Mechanical Engineering - Applied Metallurgy

Part B questions (AY 2022-23)

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Department of Mechanical Engineering Applied Metallurgy: Part B Questions Author: Fabio SantoroAdvisor: Prof. Fabrizio D’Errico Academic year: 2022-2023Abstract: This article contains the possible answers to the questions that can be assigned to the Part B of the exam of Applied MetallurgyContents 1 Discuss the phenomena of damage in metallic materials related to the phenomenonof: brittleness3 2 Discuss the phenomena of damage in metallic materials related to the phenomenonof: fatigue6 3 The surface treatments for the hardening of steels and the phenomenon of contactfatigue9 4 The phenomenon of damage due to corrosion of metals and the choice of stainlesssteels for counteracting this phenomenon13 5 The phenomenon of damage due to wear and the choice of tool steels for counter-acting this phenomenon17 6 The phenomenon of creep and the choice of special steels for using at high temper-atures21 7 Magnesium Alloys25 8 Titanium alloys28 9 Steels for special applications (Specialty Steels): HSLA, TRIP and Dual Phasesteels31 10 The steel making processes34 11 Aluminum alloys and main casting processes37 this page left 1 Discuss the phenomena of damage in metallic materials related to the phenomenon of: brittleness A metallic material is characterized by a regular structure named crystal structure. The crystal structure is composed by the repetition of elementary (or unit) cells. However the real crystal lattice is not perfect and it presents defects (can be vacancies, interstitial atoms, substitutional atoms, angle of misalignment at grain boundaries and dislocations. In particular, dislocations determine the capability of a material to be plastically deformed (with macroscopic effects). This property distinguishes two main categories of metallic materials: ductile and brittle. From now on we will simplify the notation by brittle materials (instead of brittle metallic materials) and ductile materials (instead of ductile metallic materials). Brittle materials are characterized by high value of Young modulus (elastic stiffness), low toughness, high hardness, high Yield Strength and high UTS. All these properties define a material with low ductile property. Brittle fracture phenomenon is one of the two main type of fractures that can happen in metallic materials. Brittle fracture occurs with no warning because it isn’t present any gross plastic deformation of the metal component (at least can be a very low plastic deformation). There are two many reasons to increase the possibility of a brittle fracture, also for ductile materials: stopping dislocation movement and/or reduce the slip plane. For example there are some phenomena that can help to observe brittle fractures:•Operating Temperature: by decreasing the temperature we reduce the vibrating motion of atoms, all the crystal lattice reduces its volume (becomes more compact) and, as consequence, dislocations can be hardly moved. It is possible to observe this phenomenon with the Charpy test. This experiment is used to measure the toughness (energy necessary to break the material) of a material specimen, with standard form (notched) and dimensions, by the impact of an hammer on the specimen. We can observe the toughness is reduced by decreasing the temperature, so we need less energy to break the specimen and we can observe also less macroscopic deformations before fracture. For each material it is evaluated by experiments (Charpy test) theDuctile to Brittle Transition Temperature (DBTT). In figure1it is possible to see the behaviour of some mechanical properties of a material decrease with temperature. The range of temperature where it is possible to see that big variation is the DBTT. In general, adding Ni or reducing C, it is possible to reduce the DBTT.Figure 1:Ductile to Brittle Transition Temperature (DBTT) diagram Knowing the DBTT of each material, it is possible to select the best material to use in those situations where there is high (desert) or low (Arctic) value of temperature. We have to take in consideration that there are some metal alloys that do not exhibit a DBTT (i.e. Special Steels and Titanium alloys).