A Comprehensive Guideline: Titanium and Titanium Alloys
INTRODUCTION
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 has 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 the fourth most abundant structural metal, the current global supply of raw ore for the production of titanium metal is virtually unlimited. The vast global unused titanium sponge, smelting and processing capacity can accommodate the continued growth of new high-volume applications. In addition to its attractive high-strength-density properties in aerospace applications, the superior corrosion resistance of titanium’s protective oxide film has driven its widespread use in seawater, marine, brines and corrosive industrial chemical services over the past fifty years. Today, titanium and its alloys are used in a wide range of aerospace, industrial and consumer applications. In addition to aircraft engines and airframes, titanium is 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, deep-sea applications and naval vessel components; armor plate applications; anodes, automotive components, food and pharmaceutical processing; recreational and sports equipment; medical implants and surgical devices; and many other areas.
Titanium in Periodic Table
Why Titanium?
Attractive Mechanical Properties
Titanium and its alloys have unique mechanical and physical properties and corrosion resistance, making them ideal for use in demanding aerospace, industrial, chemical, and energy industries. Of the key properties of these alloys listed in 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))
titanium’s high strength-to-density ratio is the traditional primary motivation for selection and design for aerospace engine and airframe structures and components. Its excellent corrosion/erosion resistance is the primary motivation for chemical process, marine, and industrial uses. Figure 1 shows that high-strength titanium alloys offer superior structural efficiency compared to structural steels and aluminum alloys, especially at elevated service temperatures. Titanium alloys also have good high-temperature properties, making them suitable for hot gas turbines and automotive engine components, where more creep-resistant alloys can be selected to withstand temperatures up to 600°C (1100°F) [see Figure 2].
The titanium alloy family offers a wide range of strengths and combinations of strength and fracture toughness, as shown in Figure 3. This allows alloy selection to be optimized and tailored for critical components based on whether they are controlled by strength and S-N fatigue or by toughness and crack growth (i.e., critical flaw size) during service. Titanium alloys also exhibit excellent S-N fatigue strength and life in air, which is relatively unaffected by seawater (Figure 4) and other environments. If desired, most titanium alloys can be processed to high fracture toughness with minimal environmental degradation (i.e., good SCC resistance). In fact, 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 meet many design specifications and offer attractive design allowances up to 315°C (600°F), as shown in Figure 5. Some common pressure design codes include the ASME Boiler and Pressure Vessel Code (Sections I, III, and VIII), ANSI (ASME) B31.3 Pressure Code, BS-5500, CODAP, Stoomwezen and Merkblatt Eurocodes, as well as Australian AS 1210 and Japanese JIS codes.
Corrosion and Erosion Resistance
Titanium alloys have excellent corrosion resistance in a wide range of chemical environments and conditions due to a thin, invisible, but extremely protective oxide film on their surface. This oxide film, composed primarily of TiO2, is very tough, adherent, and chemically stable, and can spontaneously and instantly self-repair even when mechanically damaged if the smallest amount of oxygen or water (moisture) is present in the environment. This metal protection ranges from mild reduction to severe oxidation, from highly acidic to moderately alkaline environmental conditions; this is true even at elevated temperatures. Titanium alloys are particularly well known for their resistance to localized corrosion and stress corrosion in aqueous chlorides (e.g., brines, seawater) and other halides and wet halogens (e.g., wet Cl2 or saturated Cl2 brines), as well as hot, highly oxidizing acidic solutions (e.g., FeCl3 and nitric acid solutions), where most steels, stainless steels, and copper- and nickel-based alloys may suffer severe corrosion. Titanium alloys are also known for their excellent resistance to erosion, erosion-corrosion, cavitation, and impact in flowing, turbulent fluids. This excellent resistance to wrought metal corrosion and erosion is shared by corresponding welds, heat-affected zones and castings of most titanium alloys due to the formation of the same protective oxide surface film.
