deforming processes. In practice the following two types of rolling processes are used and recrystallization temperature is the basis for process selection and the processes are hot rolling and cold rolling

7 Hot rolling This process is carried out for the materials above re-crystallization temperature. The usual range of hot rolling is around more than C for aluminium. In hot rolling re-crystallization have direct influence on the formation of grain size and aluminium sheets texture, which will have control on the properties of products supplied in hot rolled condition. The alloy type and the conditions of processing are mainly used to determine re-crystallization. The mobility of grain boundary is very high in pure aluminium and the material immediately exhibits rapid re-crystallization during processing Cold rolling This process is carried out below the recrystallization temperature (usually at ambient temperature) of the material. This process is used for the production of sheet and strip with better surface finish and dimensional allowances as compared hot rolled strip. Further, the strength may be increased (around 20%) by strain hardening resulting from the cold reduction compared to rolled steel products. Percentage of rolled non-ferrous metals finished by cold rolling are very much more. 1. Thickness of work piece by cold rolling cannot be minimized as compared to that of hot rolling. 2. Generally a total reduction obtained by cold rolling varies between about 50 to 90%. 3. While working under this process in industry, for steel sheet, aluminium and copper alloys, high-speed for high tandem mills with around five stands are used. Cold working regime is less than C in aluminium and for making cans, food packing, cold rolled aluminium strip finds application traditionally.

8 8 1.6 HEAT TREATMENT OF ALLUMINIUM ALLOYS Heat treatment for strength In aluminium alloys 2xxx and 7xxx are heat treatable alloys. The strength can be increased by solutionization and age hardening Solution heat treatment In this process the material is heated to a specific temperature to dissolve soluble hardening elements, and process is solution heat treatment. The process involves soaking of alloy at a high temperature for required time to get homogeneous solid solution Quenching Quenching involves cooling the alloy from solutionization temperature to room temperature to preserve solute atoms in solid solution and to maintain certain number of vacant lattice sites which promotes the low temperature diffusion that is required for formation of zone Process of age hardening Ageing increases strength of aluminium alloys to a greater extent and this can be achieved at natural ageing (i.e. room temperature.) or with artificial ageing (i.e. precipitation heat treatment.). Natural ageing: The series of alloys that can be aged by natural ageing are 3xxx, 4xxx, and 5xxx series. Precipitation hardening through natural ageing such as T3 and T4 conditions are used for 2xxx, 6xxx and 7xxx alloys. a) Process of precipitation heat treatment Precipitation hardening generally is carried out for longer time at low temperature. The usual range for precipitation hardening is between C to C, and in practice precipitation hardening time is between 5 to 48 hrs. The desirable

9 9 properties will decide precipitation hardening temperature and the precipitation hardening time. 1.7 GRAIN GROWTH Grain growth can be controlled during solution treatment or annealing. Grain growth depends on the following: alloy composition, micro structure of the material, chemical composition etc. The usual cold work critical range is between 5 and 15%, with temperature of around C. T6 type tempers alloys without affecting certain level of other properties possesses better strength in practice. Whereas alloys in T7 tempers are over-aged, this implies that to improve one or the other characteristics some percentage of strength has been sacrificed. Precipitation strengthening is carried out at 250F for 24 hrs in aluminium 7075 alloys and air cooled at T 6 condition. Whereas in case of T73 temper precipitation hardening is carried out as follows: heating to 225 F for 8 hrs followed by 325 F for 24 hrs and then air cooling. 1.8 PHASE DIAGRAMS Information regarding thermodynamic stability can be obtained from phase diagrams, which helps in developing new and better alloys from time to time for various present applications. Further phase diagrams help in developing alternative better alloys or alloys with various substitute alloying elements to substitute those, which are not easily available, costly and harmful Aluminium-zinc system Among cast alloys to be developed cast aluminium alloys were the first alloys. Incidentally in 1920s the alloys that were developed are aluminium-zinc- magnesium

