The Material And Process Requirements For Airframe Engineering Essay
First of all, I would like to introduce this assignment by giving some indication as to the airframe designer’s responsibility in the total field of the aeronautical industry, embracing design, manufacture and operation and subsequently to outline the manner in which the characteristic properties of the materials selected which able to contribute to the fulfillment of the designer’s responsibility.
Based on my research, the word of “designer’s responsibility” can be explained in the following terms: “the designer’s responsibility covers the whole process from conception to the issue of detailed instructions for production and his interest continues throughout the designed life to the product in service.” Although this definition of a designer’s responsibility was specifically agreed in the context of an enquiry into the mechanical engineering industry, few would dispute that it equally applies to any branch of engineering design. However, in today’s environment in the aeronautical field, it could well be slightly amended to give recognition to the fact that the actual life in service is tending to increase significantly relative to the nominal life envisaged at the design stage.
There are many aircraft in service today which are more than 20 years old and operators of commercial aircraft are looking for service lives of up to 30 years in the future. It is not suggested that these lives have been achieved, or will be expected to be achieved, without replacement of some parts, possibly more than once, during relevant period. However, the main airframe is expected to last for such periods. Thus, the duration of the period over which the designer’s interest is expected to be maintained. The ability to achieve these lives is very much influenced by the characteristics of the material chosen for the airframe structure.
Besides that, consideration of these is as vital a part of the design process as is consideration of the geometrical configuration. For economic considerations, of course, it plays a significant part in the continuing demand for longer lives. It is becoming increasingly difficult for new designs to show major prospect of economic advantage over the already established design.
Next, structural developments using already established materials have tended to asymptote in terms of structural weight. Structural weight is a major parameter in the operation economics of an aircraft. If the airframe structural designer is to make his contribution to an advance in the operating economics, he can only do so through the advent of new material offering the prospect of improved structural efficiency and reliability.
Therefore, selected a proper and suitable material for an airframe need to be aware based on the basic requirement of airframe as well.
Before I go in detail about the material and process requirement for an airframe, I would like to discuss about the basic requirement for an airframe.
According to Dr. James from British Aircraft Corporation Limited, Commercial Aircraft Division, Weybridge Surrey, he has talk about the safety standards for commercial aircraft are defined in the form of general requirement, such as the British Civil Airworthiness Requirements in the U.K. or the Federal Aviation Requirements in the U.S.A. or as typically, the case with military aircraft, in the aircraft specification itself. These will define the loading conditions which the aircraft structure must be designed to withstand in order to perform its specified functions. These will, in part, be in terms of conditions directly under the control of the pilot and in part, determined by the operating environment.
These requirements will be such that under reasonably foreseeable conditions, not only will the aircraft structure and the essential systems remain intact but also its flying characteristics will not be adversely affected. Two conditions are normally defined, one a limit of proof load condition requiring that the loads be sustained without deformation of the structure such that its continued airworthiness is in doubt, and the other and ultimate load condition requiring that the strength of the structure be such that collapse or critical rupture of the structure shall not occur before such loads have been sustained for at least a few seconds.
The relation between these two conditions is normally such that the ultimate load is 1.5 times limit load or in the case of military aircraft, sometimes 1.33 times proof load. The two static strength conditions thus defined are expected to be met throughout the operational life of the aircraft. It is necessary, therefore, to ensure that the material used is such that the strength conditions can be satisfied, not only with the newly constructed aircraft but throughout the long service life during which it will be subjected to the environmental conditions typical of aircraft operations.
Thus, different aspect must be take consideration in order to choose the material that able to achieve the safety requirements. Moreover, in discussing the requirements for airframe materials, it will be convenient to discuss the basic mechanical properties which traditionally have been regarded as providing a measure of the relative structural efficiencies of materials and then to discuss the impact of operational and environmental effects.
Basic Mechanical Properties of an Airframe---Strength and Stiffness
The normal basis of assessing the relative efficiencies of materials from the structural strength point of view is in terms of the ratio density. For components loaded in tension and robust components loaded in shear or compression force will be the ultimate tensile strength of the material if ultimate design strength is the criterion.
Figure A has show an airframe places high demands on its materials often in complex load cases such as shear stress, impact, high local loads, bending and others which has stat in the figure. If the limit or proof design strength is the criterion then the force will be relevant proof or offset yield strength. For less robust components loaded in shear or compression, instability may be the cause of collapse. The tangent modulus of elasticity, which for failure at stresses below the elastic limit but for failures at stresses above the elastic limit, will be a function of the stress at failure.