The useful resistance of titanium alloys is limited in strongly reducing acidic media, such as moderately or highly concentrated solutions of HCl, HBr, H2SO4, and H3PO4, and in HF solutions of all concentrations, especially with increasing temperature. However, even at concentrations as low as 20-100 ppm, the useful performance limit of titanium in dilute to moderately strong reducing acidic media can often be maintained or significantly extended in the presence of common background or contaminating oxidizing species (e.g., air, oxygen, ferrous alloy metal corrosion products, and other metal ions and/or oxidized compounds). When enhanced resistance to dilute reducing acid and/or crevice corrosion in hot (≥75°C) chloride/halide solutions is required, titanium alloys containing small amounts of palladium (Pd), ruthenium (Ru), nickel (Ni), and/or higher molybdenum contents (>3.5 wt.% Mo) 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 alloying additions can also inhibit the susceptibility of high-strength titanium alloys to stress corrosion cracking when exposed to hot, sweet, or acidic brines. As a result, titanium alloys are generally able to withstand a wider range of chemical environments (i.e., pH and redox potentials) and temperatures than steel, stainless steel, and aluminum, copper, and nickel-based alloys. Table 3 (see page 5) summarizes the various chemical environments in which titanium alloys have been successfully used in the chemical process and energy industries.
Other Attractive Properties
Table 2. Other Attractive Properties of Titanium Alloys
Exceptional erosion and 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
· Essentially nonmagnetic
· High intrinsic shock resistance
· High ballistic resistance-to-density ratio
· Nontoxic, nonallergenic and fully biocompatible
· Very short radioactive half-life
· Excellent cryogenic properties
Titanium’s relatively low density, only 56% of steel and half that of nickel and copper alloys, means that compared to other metals, titanium has twice the volume of metal per unit weight, and the cost of titanium rolled products is more attractive given the size. Combined with higher strength, this obviously means lighter and/or smaller components for static and dynamic structures (aero engines and airframes, mobile military equipment), and lower stresses in rotating and reciprocating components (e.g. centrifuges, shafts, impellers, agitators, mobile engine parts, fans). Titanium alloys’ reduced component weight and suspension loads are also key for hydrocarbon production tubing strings and dynamic offshore risers, as well as naval vessel and submersible structures/components. Titanium alloys have a low modulus of elasticity, approximately half that of steel and nickel alloys. This increased elasticity (flexibility) means less bending and cyclic stresses in deflection control applications, making them ideal for springs, bellows, body implants, dental devices, dynamic offshore risers, drill pipe, and a variety of sports equipment. Titanium’s inherent non-reactivity with the body and tissues (non-toxic, non-allergenic and fully biocompatible) has led to its widespread use in body implants, prosthetics and jewelry, as well as in food processing. Titanium alloys are inherently more resistant to impact and blast damage (e.g. military applications) than most other engineering materials due to their unique combination of high strength, low modulus and low density. These alloys have significantly lower coefficients of thermal expansion than aluminum, iron, nickel and copper alloys. This low expansion improves interfacial compatibility with ceramic and glass materials and minimizes warping and fatigue effects during thermal cycling.
Titanium is inherently non-magnetic (very slightly paramagnetic), making it ideal for use where electromagnetic interference must be minimized (e.g. electronic equipment housings, well logging tools). When irradiated, titanium and its isotopes have an extremely short radioactive half-life and will not remain “hot” for more than a few hours. Its fairly high melting point gives it good resistance to ignition and combustion in air, while its inherent ballistic resistance reduces melting and burning susceptibility during ballistic impact, making it a top choice for lightweight armor materials for military equipment. Alpha and alpha-beta titanium alloys have very low ductile-to-brittle transition temperatures, making them ideal materials for cryogenic vessels and components.
Heat Transfer Properties
Titanium is a very attractive and established heat transfer material for use in shell-and-tube, plate-and-frame, and other types of heat exchangers for heating or cooling process fluids, especially seawater chillers. Exchanger heat transfer efficiency can be optimized due to the following beneficial properties of titanium:
· Excellent resistance to corrosion and fluid erosion
· Very thin conductive oxide film
· Hard, smooth, difficult-to-adhere surface
· Surface that promotes condensation
· Reasonably good thermal conductivity
· Good strength
While the inherent thermal conductivity of unalloyed titanium is lower than that of copper or aluminum, it is still about 10-20% higher than that of typical stainless steel alloys. Due to titanium’s good strength and ability to completely resist corrosion and erosion by flowing turbulent fluids (i.e., zero corrosion margin), titanium walls can be significantly thinned to minimize heat transfer resistance (and cost). Titanium’s smooth, non-corrosive, difficult-to-adhere surface maintains high cleanliness for long periods of time. This surface promotes drop-by-drop condensation of water vapor, thereby increasing the condensation rate of the cooler/condenser compared to other metals, as shown in Figure 6. The ability to design and operate with high process or cooling water side flow rates and/or turbulence further improves overall heat transfer efficiency. All of these attributes allow for reductions in the size, material requirements, and overall initial life cycle costs of titanium heat exchangers, making titanium heat exchangers more efficient and cost-effective than heat exchangers designed with other common engineering alloys.