(Contents link)Applied Metallurgy: Part B Questions Pag. 3 • Strain rate: this phenomenon is called strain-hardening mechanism and it consists into the fact it is difficult to deform a material increasing the rate of the deformation. Dislocations are locked and they hardly move •No shear stress: if there is no shear stress, dislocations can’t be moved (can’t slip). It usually happened with triaxial stress condition A brittle fracture is characterized by the initiation of a crack (usually where stress is concentrated due to an important defect in the crystal lattice and usually it starts on material surface) and its propagation inside the material (usually the propagation speed is comparable with the sound speed inside the material). There is an analytical method (energetic method of Griffith’s experience) that tries to explain the advancement of a crack in a ductile material and the consequently final break of material. Let’s consider the model represented in figure (2). It consists into a plate of thicknesstwhere it is localized a crack of lengthaand the plate is stressed by the forceF.Figure 2:Griffith’s experiment - fast fracture in a plane This criterion states that the component will brake with a fast fracture when the energy necessary to the advancement of the crack is equal to the energy stored in the material in form of elastic energy (it is necessary to remember that only the continuous material is sub jected to elastic deformation and the zone where the material is already separated y the crack is unstressed). This phenomenon happens when is reached the condition expressed in the following equation: σ·√π ·a=pE ·G c(1.1) Each element of this equation is:•σ Nm 2 is the stress imposed by the forceF •a[m]is the length of the crack •E Nm 2 is the Young modulus •G c Jm 2 is the Toughness The crack can propagate with two mechanisms: •Ductile tearing: In a ductile material the crack advance following first the elastic deformation, then when it is reached the YS value of stress, the material starts to deform plastically. There are forming few voids in the crack advancement direction for a certain depth (plastic radius, that depends on the stress, YS and the crack length as expressed in equation1.2), then the crack advances. This type of propagation of a crack can be observed when the material is sub jected to fatigue stress of continuous opening and closing of the crack ry=σ 2 ·a2 σ2 y(1.2) •Cleavage: this mechanism of propagation is typical of materials characterized by high YS (like ceramics). In this case there is a very small elastic and plastic deformation at the tip of the crack. The local stress is still very high that allows to break, one by one, interatomic bond (In this case the radius of propagation is comparable with atoms distance)Pag. 4 Applied Metallurgy: Part B Questions(Contents link) The type of advancement of the crack can be observed, after the fracture, by naked eye or with optical devices. The crack can propagate in two different ways, by parallel, or better, mono-dimensional direction (calledriver pattern3a) or V-shape propagation (Chevron pattern3b). In the second case the crack propagates with radial directions from its origin.(a) River pattern(b) Chevron pattern Figure 3:Crack propagation patterns Normally the crack propagation doesn’t follow the geometry of the crystal lattice and grain boundaries, but it pass through the grains. This phenomenon is calledTransgranularfracture, can be observed by using scanning electronic microscope and it also callednormal brittle fracture. However in some materials can happen that the crack propagation path follows the grain boundaries. This is called Intergranularfracture, it occurs when grain boundaries are abnormally weak and, as consequence, this is calledabnormal brittle fracture. This fracture can be observed in all those materials that don’t contain the correct alloying element or if they are sub jected to the wrong heat treatment. For example, in quenching-tempering steels it is necessary the presence of Molybdenum (added during the steel-making process). Molybdenum is used to prevent the high brittleness of grain boundaries because to to specific small cooling rate, some precipitate particles can migrate to the grain boundary increasing its brittleness. This kind of fracture can be also observed ifH 2atoms has not been removed during steel making process (in the ladle furnace). During previous steps of steel making, the molted material can absorb water from the atmosphere (in particular if it is humid) and if it is not completely removed, H2can easily combine with other elements and forming brittle precipitations at grain boundariesFigure 4:Transgranular vs Intergranular brittle fracture (Contents link)Applied Metallurgy: Part B Questions Pag. 5 2 Discuss the phenomena of damage in metallic materials related to the phenomenon of: fatigue Fatigue is the most common form of failure in metals. It occurs when a mechanical component is sub jected to dynamic (cycling) stress. In this case the component can fail also if the level of stress is way below the value of YS and UTS. The first case reported of this strange and unknown failure phenomenon is located in France in 1842 with a