The critical value of the stress in these cases is not only a function of the tangent modulus but is also a function of the geometry of the component. From the materials point of view, the efficiency parameters indicate that the basic requirements are for high yield and ultimate tensile strengths, high modulus of elasticity and low density. This statement of requirements is indeed a platitude. In the context of airframe construction, its truth has been recognized from the earliest days.
They are the factors which extended experience has taught us must often take precedence over simple structural efficiency, as measured in terms of basic mechanical properties, if the requirements for safety, reliability, and economic operation over a long service life are to be realized. They reflect the response of materials to their service environment and highlight the need for the designer to ensure that the design is such as to cater for the potential problems, either directly in the geometric detail of the design, or indirectly by providing for adequate preventative methods.
For service problems, which have until very recently detracted from the attractions of the high structural efficiency potential of aluminum alloys for airframe application in previous, have arisen mainly from three causes, which may often have occurred in combination. There are fatigues, notch sensitivity and stress corrosion and I will discussed it separately as below:
The basic cause of fatigue is too well known to need further comment. Suffice it to say that, except when standing idle, the airframe is continuously exposed to fluctuating loads and hence to a fatigue inducing environment.
The consequence is that in major parts of an airframe where fatigue considerations predominate, there is no advantage to be gained by the use of the high strength alloy. The stress limitations imposed by fatigue considerations are such that the ultimate strength potential of these alloys is not reached under normal ultimate design considerations. It is sometimes argued that there is still merit in having the excess static strength potential.
However, this potential has often to be overridden by considerations of the likely behavior of the structure in which a fatigue crack has developed. This will be determined by the notch sensitivity and fracture toughness of the material.
Notch Sensitivity and Fracture Toughness
The term notch sensitivity is used to describe the deterioration in the behavior under stress of a material in the presence of a stress raiser, as compared with its behavior in the absence of a stress raiser. Premature failures have occurred under static stress conditions due to the presence of what, in general engineering terms, might be regarded as superficial imperfections.
Such sensitivity to imperfections is obviously undesirable. The deterioration, be it in static strength of fatigue endurance, will be dependent on the sharpness of the notch and it most cases on its orientation relative to the local direction of grain flow.
In order to compare to notch sensitivity of materials, it is obviously necessary that the notched tensile strength be established on the basis of a standard notch. An associated characteristic of materials is that which determines the rate of growth of cracks at stress below the critical level, namely its fracture toughness.
Besides that, the modern aircraft structure is designed to ensure that the risk of collapse of the structure due to failure of a single structural element is extremely remote. Structures designed on such principles are called “fail-safe” structures. It is recognized that it is not always possible to detect defects, especially fatigue cracks, when they occur. It is highly desirable, therefore that the materials used have the characteristic of toughness.
Therefore, the materials selection for the airframe is a very important sections that designer should take more consideration on it carefully with different assumption to make sure it able to produce a greater airframe and reduce the cost at the same time.
Stress corrosion failures have, perhaps been the most difficult of the material related failure modes that the airframe designer has to contend with in the last few decades. Every airframe manufacturer is able to quote examples of failures which have occurred, not only very early in the service life of an aircraft but sometimes before entry into service as well.
Occasionally, failures have occurred on parts in store before assembly to components. Such failures have been established as being due to stress corrosion. The failure has been due to the presence of a sustained tensile stress in the component while it is exposed to a corrosive environment. The most disturbing aspect of such failures has been that the extent of corrosion present, as determined by normal methods of assessment, has been negligible.
The sustained stress may arise from one or more of three causes, heat treatment quenching stresses, machining stresses or assembly stresses. Rarely does it arise from the normal service stress on the main airframe. It must be accepted in the context of airframe manufacture and aircraft operation, that all environments are corrosive and that a sustained stress, therefore, means a stress corrosion hazard if the material is susceptible.
It must also be accepted that it is not possible to design and buid a structure to such closely controlled dimensions as to completely eliminate all assembly stress. Stress corrosion potential is therefore, unavoidable and the need exists for non-susceptible alloys. Material manufacturers have made great progress in this field too. Initially reduced susceptibility was achieved by modified heat treatment procedures which carried with them deterioration of the basic mechanical properties.
More recently, advances have been made in alloy developments which have effectively restored the basic mechanical properties while maintaining good stress corrosion resistance. This has been achieved by a combination of changes in the balance of the major alloying elements and the introduction of new secondary alloying elements.
Based on the service problem which occurs when using aluminum alloy as material of airframe in previous, I would like to say that this service problem able to be solve by using other more suitable materials and it will be discuss material selection section.
Creep and Temperature Effect
The need for a long life at high temperature has resulted in very considerable effort being devoted to establishing the response of the material to prolonged exposure to high temperature for the airframe. Several aspects have to be considered in relation to continued structural integrity. The deterioration of mechanical properties of materials with rise in temperature has been known for many years mainly as a result of the development work carried out in support of engine applications.