10 10 alloys. Over a period of around 82 years, Al-Zn was one of the most investigated equilibrium diagram. In this system the solid solubility of zinc increases with temperature. In the range of 620k and 525k, the aluminium breaks up in to two phases such as η 1 and η 2, both of which are in the form of solid solutions of zinc in aluminium. At 57% Zn lattice parameter of aluminium linearly decreases. Increase in Zn content enhances thermal coefficient also. In age hardening of Al-Zn alloy solid solution will have more number of vacancies. Guinier Perston (GP) zones are usually of spherical shape with a diameter ranging from 10 E-6 to 60 E-6. Ageing time and/or temperature will have bearing on diameter. The number of zones is affected by the presence of Zn content. GP zones do not form at high temperature, from the start of GP zones formation hardness increases continuously to reach a maximum just before the formation of intermediate phase. A small absorption of heat is associated with the intermediate phase formation, which is represented by solution of GP zones at the transition phase due to independent nucleation. At this point, softening begins with reduction in internal friction. Aluminium- zinc phase diagram is shown in figure 1.1. Fig.1.1 Phase Diagram of aluminium-zinc

11 Al Zn-Mg alloys The phase diagram of Al Zn-Mg alloy is shown in fig 1.2. The primary alloying element is zinc in 7075 aluminium alloy. Among commercial alloys of aluminium, aluminium zinc alloys were the first. In place of castings and wrought products after 1900 the binary alloys were used. All these commercial Al-Mg alloys had more zinc than magnesium. Both for castings and wrought products these alloys can be used and the major alloying elements being zinc and magnesium. Higher ratios of Zn, Mg gives better strength and helps in heat treatment process. The amount of zinc and magnesium along with copper controls the properties and hence its applications. With total of around 6-8%, formability and weldability are much better whereas strength is still high. Less than 5-6% susceptibility to stress corrosion tends to disappear. The amount of zinc, copper and magnesium are in solid solution and only small amounts of their phases are visible in properly homogenized solution. Solid solubility of various phases of the corner Al-Mg-Zn system is shown in fig1.2. The alloy used in the present study is 7075 aluminium alloy. It is classified under 7xxx series of alloys and major alloying element is zinc. This finds application in aircraft parts, valves, shafts, spindles and missile components and other parts which are important from strength point of view. This is being used since from a long time to a larger extent by the leading manufacturer HAL, Bangalore.

12 12 Fig.1.2 Aluminum corner showing solid solubility of various phases of Al-Mg-Zn system. Now for the present investigation, tests were carried out on 7075 Al alloy subjected to T6 treatment for various chosen parameters involving solution temperature, ageing time and temperature and re-ageing temperature along with microstructure analysis. The tests were conducted to study mechanical properties such as UTS, ductility, and hardness. Some studies on dry sliding abrasive wear were also made for different loads, sliding distance in addition to sliding velocities. ASTM (American Society for Testing Materials) norms were used for conducting these tests and the results obtained were compared with that of as-cast material. Even microstructure observation was made for as-cast as well as heat treated alloys and LOM photographs were taken. To observe the type of fracture, photographs of fractured surfaces in tension and fatigue were taken. To know the mechanism of failure photographs of worn surfaces of the alloy specimens were taken. The conventional heat treatment for the material involves solution heat treatment, quenching followed by ageing. The AA 7075 alloy procured in rolled

13 13 condition was subjected to homogenization, and solutionization for 2 hours at 465 C followed by water quenching. The heat treatment process was as follows: The alloys were subjected to solutionization for 2 hours at C followed by quenching (water at room temp) and aged to C for the chosen ageing time of 16, 20, and 24 hours respectively and then cooled to room temperature in air. These specimens were designated as h, h, and h where, 465 was the solution temperature chosen and 16h,20h, and 24h are the hours of ageing. The above specimens were resolutionised at chosen retrogression temperatures of C, C, C and C and for chosen retrogression time of 5 and 10 minutes. After quenching back to room temperature re-ageing at C was carried out. Finally out of these specimens the one with high hardness were chosen for the tests such as fatigue strength, tensile strength, wear property, hardness, and microstructural behaviour. Above specified properties and wear resistance have improved with heat treatment when compared to as-cast material. To reduce the weight of components and structures aluminium and magnesium are considered as they are traditionally known as light metals. In aerospace applications Al-Zn castings are widely used due to their excellent combined properties such as better castability, low coefficient of thermal expansion, better strength-to-weight ratio, and energy required for recycling is less.