really big train accident. No one could explain why one train axle tailed without showing any kind of plastic deformation. Wohler was the first who tried to give an explanation to this phenomenon by performing laboratory experiments. These experiments consists into the observation of the response of a material speciment to a dynamic stress. The speciment has very strict requirements, one of them is the surface quality that must not show any defect/polished mirror-like surface). It is possible to test a speciment to fatigue with: axial (5a), bending (5b) and torsional. The easiest and (relatively) cheapest test is the rotating-bending fatigue test. In figure (5b) is represented also the equipment used to perform the test(a) Axial(b) (rotating) Bending Figure 5:Fatigue Tests A real component is generally stressed by random loads, but these experiments are performed with a sinusoidal-shape stress (6a). The stress-time curve is characterized by:•σ max: maximum stress •σ min: minimum stress •σ m=σ max+ σ min2 : mean stress •σ a=σ max− σ min2 : stress amplitude •R=σ minσ max: fatigue ratio(a) Stress-Time curve(b) withσ D(c) withoutσ D Figure 6:Fatigue stress curve and Wholer diagrams The experiment consists to performs several tests varying the parameter and applying a statistical approach, it is possible to obtain a diagram similar to (6b). On the vertical axis is represented the amplitude stressσ a, while on the horizontal axis the number of cycles. These tests can last many weeks or months, so the cycle axis is usually represented in logarithmic scale. For some materials, there is a limit calledσ D(6b). Below of that value the speciment is not sub jected anymore to fatigue failure, but there are also materials that don’t present that characteristic and they are always sub jected to fatigue failure (6c). There are some factors that can modify the experimental diagrams:•Notch effect(7a): the tests are performed with an unnotched speciment. The presence ofPag. 6 Applied Metallurgy: Part B Questions(Contents link) notches push the fatigue curve down, so sigma Ddecreases. Notches are typical the origin of cracks due to fatigue •Mean stress(7b): if the component or the speciment is sub jected to mean stress (σ m̸ = 0), the fatigue limit curve moves down •Material strength(7c): increasing the material strength, the endurance limit is increased. For low values of hardness it is possible to observe a linear correlation •Manufacturing stress(7d): manufacturing processes tends to move down the fatigue limit curve because all the fatigue experiments are performed with mirror polishing. A low quality process will reduce more the limit •Environment(7e): if the component is exposed to a corrosive environment, also materials that originally showed a value ofσ Dcan behave like materials that don’t have that property.(a) Notch(b) Mean stress(c) Strength(d) Manufacturing(e) Environment Figure 7:Fatigue factors The fatigue failure is characterized by 3 stages:(a) Nucleation(b) Growing(c) Fast Fracture Figure 8:Fatigue Failure - Steps ▶Stage 1:Fatigue Crack Nucleation: Cracks usually start on the material surface (or near surface) (8a), in particular where there is a material discontinuity (like notches) but, if there aren’t any defects and the stress level reaches the value necessary to move dislocations, the material starts to deform and the surface becomes to show some irregularities like extrusions and intrusions◦Intrusions are very small notches (dimensions comparable to crystal lattice) where cracks can nucleate ▶Stage 2:Fatigue Crack Propagation: the crack starts to propagate inside the material under the continuous action of opening and closing (8b). This way of propagation is called ductile tearing. When it is reached a minimum value of stress, a limited zone near the crack(Contents link)Applied Metallurgy: Part B Questions Pag. 7 tip starts to deform plastically forming micro-voids, then this zone breaks and the mechanism restart. The dimension of this plastic zone is equal to the propagation radiusr ythat depends on theFracture ToughnessK=Y·σ·√π ·a, the crack lengthaand the value of the yield strength of the materialσ y: ry=K 22 πσ2 y(2.1) This mechanism start with a initial crack lengtha 0until the final length a cfollowing a function of the number of cycles (9a). The slope of this curve is thecrack growth rateda/dn. During the component life it is necessary to perform periodically inspections (externally by vision, eddy current or luminescent fluid contrast, or internally by ultrasound,xorγrays). The crack growth rate is dependent to the fracture toughness∆K=Y·∆σ·√πa to make the crack of lengtha to advance of a displacement∆a. This relation is represented in a bi-logarithmic diagram called Paris Law(9b). This was extracted by an experiment with a plate with a crack stressed with an axial cycle load (9c).