By the way, the special effort has been devoted to examining the interaction between creep and fatigue behavior in previous when using aluminum alloy as material of airframe. It has been shown that the effect of the combination is adverse on both counts. Another phenomenon which has been revealed is that under certain conditions subsurface creep deformations can give rise to the development of subsurface fatigue crack initiation and propagation.
2.0 Material Selection
Based on the research about the airframe, I would like to choose Titanium Alloys as the material of airframe. Titanium is attractive to the airframe industry due to its mechanical properties and certain behavior which able to fulfill the requirement of an airframe.
2.1 Introduction of Titanium Alloys
Titanium has been recognized as an element (Symbol Ti; atomic number 22; and atomic weight 47.9) for at least 200 years. However, commercial production of titanium did not begin until the 1950’s. At that time, titanium was recognized for its strategic importance as a unique lightweight, high strength alloyed structurally efficient metal for critical, high-performance aircraft, such as jet engine and airframe components.
The worldwide production of this originally exotic, “Space Age” metal and its alloys have since grown to more than 50 million pounds annually. Increased metal sponge and mill product production capacity and efficiency, improved manufacturing technologies, a vastly expanded market base and demand have dramatically lowered the price of titanium products.
Today, titanium alloys are common, readily available engineered metals that compete directly with stainless and specialty steels, copper alloys, nickel based alloys and composites.
As the ninth most abundant element in the Earth’s Crust and fourth most abundant structural metal, the current worldwide supply of feedstock ore for producing titanium metal is virtually unlimited. Significant unused worldwide sponge, melting and processing capacity for titanium can accommodate continued growth into new, high-volume applications. In addition to its attractive high strength to- density characteristics for aerospace use, titanium’s exceptional corrosion resistance derived from its protective oxide film has motivated extensive application in seawater, marine, brine and aggressive industrial chemical service over the past fifty years.
Today, titanium and its alloys are extensively used for aerospace, industrial and consumer applications. In addition to aircraft engines and airframes, titanium is also used in the following applications: missiles; spacecraft; chemical and petrochemical production; hydrocarbon production and processing; power generation; desalination; nuclear waste storage; pollution control; ore leaching and metal recovery; offshore, marine deep sea applications, and Navy ship components; armor plate applications; anodes, automotive components, food and pharmaceutical processing; recreation and sports equipment; medical implants and surgical devices; as well as many other areas.
2.2 Reason Selecting Titanium Alloys as Material of Airframe
The main reason of selecting titanium alloys as material of airframe is because of its attractive mechanical properties. Titanium and its alloys exhibit a unique combination of mechanical and physical properties and corrosion resistance which have made them desirable for critical, demanding aerospace, industrial, chemical and energy industry service. The primary attributes of these alloys listed in Table 1, titanium’s elevated strength-to-density represents the traditional primary incentive for selection and design into aerospace engines and airframe structures and components.
Table 1: Primary Attributes of Titanium Alloys
Elevated Strength-to-Density Ratio (high structural efficiency)
Low Density (roughly half the weight of steel, nickel and copper alloys)
Exceptional Corrosion Resistance (superior resistance to chlorides seawater and sour and oxidizing acidic media)
Excellent Elevated Temperature Properties ( up to 600◦C (1100◦F))
Its exceptional corrosion/erosion resistance provides the prime motivation for chemical process, marine and industrial use. Figure 1 reveals the superior structural efficiency of high strength titanium alloys compared to structural steels and aluminum alloys, especially as service temperature increase.
Titanium alloys also offer attractive elevated temperature properties for application in hot gas turbine and auto engine components, where more creep resistant alloys can be selected for temperature as high as 600◦C (1100◦F) as shown in Figure 2.
The family of titanium alloys offers a wide spectrum of strength and combinations of strength and fracture toughness as shown in Figure 3.
This permits optimized alloy selection which can be tailored for a critical component based on whether it is controlled by strength and S-N fatigue, or toughness and crack growth in service.
Titanium alloys also exhibit excellent S-N fatigue strength and life in air, which remains relatively unaffected by seawater (Figure 4) and other environments. Most titanium alloys can be processed to provide high fracture toughness with minimal environmental degradation if required.
In fact, the lower strength titanium alloys are generally resistant to stress corrosion cracking and corrosion-fatigue in aqueous chloride media. For pressure-critical components and vessels for industrial applications, titanium alloys are qualified under numerous design codes and offer attractive design allowable up to 315◦C (600◦F) as shown in Figure 5.