Table 3. Chemical Environments Where Titanium Alloys Are Highly Resistant and Have Been Successfully Applied
| Generic Media | Typical Examples | Guideline for Successful Use |
| Acids (oxidizing) | HNO₃, | — |
| Acids (reducing) | HCl | Observe acid conc |
| Alcohols | Me | Avoi |
| Alkaline solutions (strong) | NaOH | Exc |
| Alkaline solutions (mild) | Mg( | — |
| Bleachants | C | (1) |
| Chloride brines | NaCl, KCl, LiCl | (1) |
| Gases | O₂, Cl₂, Br₂, | Ignition |
| Gases (other) | H₂, N₂, CO₂, CO, SO₂, H₂S, NH₃, NO | Excessive hydrogen absorption in dry H₂ gas at higher temps. and pressures. |
| Halogens | Cl₂, Br₂, I₂, F₂ | Avoid dry halogens, need to be moist (wet) for good resistance. Avoid F₂ and HF gases. |
| Hydrocarbons | Alkanes, alkenes, aromatics, etc. sweet and sour crude oil and gas | — |
| Halogenated hydrocarbons | Chloro-, chloro-fluoro-, or brominated alkanes, alkenes, or aromatics | Need at least traces of water (>10-100 ppm) for passivity, (1) |
| Liquid metals | Na, K, Mg, Al, Pb, Sn, Hg | Observe temp. limitations. Avoid molten Zn, Li, Ga, or Cd. |
| Hydrolyzable metal halide solutions | MgCl₂, CaCl₂, AlCl₃, ZnCl₂ | Observe temp./conc. guidelines, (1) |
| Oxidizing metallic halide solutions | FeCl₃, CuCl₂, CuSO₄, NiCl₂, Fe₂(SO₄)₃ | (1) |
| Organic acids | TPA, acetic, stearic, adipic, formic, tartaric, tannic acids | Observe temp./conc. guidelines for formic acid, and select Pd- or Ru-enhanced alloys if necessary. |
| Other organic compounds | Aldehydes, ketones, ethers, esters, glycols | — |
| Salt solutions | Sulfates, phosphates, nitrates, sulfites, carbonates, cyanates, etc. | — |
| Seawater | Aerated, deaerated, contaminated, or slightly acidified condition | (1) |
| Note (1): The note “(1)” may refer to a specific guideline, limitation, or requirement not fully described in the table. | ||
A Guide to Commercial Titanium Alloys and Their Mill Product Forms
| Alloy Composition (ASTM Grade) |
Alloy Description | Available Product Forms |
| Ti Grade 1 | Lower strength, softest, unalloyed Ti grade with highest ductility, cold formability, and impact toughness, with excellent resistance to mildly reducing to highly oxidizing media with or without chlorides and high weldability. | Ingot/Bloom, Bar*, Billet, Plate, Strip, Welded Tubing, Welded Pipe, Wire* |
| Ti Grade 2 | Moderate strength unalloyed Ti with excellent weldability, cold formability, and fabricability; “garden variety” Ti grade for industrial service with excellent resistance to mildly reducing to highly oxidizing media with or without chlorides. Approved for sour service use under the NACE. | Ingot/Bloom, Bar*, Billet, Plate, Strip, Welded Tubing, Welded Pipe, Seamless Tubing*, Wire*, Foil* |
| Ti Grade 3 | Slightly stronger version of Gr. 2 Ti with similar corrosion resistance with good weldability and reasonable cold formability/ductility. | Ingot/Bloom, Bar*, Billet, Plate, Strip, Welded Tubing, Welded Pipe |
| Ti Grade 4 | Much stronger, high interstitial version of Grades 2 and 3 Ti with reasonable weldability, and reduced ductility and cold-formability. | Ingot/Bloom, Bar*, Billet, Plate, Strip |
| Ti-0.15Pd
(Grade 7) |
Most resistant Ti alloy to corrosion in reducing acids and localized attack in hot halide media, with physical/mechanical properties equivalent to Gr. 2 Ti, and excellent weldability/fabricability. | Ingot/Bloom, Bar*, Billet, Plate, Strip, Welded Tubing, Welded Pipe |
| Ti-0.15Pd
(Grade 11) |
Most resistant Ti alloy to corrosion in reducing acids and localized attack in hot halide media, with physical, mechanical, formability properties equivalent to Gr. 1 Ti (soft grade) and excellent weldability. | Ingot/Bloom, Bar*, Billet, Plate, Strip, Welded Tubing, Welded Pipe, Wire* |
| Ti-0.05Pd
(Grade 16) |
Lower cost, leaner Pd version of Ti Gr. 7 with equivalent physical/mechanical properties, and similar corrosion resistance. | Ingot/Bloom, Bar*, Billet, Plate, Strip, Welded Tubing, Welded Pipe |
| Ti-0.05Pd
(Grade 17) |