(a) Crack length vs # of cycles(b) Paris Law(c) Experi- ment Figure 9:Crack growth rate advancement If it is applied a too small load, no crack growth would be visible. Increasing∆Kis possible to observe a constant crack grows rate following the relation: dadn = C·∆Km (2.2) whereCandmare material constants. It is possible to determinate an expression to calculate the maximum number of cycle the material speciment can support before break ▶Stage 3:Fatigue Fast Fracture: when the crack length reaches a critical valuea cit occurs the quasi-instantaneous break of the component. The phenomenon is calledCleavageand it consists a really quick separation of atoms bond with a crack propagation rate comparable to the speed of sound in metal. It is a brittle fracture. In the last part of the curve of (9b) it is possible to observe a rapid and unstable crack growth. The critical crack lengtha ccan be estimated with the expression when the fracture toughness reaches the critical valueK=K c: ac=K 2 cπY 2 ·σ2 max(2.3) There are different methodologies to design a component sub jected to fatigue:•Infinite-Life: design the component to work under the endurance limit •Safe-Life: the component doesn’t contain any defect, so it cannot develop a critical crack size •Fail-Safe: it is assumed that the component contains defects. During maintenance, cracks will be detected and repaired before the component fail •Damage-Tolerant: it is assumed that the component contain cracks. Their length are kept under control to determine the growth rate. During programmed inspection, it is decided if the component would be repaired or replaced when the crack is next to reach the critical lengthPag. 8 Applied Metallurgy: Part B Questions(Contents link) 3 The surface treatments for the hardening of steels and the phe- nomenon of contact fatigue For all the components that are sub jected to contact stresses (gears and cams) it is required to have an high surface hardness. It is possible to choose directly hard materials like steels with high carbon con- tent (next to eutectoidic concentration) and perform on them classic hardening treatment (quenching and tempering) to increase the hardness. These components will be globally hard (from the surface to the core) and it could be a problem because they loose all ductile properties.These materials are also difficult to machine. The other possibility is to use material with medium low carbon content and per- forms hardening treatments just on the surface, in this way we maintain the properties of the original material in the core. They are easy to machine and we perform the surface hardening treatments after we reach the final shape. The surface hardening treatments are divided in two families:ThermalandThermo-Chemical. The first one consists into just heat up the material just next to the surface without changing the chem- ical composition in that zone. The surface layers are heat up to obtain austenitic crystal structure, they are quickly cooled down in water (obtaining martensite BCT structure, quenching) and finally they are sub jected to tempering (temperature up to maximum 100°C) for a limited time just to release the residual stress from the quenching treatment without allow carbon diffusion from the surface to the core. There are three treatments and they are different by the kind of heat source they use:•Flame heating: it consists to heating up the material with the direct contact of a flame (for example using an acetylene torch). This is a very simple procedure and it is used on low con- ductive materials because we are able to heat up quickly the surface thank to high power source (direct flame). This procedure can be performed by an operator or by an automatic system (more controlled in terms of quality) and it is possible to achieve depth of 5-8mm of hard materialFigure 10:Schematic of flaming surface hardening process •Induction: this process uses the properties of conductive materials to be heat up if they are placed into a magnetic field. This procedure allows, without direct contact, to heat up the surface layers of a component that is placed into a coil (cylindrical or circular) that is carrying a current with high frequency (from 2 to 500kHz). Usually the system is already provided of a cooling system like water-jets and the depth of the treatment depends on current intensity and frequency and also from exposition time to the magnetic field(a)(b) Figure 11:Induction surface hardening process examples(Contents link)Applied Metallurgy: Part B Questions Pag. 9 • Laser: of all three, this is the most precise treatment. In this