2.3 Corrosion and Erosion Resistance
Titanium alloys exhibit exceptional resistance to a vast range of chemical environments and conditions provided by a thin, invisible but extremely protective surface oxide film. This film, which is primarily TiO2, is highly tenacious, adherent and chemically stable, and can spontaneously and instantaneously reveal itself if mechanical damaged if the least traces of oxygen or water (moisture are present in the environment.
This metal protection extends from mildly reducing to severely oxidizing and from highly acidic to moderately alkaline environmental conditions; even at high temperatures. Titanium is especially known for its elevated resistance to localized attack and stress corrosion in aqueous chlorides and other halides and wet halogens, highly-oxidizing, acidic solutions where most steels, stainless steels, copper and nickel based alloys can experience severe attack.
Titanium alloys are also recognized for their superior resistance to erosion, erosion-corrosion, cavitations, and impingement in flowing, turbulent fluids. This exceptional wrought metal corrosion and erosion resistance can be expected in corresponding elements, heat affected zones and castings for most titanium alloys, since the same protective oxide surface film is formed.
The useful resistance of titanium alloys is limited in strong, highly reducing acid media, such as moderately or highly concentrated solutions of HCl, HBr, H2SO4, and H3PO4, and in HF solutions at all concentrations, particularly as temperature increases. However, the presence of common background or contaminating oxidizing species, even in concentrations as low as 20-100 ppm, can often maintain or dramatically extend the useful performance limits of titanium in dilute-to-moderate strength reducing acid media.
Where enhanced resistance to dilute reducing acids or crevice corrosion in hot chloride solutions is required, titanium alloys containing minor levels of palladium, ruthenium, nickel or higher molybdenum should be considered. Some examples of these more corrosion-resistant titanium alloys include ASTM GRADES 7, 11, 12, 16, 17, 18, 19, 20, 26, 27, 28, and 29. These minor alloy additions also inhibit susceptibility to stress corrosion cracking in high strength alloys exposed to hot, sweet or sour brines.
Therefore, titanium alloys generally offer useful resistance to significantly larger ranges of chemical environments and temperature compared to steel, stainless steels and aluminum-, copper- and nickel-based alloys. Table 3 (page 14) provides an overview of a myriad of chemical environments where titanium alloys have been successfully utilized in the chemical process and energy industries.
Thus, titanium alloys is more suitable materials that can be applied in airframe due to its attractive corrosion and erosion resistance which is much better than aluminum alloys or others materials.
2.4 Others Attractive Properties of Titanium Alloys
Table 2: Other Attractive Properties of Titanium Alloys
Exceptional erosion and erosion-corrosion resistance
High fatigue strength in air and chloride environments
High fracture toughness in air and chloride environments
Low modulus of elasticity
Low thermal expansion coefficient
High melting point
High intrinsic shock resistance
High ballistic resistance-to-density ratio
Nontoxic, non-allergenic and fully biocompatible
Very short radioactive half-life
Excellent cryogenic properties
Titanium’s relatively low density, which is 56% of steel and half that of nickel and copper alloys, means twice as much metal volume per weight and much more attractive mill product costs when viewed against other metal on a dimensional basis. Together with higher strength, this obviously translates into much lighter or smaller components for both static and dynamic structures (aerospace engines and airframes, transportable military equipment), and lower stress for lighter rotating and reciprocating components. Reduced component weight and hang-off loads achieved with Ti alloys are also key for hydrocarbon production tubular strings and dynamic offshore risers and Navy ship and submersible structures.
Titanium alloys exhibit a low modulus of elasticity which is roughly half that of steels and nickel alloys. This increased elasticity (flexibility) means reduced bending and cyclic stresses in deflection-controlled applications, making it ideal for springs, bellows, body implants, dental fixtures, dynamic offshore risers, drill pipe and various sports equipment.
Moreover, titanium’s inherent no reactivity (nontoxic, non-allergenic and fully biocompatible) with the body and tissue has driven its wide use in body implants, prosthetic devices and jewelry. Thus, this situation able to protect the airframe from chemical reaction when it contact with air, rain water and environmental temperature.
Stemming from the unique combination of high strength, low modulus and low density, titanium alloys are intrinsically more resistant to shock and explosion damage than most other engineering materials. These alloys possess coefficients of thermal expansion which are significantly less than those of aluminum, ferrous, nickel and copper alloys. This low expansivity allows for improved interface compatibility with ceramic and glass materials and minimizes warp age and fatigue effects during thermal cycling.