Lower cost, leaner Pd version of Ti Gr. 11 with equivalent physical/mechanical properties (soft grade) and similar corrosion resistance. | Ingot/Bloom, Bar*, Billet, Plate, Strip, Welded Tubing, Welded Pipe, Wire* |
| Ti-0.1Ru
(Grade 26) |
Lower cost, Ru-containing alternative for Ti Gr. 7 with equivalent physical/mechanical properties and fabricability and similar corrosion resistance. | Ingot/Bloom, Bar*, Billet, Plate, Strip, Welded Tubing, Welded Pipe |
| Ti-0.1Ru
(Grade 27) |
Lower cost, Ru-containing alternative for Ti Gr. 11 with equivalent physical/mechanical properties (soft grade) and fabricability and similar corrosion resistance. | Ingot/Bloom, Bar*, Billet, Plate, Strip, Welded Tubing, Welded Pipe |
| Ti-6Al-4V
(Grade 5) |
Heat treatable, high-strength, most commercially available Ti alloy (“workhorse” alloy for aerospace applications), for use up to 400°C offering an excellent combination of high strength, toughness, and ductility along with good weldability and fabricability. | Ingot/Bloom, Bar*, Billet, Plate, Sheet, Seamless Pipe, Wire*, Seamless Tubing*, Foil* |
| Ti-6Al-4V ELI
(Grade 23) |
Extra low interstitial version of Ti-6Al-4V offering improved ductility and fracture toughness in air and saltwater environments, along with excellent toughness, strength, and ductility in cryogenic service as low as -255°C. Typically used in a non-aged condition for maximum toughness. | Ingot/Bloom, Bar*, Billet, Plate, Sheet, Wire*, Seamless Tubing*, Foil* |
| Ti-6Al-4V-0.1Ru
(Grade 29) |
Extra low interstitial, Ru-containing version of Ti-6Al-4V offering improved fracture toughness in air, seawater, and brines, along with resistance to localized corrosion in sweet and sour acidic brines as high as 330°C. Approved for sour service use under the NACE MR-01-75 Standard. | Ingot/Bloom, Bar*, Billet, Plate, Sheet, Seamless Pipe, Wire* |
| Ti-6Al-7Nb | High strength Ti alloy with good toughness and ductility, used primarily for medical implants stemming from its excellent biocompatibility. | Ingot/Bloom, Bar*, Billet, Wire* |
| Ti-6Al-6V-2Sn
(Grade 6-2) |
Heat-treatable, high-strength Ti alloy with higher strength and section hardenability than Ti-6Al-4V, but with lower toughness and ductility, and limited weldability. Can be used in mill annealed or in the aged (very high strength) condition. | Ingot/Bloom, Bar*, Billet, Plate, Sheet |
| Ti-6Al-2Sn-4Zr-6Mo
(Grade 2-4-6) |
Heat-treatable, deep-hardenable, very high strength Ti alloy with improved strength to temperatures as high as 450°C, with limited weldability. Approved for sour service under the NACE MR-01-75 Standard. | Ingot/Bloom, Bar*, Billet |
| Ti-4Al-4Mo-2Sn-0.5Si
(Grade 550) |
Heat-treatable, high strength forging alloy with good strength and creep resistance to temperature as high as 400°C. | Ingot/Bloom, Bar*, Billet |
| Ti-6Al-2Sn-2Zr-2Mo-0.15Si (Grade 22-22) | Heat-treatable, high strength Ti alloy with strength and fracture toughness-to-strength properties superior to those of Ti-6Al-4V, with excellent superplastic formability and thermal stability. | Ingot/Bloom, Bar*, Billet, Plate, Sheet, Wire* |
| Ti-4.5Al-3V-2Mo-2Fe
(SP-700) |
Heat-treatable, high strength Ti alloy with superior strength and exceptional hot and superplastic formability compared to Ti-6Al-4V, combined with good ductility and fatigue resistance. | Ingot/Bloom, Bar*, Billet, Plate, Sheet |
| Ti-5Al-4Cr-4Mo-2Sn-2Zr
(Grade 17) |
Heat-treatable, deep section hardenable, very high strength Ti alloy with superior strength and creep resistance over Ti-6Al-4V to temperatures as high as 400°C, and limited weldability. | Ingot/Bloom, Bar*, Billet |
| Ti-10V-2Fe-3Al
(Grade 10-2-3) |
Heat-treatable, deep hardenable, very high strength Ti alloy possessing superior fatigue and strength/toughness combinations, with exceptional hot-die forgeability, but limited weldability. | Ingot/Bloom, Bar*, Billet |
| Ti-3Al-8V-6Cr-4Zr-4Mo
(Grade 19) |
A heat-treatable, deep section hardenable, very high strength Ti alloy possessing good toughness/strength properties, low elastic modulus and elevated resistance to stress and localized corrosion in high temperature sweet and sour brines. Approved for sour service under the NACE MR-01-75 Standard. | Ingot/Bloom, Bar*, Billet, Seamless Pipe, Wire* |