case the heat source is a concen- trated (by optical system) light beam. It is possible to reach 2mm of depth and it is not necessary to perform another heat treatment (like tempering) to reduces the internal stresses because just 0,2mm of surface layers are sub jected to quenching distortion. This makes laser treatment very quickly and productive.Figure 12:Schematic of Laser surface hardening process The thermo-chemical surface treatments consists into two procedures:CarburizingandNitriding. Both of them change the material chemical composition on the material surface:•Carburizing: It consists into heating up a low carbon steel (max 0,2%C) into a furnace high carbon concentration atmosphere at the temperature closed to 900°C at austenitic structure (this temperature must not be too high otherwise the material will start recovery process and loose hardness). This temperature allows the diffusivity of carbon from the atmosphere into the material by occupying the interstitial spaces of the crystal lattice. The diffusion depth of carbon atoms depends on the time of exposition of the component at that temperature and the carbon content of the surface is about the eutectoidic concentration. At a certain point, the furnace is turned off and it cool down with the piece just to reach the quenching temperature of the surface. Then, the component is quenched in oil. It is necessary to release the residual stresses on the surface with tempering-like heat treatment. It is performed with a lower value of temperature than the usual one for tempering to inhibit the carbon diffusion from the surface to the core. After the carburizing process it is possible to perform finishing machining operationsFigure 13:Carburizing process: temperature-time plot •Nitriding: This process is very similar to the carburizing process, but, instead of increasing the carbon content on the material surface, it is used nitrogen (N). Usually, the row material is a quenching and tempering steel (low %C) but with low or at least none Nickel (the other alloys are normal). The nitriding process takes place at low temperature (450÷500°C) otherwise it will start the phenomenon of nitrogen diffusivity in the material core creating a crystal structure of nitrogen lamellas and the material becomes very brittle. The maximum depth is very low (0,8mm) and the the process time is very longer than carburizing (can last 100h) with the consequently high costs of energy and furnace. With nitriding process it is possible to reach higher hardness value (900-1200HV), way more than carburizing, so this treatment takes place after finishing operations because of the too high hardness it is impossible to machinePag. 10 Applied Metallurgy: Part B Questions(Contents link) Contact fatigue is a failure mechanism that occurs on the surface of bodies in continuous rolling and/or sliding contact. The typical components sub jected to this phenomenon are gears and bearings. The two main damaging modes of this phenomenon are: •Pitting: this consists into the damage of the surface by the formation of voids (craters) on it (14a). This phenomenon is caused mainly by wear of the two surfaces. The crack nucleates on the surface and propagates in the near layers till part of the material on the surface is removed. Depending of the depth of these caters there are:pittingfor depth0,5·T mand it takes place at grain boundaries due to the fact that there are a lot of spaces. Atoms can diffuse from a grain to another, so grain boundaries are modified (grains tend to enlarge, so small grains tend to disappear).Figure 35:Diffusion creep mechanisms Pag. 22 Applied Metallurgy: Part B Questions(Contents link) Is we want to use a new alloy, maybe we don’t have enough data to study the creep phenomenon and also the test requires a very lot of time. It is possible to extrapolate these data by performing an accelerated creep test at lower temperature and for a short period of time. It was developed an empirical model by Larson-Miller. The relation is: P=T·(lnt+C) =QR (6.2) where:•Tis the absolute temperature[K] •tis the time to rupture[h] •Qis the activation energy for creep[J/mol] •Ris the universal gas constant[J/mol·K] •Cis the Larson-Miller constantC∈[30,65] To resist to creep damage phenomenon it is convenient to choose a material with the following prop- erties:•High melting temperature (T m) •Select anF C Cmetal instead ofBC C. This is explained by the factF C Ccrystal structure is characterized by a lower diffusivity thanBC C •Fine inert particles (like carbides) distributed at grain boundaries to stop creep diffusion and inside grains to stop dislocation creep These materials are called high-temperature resistant alloys and they are characterized by the