Titanium is essentially nonmagnetic (very slightly paramagnetic) and its ideal where electromagnetic interference must be minimized. When irradiated, titanium and its isotopes exhibit extremely short radioactive half-lives, and will not remain “hot” for more than several hours. Its rather high melting point is responsible for its good resistance to ignition and burning in air, while its inherent ballistic resistance reduces susceptibility to melting and booming during ballistic impact, making it a choice lightweight armor material for airframe. Alpha and alpha-beta titanium alloys possess very low ductile-to-brittle transition temperatures and have, therefore been attractive materials for cryogenic vessels, airframe and components.
2.5 Heat Transfer Characteristic
Titanium has been a very attractive and well established heat transfer material in shell or tube, plate or frame, and other types of heat exchangers for process fluid heating or cooling, especially in seawater coolers. Exchanger heat transfer efficiency can be optimized because of the following beneficial attributes of titanium:
Exceptional resistance to corrosion and fluid erosion
An extremely thin, conductive oxide surface film
A hard, smooth, difficult-to-adhere to surface
A surface that promotes condensation
Reasonably good thermal conductivity
Although unalloyed titanium possesses an inherent thermal conductivity below that of copper or aluminum, its conductivity is still approximately 10-20% higher than typical stainless steel alloys. With its good strength and ability to fully withstand corrosion and erosion from flowing, turbulent fluids, titanium walls can be thinned down dramatically to minimize heat transfer resistance and reducing the cost at the same time.
Titanium’s smooth non-corroding, hard-to-adhere to surfaces maintains high cleanliness factors over time. This surface promotes drop-wise condensation from aqueous vapors, thereby enhancing condensation rates in cooler or condensers compared to other metals as indicated in Figure 6.
The ability to design and operate with high process or cooling water side flow rates and turbulence further enhances overall heat transfer efficiency.
All of these attributes permit titanium heat exchanger size, material requirements and overall initial life cycle costs to be reduced making titanium heat exchangers more efficient and cost-effective than those designed with other common engineering alloys.
According to Dr. Roger Digby, CEng FIMMM, Head of M&P Integration, New Product Airbus, November 2007, he has comes out the information as below which is quite similar with the information that has been presented by Dr. James at the beginning of this assignment.
Dr. Roger Digby has stat that Titanium Alloys has high strength to weight ratio which able to contribute some weight saving and used for highly loaded structure in geometrically constrained areas (wing root and landing gear bay).
Moreover, the inherent resistance to corrosion and compatibility with composites able to removes requirement for coating and simplifies the design of the airframe as well. Furthermore , the property stability at elevated temperatures able to replace Aluminum in high temperature area such as APU bay of the airframe.
Based on research information by Dr. Roger Digby, the composites use over time can be expressed by diagram above and I realized that the result of the research is titanium have replacing aluminum structure in airframe.
Diagram above is the principle Titanium application on A380. Based on the diagram, we able to see that titanium alloy is an important material in airframe industrial and due to its attractive mechanical properties, it has been chosen for the material of airframe.
Even thought the price of the titanium might be more expensive if compare with others material such as aluminum alloys, but when we comparing at the mechanical properties between titanium alloys and others materials, I found out that titanium alloy is still the most suitable material for airframe.
Besides that, using titanium alloy able to reduce the maintenance cost or service course if compare to other material such as aluminum alloy that facing service problems that has been discuss in service problems section (page 4).
3.0 Fabrication process of Titanium Alloys for Airframe
Before I goes in detail about the fabrication process or processes about titanium alloys, I would like to discuss about the basic titanium metallurgy.
3.1 Basic Titanium Metallurgy
Titanium mill produces that available in both commercially pure and alloy grades can be grouped into three categories according to the predominant phase or phases in their microstructure, there are alpha, alpha-beta, and beta.
Although each of these three general alloy types requires specific and different mill processing methodologies, each offers a unique suite of properties which may be advantageous for a given application. In pure titanium, the alpha phase can be characterized by a hexagonal close packed crystalline structure which is stable from room temperature to approximately 882°C (1620°F). The beta phase in pure titanium has a body-centered cubic structure and is stable from approximately 882°C (1620°F) to the melting point of about 1688°C (3040°F).
3.2 Machining Titanium
Titanium can be economically machined on a routine production basis if shop procedures are set up allow for the physical characteristics common to the metal. The factors which must be given consideration are not complex, but they are vital to successfully machining titanium.
The different grades of titanium such as commercially pure and various alloys do not have identical machining characteristic, any more than all steels, or all aluminum grades have identical characteristics. As example, stainless steel has low thermal conductivity of titanium inhibits dissipation of heat within the work piece itself, thus requiring proper application of coolants.
Good tool life and successful machining of titanium alloys can be assured if the following guidelines are observed:
Maintain sharp tools to minimize heat buildup and galling
Use rigid setup between tool and workpiece to counter workpiece flexure
Use a generous quantity of cutting fluids to maximize heat removal
Utilize lower cutting speeds
Maintain high feed rates
Avoid interruptions in feed (positive feed)
Regularly remove turnings from machines
For the machining titanium section, it can be divided into different machinery way. They are as below:
Commercially pure and alloyed titanium can be turned with little difficulty. Carbide tools should be used wherever possible for turning and boring since they offer higher production rates and longer tool life.
Where high speed steels are used, the super high speeds are recommended. Tool deflection should be avoided and a heavy and constant stream of cutting fluid applied at the cutting surface. Live centers must be used since titanium will seize on the dead center.
The milling of titanium is more difficult operation than that of turning. The cutter mills only part of each revolution and chips tend to adhere to the teeth during that portion of the revolution that each tooth does not cut. On the next contact, when the chip is knocked off, the tooth may be damaged.
This problem can be alleviated to a great extent by employing climb milling, instead of conventional milling. In this type of milling, the cutter is in contact with the thinnest portion of the chip as it leaves the cut, minimizing chip “welding”
For slab milling, the work should move in the same direction as the cutting teeth and for face milling, the teeth should emerge from the cut in the same direction as the work is fed.
In milling titanium, when the cutting edge fails, it is usually because of chipping. Thus, the results with carbide tools are often less satisfactory than with high speed steel. The increase in cutting speeds of 20-30% which is possible with carbide tools compared with high speed steel tools does not always compensate for the additional tool grinding costs.
Consequently, it is advisable to try both high speed steel and carbide tools to determine the better of the two for each milling job. The use of a water-base coolant is recommended as well.
Successful drilling is accomplished by using sharp drills of proper geometry and by maintaining maximum drilling force to ensure continuous cutting. It is important to avoid having the drill ride the titanium surface since the resultant work hardening makes it difficult to reestablish the cut.
Another important factor in drilling titanium is the length of the unsupported section of the drill. This portion of the drill should be no longer than necessary to drill the required depth of hole and still allow the chips to flow unhampered through the flutes and out of the hole. This permits application of maximum cutting pressure, as well as rapid drill removal to clear chips and drill re-engagement without breakage. An adequate supply of cutting fluid to the cutting zone is also important.
High speed steel drills are satisfactory for lower hardness alloys and for commercially pure titanium but carbide drills are best for most titanium alloys and for deep holes drilling.
Titanium is successfully ground by selecting the proper combination of grinding fluid, abrasive wheel, and wheel speeds. Both aluminum oxide and silicon carbide wheels are used. Considerably lower wheel speeds than in conventional grinding of steels are recommended. Feeds should be light and particular attention paid to the coolant.
A water-sodium nitrite coolant mixture gives good results with aluminum oxide wheels. Silicon carbide wheels operate best with sulfochorinated oils, but these can present a fire hazard, and it is important to flood the work when using these oilbase coolants.
For the others method that can be applied in machining titanium including tapping, sawing, water jet, electric discharge machining and chem. milling as well. All this method is useful and important in the progress of titanium alloy fabrication process.
3.3 Forming Titanium
Titanium and its alloys can be cold and hot formed on standard equipment using techniques similar to those of stainless steels. However, titanium possesses certain unique characteristics that affect formability, and these must be considered when undertaking titanium forming operations.
The room temperature ductility of titanium and its alloys is generally less than that of the common structural metals including stainless steels. This necessitates more generous bend radii and less allowance for stretch formability when cold forming.
Titanium has a relatively low modulus of elasticity, about half that of stainless steel. This results in greater spring back during forming and requires compensation either during bending or in post bend treatment.
Titanium in contact with itself or other metals exhibits a greater tendency to gall than does stainless steel. Thus, sliding contact with tooling surfaces during forming calls for the use of lubricants. Effective lubricants generally include grease, heavy oil or waxy types, which may contain graphite or molly disulfide additives for cold forming and solid film lubricants or glassy coatings for higher temperature forming.
The following is basic information on forming titanium. A great deal of published information exists on titanium forming practices in the common commercial forming processes.
Before titanium sheet is formed, it should be clean and free of surface defect such as nicks, scratches or grinding marks. All scratches deeper than the finish produced by 180-grid emery should be removed by sanding. To prevent edge cracking, burred and sharp edges should be radius. Surface oxides can lead to cracking during cold forming and should be removed by mechanical or chemical methods.
Plate products should be free of gross stress raisers, very rough, irregular surface finishes, and visible oxide scale and brittle alpha case (diffused-in oxygen layers) to achieve reasonable cold or warm formability. Experience has shown that pickled plate often exhibits enhanced formability compared to plate with as-grit blasted or as ground surface finishes.