formation of a oxide protective layer on the surface:•Chromium-Oxide(C r 2O 3): this is effective for operative temperatures below 980°C. The high content ofC ris typical of stainless steels, but alsoN i−alloyscan content8÷48%ofC r •Aluminium-Oxide(Al 2O 3): this is effective for operating temperatures above 980°C. At these temperatureAl−oxideis more effective thanC rto counteract hot corrosion These alloys are mainly divided in two categories distinguished in:•Ferrous alloys: ◦Chromium-MolybdenumandChromium-Molybdenum-Vanadium: they are a low alloyed steels with presence ofC rfor oxidation resistance,M ois added to prevent the carbide decomposition up to550°C. These alloys are mostly used for the production of seamless pipes (pipes of small dimensions manufactured by extrusion and not by sheet bending and longitudinal bending) used in petrochemical industry and in boilers. They are also weldable to ferritic steels. The low content ofCinhibits the formation ofC r-carbides, soC ris mostly "applied" to prevent corrosion ◦Stainless steels: there are three 1.Martensitic: used for temperatures below540÷650°Cand they are tempered at 55°Chigher than the operating temperature to prevent softening by precipitations growing 2.Ferritic: they are used at lower temperature, less than370°C, otherwise in the tem- perature range400÷800°Cthere is the formation of the brittleσstructure. They can also be used at temperatures higher than800°Cbut, due to low mechanical resistance, they can only be used in low pressure exhausted gas piping 3.Austenitic: they can be used for temperature up to870°Cbut with limited stress thanks also to high thermal expansion. Thanks toF C Cstructure, there is a lim- ited diffusivity and with the precipitation ofC-nitrides able to stop creep dislocation movement.(Contents link)Applied Metallurgy: Part B Questions Pag. 23 • Non-Ferrous alloys: ◦Nickel and Nickel alloys: have an excellent combination of corrosion, oxidation, and heat resistance, combined with good mechanical properties. The can be used in aggressive environments. ThanksF C Cstructure they have high ductility and toughness. They can be strengthened by: solid solution, precipitation and work hardening ◦Ni-Cr-Fealloys: the chemical composition is50÷80%N ito increase alloying elements quantity and maintain high toughness,14÷30%C rto formC r 2O 3layer to resist also in severe environments (chloride and sulfured). These alloys are calledincoloy ◦Ni-Cr-Al-Fealloys: same properties ofNi-Cr-Fealloys but it containsAland they are used at temperatures above955°C. The oxidation protection layer is based inAl 2O 3 ◦Nickel superalloys: characterizedF C Cstructure strengthened bysolid solution(by al- loying elements) andprecipitates(γ′ carbides inγmatrix) that maintain high mechanical properties at high temperature. Large carbides are on grain boundaries to stop creep diffu- sion and small precipitates in the matrix to stop dislocations. They are also used for turbine blades up to temperature of1290°C(single grain investment casting)Pag. 24 Applied Metallurgy: Part B Questions(Contents link) 7 Magnesium Alloys Magnesium is the most light weight metal material with a density of1,78g/cm3 (1/4of steel≈ 7,85g/cm3 ). It is a very "elastic" material thanks to a very low Young Modulus (45GP avs≈205GP a of steel and it has a very low tensile strength (from140to345M P a). Magnesium is characterized by a medium-high thermal conductivity (3×4times higher than steel but still approximately half of aluminium) It has also low toughness, that means it is a very brittle material and this fact is directly correlated to its crystal structure (figure36) that isH C P(High Compact Closed Packed)Figure 36:Magnesium crystal structure This structure makes the magnesium very brittle because the dislocation slip plane (at ambient tem- perature) can be only parallel to the basal plane. As always, by increasing temperature and the consequence atoms’ vibration, it is possible to have different slip plane. These additional plane can be activated above205°C. It is necessary to remind thatM ghas a very low melting temperature (650°C) and, as consequence, some of its alloys suffer a lot to creep phenomenon (around95°C). Thanks to its lightweight characteristic,M gwas succesfully applied by Nazi Germany during WW2. The external panels of the aircraft were made inM galloys and the Luftwaffe aircraft were able to bomb London taking off directly from Germany and not from France. (Low weight→low fuel consumption →longer flight distance and more quantity of weapons). M galloys are designated by chemical composition, level of purity and hardening process: AB ab C D(7.1) Each element is:•Ais the first alloying element with mass content ofa(the higher