Cold Versus Hot Forming
Commercially pure titanium, the ductile, low-alloy alpha and unpaged beta titanium alloys can be cold formed within certain limits. The amount of cold forming either in bending or stretching is a function of the tensile elongation of the material.
Heating titanium increases its formability, reduces spring back and permits maximum deformation with minimum annealing between forming operation. Heated forming dies or radiant heaters are occasionally used for low temperature forming while electric furnaces with air atmospheres are the most suitable for heating to higher temperature. Gas fired furnaces are acceptable if flame impingement is avoided and the atmosphere is slightly oxidizing.
Any hot forming or annealing of titanium products in air at temperature above approximately 590-620◦C produces a visible surface oxide scale and diffused-in oxygen layer (alpha case) that may require removal on fatigue- and fracture critical components. Oxide scale removal can be achieved mechanically or by chemical descale treatment.
Stress Relief and Hot Sizing
Cold forming and straightening operations produce residual stresses in titanium that sometimes require removal for reason of dimensional stability and restoration of properties.
Stress relieving can also serve as an intermediate heat treatment between stages of cold forming. The temperatures employed lie below the annealing ranges for titanium alloys. They generally fall within 482-649◦C with times ranging from 30 to 60 minutes depending on the workpiece configuration and degree of stress relief desired.
Hot sizing is often used for correcting spring back and inaccuracies in shape and dimensions of preformed parts. The part is suitably fixture such that controlled pressure is applied to certain areas of the part during heating. This fixture unit is placed in a furnace and heated at temperatures and times sufficient to cause the metal to creep until it conforms to the desired shape. Creep forming is used in a variety of ways with titanium, often in conjunction with compression forming using heated dies.
Typical Forming Operation
Following are description of several typical forming operations performed on titanium. They are representative of operations in which bending and stretching of titanium occur. The forming can be done cold, warm or hot. The choice is governed by a number of factors among which are workpiece section thickness, the intended degree of bending or stretching, the speed of forming (metal strain rate), and alloy product type.
In this operation, bending is employed to form angles, z-sections, channels and circular cross sections including pipe. The tooling consists of unheated dies or heated female and male dies.
Stretch forming has been used on titanium sheet primarily to form contoured angles, hat sections, Z-sections and channels, and to form skins to special contours. This type of forming is accomplished by gripping the sheet blank in knurled jaws, loading it until plastic deformation begins, and then wrapping the part around a male die. Stretch forming can be done cold using inexpensive tooling or, more often it is done warm by using heated tooling and preheating the sheet blank by the tooling.
Spinning and Shear Forming
These cold, warm or hot processes shape titanium sheet or plate metal into seamless hollow parts using pressure on a rotating workpiece. Spinning produces only minor thickness changes in the sheet, where as shear-forming involves significant plastic deformation and wall thinning.
Other Forming Processes
Titanium alloy sheet and plate products are often formed cold, warm or hot in gravity hammer and pneumatic drop hammer presses involving progressive deformation with repeated blows in matched dies. Drop hammer forming is best suited to the less high strain rate sensitive alpha and leaner alpha-beta titanium alloys. Hot closed-die and even isothermal press forging is commonly used to produce near-net shape components from titanium alloys.
Trapped-rubber forming of titanium sheet in cold or warm (540◦C max) pressing operation can be less expensive than that utilizing conventional mating “hard die” tooling. Even explosive forming has been successfully employed to form complex titanium alloy airframe component.
The lower strength, more ductile titanium alloys can be roll-formed cold as sheet strip to produce long lengths of shape products, including welded tubing and pipe. Welded or seamless tubing can be bent cold on conventional mandrel tube benders. Seam-welded unalloyed titanium piping can also be bent cold or warm on standard equipment utilizing internal mandrels to minimize buckling, whereas higher strength alloy seamless piping can be successfully bent in steps via hot induction bending.
3.4 Welding Process
Commercially pure titanium and most titanium alloys are readily welded by a number of welding processes being used today. The most common method of joining titanium is the gas tungsten arc (GTAW) process and, secondarily, the gas metal-arc (GMAW) process. Others include electron beam and more recently laser welding as well as solid state processes such as friction welding and diffusion bonding. Titanium and its alloys also can be joined by resistance welding and by brazing.
The techniques for welding titanium resemble those employed with nickel alloys and stainless steels. To achieve sound welds with titanium, primary emphasis is placed on surface cleanliness and the correct use of inert gas shielding. Molten titanium reacts readily with oxygen, nitrogen and hydrogen and exposure to these elements in air or in surface contaminants during welding can adversely affect titanium weld metal properties. As a consequence, certain welding processes such as shielded metal arc, flux cored arc and submerged arc are unsuitable for welding titanium.
In addition, titanium cannot be welded to most other metals because of formation of embrittling metallic compounds that lead to weld cracking.
3.4.1 Welding Environment
While chamber or glove box welding of titanium is still in use today, the vast majority of welding is done in air using inert gas shielding. Argon is the preferred shielding gas although argon helium mixtures occasionally are used if more heat and greater weld penetration are desired. Conventional welding power supplies are used both for gas tungsten arc and for gas metal arc welding. Tungsten arc welding is done using DC straight polarity (DCSP) while reverse polarity (DCRP) is used with the metallic arc.
3.4.2 Inert Gas Shielding
An essential requirement for successfully arc welding titanium is proper gas shielding. Care must be taken to ensure that inert atmosphere protection is maintained until the weld metal temperature cools below 426°C (800°F). This is accomplished by maintaining three separate gas streams during welding.
The first or primary shield gas stream issues from the torch and shields the molten puddle and adjacent surfaces. The secondary or trailing gas shield protects the solidified weld metal and heat-affected zone during cooling. The third or backup shield protects the weld underside during welding and cooling. Various techniques are used to provide these trailing and backup shields and one example of a typical torch trailing shield construction is shown below. The backup shield can take many forms.
One commonly used for straight seam welds is a copper backing bar with gas ports serving as a heat sink and shielding gas source. Complex workpiece configurations and certain shop and field circumstances call for some resourcefulness in creating the means for backup shielding. This often takes the form of plastic or aluminum foil enclosures or “tents” taped to the backside of the weld and flooded with inert gas.
3.4.3 Weld Join Preparation
Titanium weld joint designs are similar to those for other metals, and the edge preparation is commonly done by machining or grinding. Before welding, it is essential that the weld joint surfaces are free of any contamination and that they remain clean during the entire welding operation. The same requirements apply to welding wire used as filler metal. Contaminants such as oil, grease and fingerprints should be removed with detergent cleaners or non-chlorinated solvents. Light surface oxides can be removed by acid pickling while heavier oxides may require grit blasting followed by pickling.
3.4.4 Weld Quality Evaluation
A good measure of weld quality is weld color. Bright silver welds are an indication that the weld shielding is satisfactory and those proper welds interpose temperatures have been observed. Any weld discoloration should be cause for stopping the welding operation and correcting the problem. Light straw-colored weld discoloration can be removed by wire brushing with a clean stainless steel brush, and the welding can be continued. Dark blue oxide or white powdery oxide on the weld is an indication of a seriously deficient purge.
The welding should be stopped, the cause determined and the oxide covered weld should be completely removed and re-welded. For the finished weld, non-destructive examination by liquid penetrate, radiography and/or ultrasound are normally employed in accordance with a suitable welding specification. At the outset of welding it is advisable to evaluate weld quality by mechanical testing. This often takes the form of weld bend testing, sometimes accompanied by tensile tests.
3.4.5 Resistance Welding
Spot and seam welding procedures for titanium are similar to those used for other metals. The inert-gas shielding required in arc welding is generally not required here. Satisfactory welds are possible with a number of combinations of current, weld time and electrode force. A good rule to follow is to start with the welding conditions that have been established for similar thicknesses of stainless steels and adjust the current, time or force as needed. As with arc welding, the surfaces to be joined must be clean. Before beginning a production run of spot or seam welding, weld quality should be evaluated by tension shear testing of the first welds.
3.5 Cost Factor of Titanium Alloys
The price of titanium alloy products results from a number of factors:
Alloying grade. (Some grades with Pd alloying component can significantly increase the price of the alloy.
The purity of the grade. (the more pure the higher the cost)
The test and inspection requirements
The procured quantities. (the more ordered the lower the specific cost)
The geometry. (Rolling or forging affects prices per volume or weight)
Demand (high defense demand for aerospace industry can result higher metal prices)
Local economy (Metal availability)
Based on my research, the price of titanium was about 13000 to 43000/tonne in year 2000 but following by 8950/tonne in year 2002. In year 2005, the price of titanium has varied between 6000 and 9000/ tonne which keep decreasing if compare with previous. This situation occurs because the needed of titanium alloys has been increasing and therefore the volume of manufacture has been increasing as well, therefore, the price able to be reduced to fulfill the requirement.
As conclusion, titanium alloys is a very good and famous material in this century due to its attractive mechanical properties and high technology that able to be applied in the fabrication process. Therefore, titanium alloys is the most suitable material which can be used for the airframe industrial.
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