content) •Bis the second alloying element with mass content ofb •Cis the purity level •Dindicates the hardening process In figure37are listed all the designation components:Figure 37:ASTM Magnesium designation table (Contents link)Applied Metallurgy: Part B Questions Pag. 25 There are two alloying elements ( F eandN i) for magnesium alloys that are very detrimental because they create impurities (micro galvanic cells) that reduce corrosion resistance. The main alloying element inM galloys is aluminium.Alincreases strength by solid-solution pre- cipitate and widens the freezing range, making the alloy easier to cast. In figure38ais represented theM g−Alphase diagram.Above approximately the8%ofAl, it will be formed an intermetallic brittle material (M g 17Al 12) (the white compound βin figure38bon grain boundaries). This brittle compound decreases the ductility.(a) Magnesium-Aluminium phase diagram(b) Metal lographic section Figure 38:Magnesium-Aluminium alloy Magnesium alloys are produced by wrought and casting processes. They, maybe, are the best castable metals butM gis very dangerously reactive with oxygen creating a not self-extinguish flame. As for Al, alsoM gflame cannot be extinguished with water becauseM gwill react with hydrogen with an explosive reaction (Twin Towers exploded because the automatic extinguish system ejected water on the burning planes and the magnesium exploded). It is necessary to control the atmosphere around the melted metal. It could be done by:•Vacuum casting •Inert gas in atmosphere (i.e. usingAr) Let’s analyse theM gcasting alloys from the oldest and cheapest to the youngest and more expensive one:•AM series(M g−Al): characterized by an high ductility and the aluminium content is very low (just little formation ofM g 17Al 12). They are susceptible at creep phenomena (operating temperature less than120C elsius) •AZ series(M g−Al−Z n): with the addition ofZ nit is possible to decreaseAl.Z nhelps the precipitation hardening also refining with ageing heat treatment •AS series(M g−Al−S iCreep resistant alloy): theS iaddition helps the formation of hard particles ofM g 2S i on grain boundaries that helps to stop the diffusion phenomenon of creep •ZK series(M g−RE−Z n−Z rRare Earth alloy): Zirconium has remarkable grain-refining ability because at the start of the solidification processZ rincreases the nucleation phenomena and, as consequence, the medium radius of the grains is smaller. It increases also the hardness. This alloy is sub jected to the heat treatments ofT5andT6.Z ris very expensive •ZE series(M g−RE−Z n−Z rRare Earth al loy): there are added Rare-Earth elements (like LaandC e) to form, during aging treatment, hard precipitations on grain boundaries that helps with creep. This alloy can be strengthened withT5treatment. RE elements are only extracted in China because that is the only place on earth where it is economically convenient to extract them •WE series(M g−Y): it contains about4×5%ofYincreasing strength and creep temperature up to300°C.Yis very expensive Wrought magnesium alloys are also produced as sheet, plates, extruded bars, billets and forging. Sheets and plates are well used in automotive sector thanks to its low density. Due to the crystal structure, it is difficult to increase the temperature to move dislocations and plastically deform the component.Pag. 26 Applied Metallurgy: Part B Questions(Contents link) The recommended temperature range is 345÷510°C. If we exceed the temperature there are problems of burning.AZ-seriesare the most used alloy to produce sheets and plate. All these components can be strengthened by: •solid-solution precipitation •grain size refining •cold working (i.e. cold rolling) Due to the high reactivity to water, all machining operations must be done dry, without lubricants. It is possible to reach high surface tolerances(Contents link)Applied Metallurgy: Part B Questions Pag. 27 8 Titanium alloys Titanium (T i) is a metal material with the following characteristics: •Density: it is,more or less, half dense than steel (4,5vs7,85g/cm3 •Strength: it has a YS from480M P aof pureT ito1100M P aof its alloys. These value are comparable to the steel •Young modulus:110GP a, approximately half of steel •Corrosion resistance:T iis very reactive with oxygen. It forms a layer ofT iO 2that is a protective oxide more stable thanC r−oxide •Bio-compatibility: as some type of stainless steel,T iis largely used in biomedic applications like dental implants and bone substitution (a bone could be realized with additive manufacturing processes).T iO 2forms a barrier from the body environment preventing immune reaction. T i can stay in human body about for50years instead of20of stainless steel •Operating temperature: depending on the specific alloy, it could be used at medium-high tem- peratures (350÷595°C) without suffering creep. Due to the absence of the DBTT it could be used in cryogenic applications •Cost: it is very expensive, not because it is rare or it is difficult to extract, but because it is not recyclable PureT ipresents two different crystal structures. At room temperature it ishexagonal close-packed (H C P), this phase is calledalpha(α)39aand thanks to this it is difficult to be cold-formed (low dislocations degree of movements). At about885°Cthere is a phase change tobody-centered cubic (BC C) orbeta-phase(β)39b. This structure is way more deformable than the previous one. The transition temperature is calledBeta-Transus Temperature(BT T)(a) α-phaseT i(α)(b) β-phaseT i(β) Figure 39:T imetallurgy structure Adding alloying elements can increase (fig40a) or decrease (fig40b) the BTT. It is also possible obtain theβ-phase at room temperature(a) α-stabilizer(b) β-stabilizer Figure 40:T ialloy phase diagramsPag. 28 Applied Metallurgy: Part B Questions(Contents link) Hydrogen must be minimize (or totally eliminated) because it causes the precipitation of hydrides and the consequently embrittlement of the material.T ialloys are classified by the crystal structure at ambient temperature (αandβphases composition)Figure 41:Titanium alloys classification phase diagram and they are designated by the list of alloying elements followed by their mass content like in the general example: T i−x T iA −x AB −x B... (8.1) There are four kinds ofT ialloys: 1-2)alpha and near-alpha: in these alloys,Alis the principal alloying element. It increases strength with solid-solution and corrosion resistance (this is still less than pureT i). Its content is about5÷8%and if it exceeds it will be detrimental for the material because the formation brittle intermetallic precipitations. This alloy is a single-phase, so it can’t be strengthened by heat treatments. The pureα-phase alloy produced is only:T i−5Al−2.5S n. The near-alpha alloys contain just a very small amount of beta phase. Both these alloys are used in cryogenic applications and they have good creep resistant (315÷595°C) 3)alpha-beta: this alloy can be solution treated by heating the material above the martensite start temperatureM s(see pointAin figure42a), so it is produced an high content ofβ-phase. Let’s consider in particular the alloyT i−6Al−4V. During the fast cooling, part ofβ-phase transforms totitanium martensiteorα′ -phase with acinular crystal structure (42b). Then, it follows the aging treatment at low temperature to obtain 2 main effects:•decomposing the unstableT i-martensite intoβ+αphases to obtain the crystal structure represented in figure (42c) •precipitation ofα 2particles ( T i 3Al ) inα-phase 4)beta: this kind of alloy is obtained by adding a lot ofβ-stabilizer alloying elements likeVor M owith an appropriate cooling rate.β-phase is metastable and thanks toα-phase particles the strength is increased. This alloy can be solution-treated like the previous one and it is possible to achieve values of1300M P a(Contents link)Applied Metallurgy: Part B Questions Pag. 29 (a) Treatment starting point(b) after-quenching(c) after-aging Figure 42:T i−6Al−4V: Solution Treatment Aging To fabricateT i-alloys it is necessary to consider its high degree of shrinking, in particular at room temperature, so it is necessary to be extensively overformed. It is also difficult to machine thanks to:•low thermal conductivity •high reactivity with oxygen •high strength at high temperature •low modulus T i-alloys can also be welded byT I G(Tungsten Inert Gas) or laserPag. 30 Applied Metallurgy: Part B Questions(Contents link) 9 Steels for special applications (Specialty Steels): HSLA, TRIP and Dual Phase steels High-Strength Low-Alloysteels (orHSLA) are the cheapest solution of all Specialty Steels. They are a hybrid between carbon steels and alloy steels. They have a very low carbon content (0,03÷0,1%) that increase weldability (comparable with carbon steels) and ductility properties. In this alloy there are a lot of alloying elements (N b,T i,V), but all of them are of the order of0,1%of content and this is the reason HSLA steels are calledmicro-al loyed steels. JustM nis present with an high percentage (1,5%).The low content of alloying elements makes this alloy competitively priced than carbon steels, but with higher strength and similar formability. The low YS and high toughness allow to apply high deformations and, because of it, one application of this steels is the production of external panels of cars. During secondary hot manufacturing processes, like hot rolling under controlled condition, are produced ultrafine ferrite grain size (