| Ti-3Al-8V-6Cr-4Zr-4Mo-0.05Pd (Grade 20) | A Pd-containing version of the Ti-38644 alloy (Beta-C/Pd) possessing equivalent physical/mechanical properties, but with significantly enhanced resistance to stress and localized corrosion in high temperature brines. | Ingot/Bloom, Bar*, Billet, Seamless Pipe |
Basic Titanium Metallurgy
Titanium mill products are available in commercially pure and alloyed grades that can be divided into three categories based on the predominant phases in their microstructures… alpha, alpha-beta, and beta. While these three general alloy types require specific and different mill processing methods, each alloy type has a unique set of properties that may be beneficial for specific applications. In pure titanium, the alpha phase… is characterized by a hexagonal close-packed crystal structure… and 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 approximately 1688°C (3040°F).
Effect of Alloying Elements
A wide range of physical and mechanical properties can be obtained by the selective addition of alloying elements to titanium. The basic effects of many alloying elements are as follows:
1. Certain alloying additions, particularly aluminum and interstitial elements (O, N, C), tend to stabilize the alpha phase, i.e., to increase the temperature at which the alloy completely transforms to the beta phase. This temperature is called the beta transition temperature.
2. Most alloying additions… such as chromium, niobium, copper, iron, manganese, molybdenum, tantalum, vanadium… stabilize the beta phase by lowering the transformation temperature (from alpha to beta).
3. Certain elements… especially tin and zirconium… behave as neutral solutes in titanium, have little effect on the transformation temperature, and act as strengtheners of the alpha phase. Titanium alloy microstructures are characterized by various alloying additions and processing. This article describes various types of alloys, with typical micrographs of various mill products manufactured.
Alpha Alloys
The single-phase and near-single-phase alpha alloys of titanium have good weldability. The aluminum content of these alloys is usually high, ensuring excellent strength properties and oxidation resistance at high temperatures (in the range of 316-593°C (600 – 1100°F)). Since alpha alloys are single-phase alloys, they cannot be heat treated to increase strength.
Microstructure
Alpha-Beta Alloys
The addition of controlled amounts of beta stabilizing alloying elements causes some beta phase to persist below the beta transformation temperature, down to room temperature…thus forming a two-phase system. Even small amounts of beta stabilizers will stabilize the beta phase at room temperature. A group of alloys designed to have large amounts of alpha stabilizers and small amounts of beta stabilizers are the alphabet alloys, often referred to as high alpha or near-alpha alloys. With the addition of large amounts of beta stabilizers, a higher percentage of the beta phase is retained at room temperature. Such two-phase titanium alloys can be significantly strengthened by heat treatment…quenching from an elevated temperature in the alpha-beta range followed by an aging cycle at a slightly lower temperature. The transformation to the beta phase…normally occurs with slow cooling…is inhibited by the quench. The aging cycle causes precipitation of fine alpha particles in the metastable beta phase, resulting in a structure that is stronger than the annealed alpha-beta structure.
α-β Microstructure
α-β Microstructure – 1
Beta Alloys
The high proportion of beta stabilizing elements in this group of titanium alloys results in a metastable beta phase microstructure after solution annealing. Precipitation of the alpha phase during aging allows for extensive strengthening.
β Microstructure
Titanium Machining
Titanium can be machined economically on a routine production basis if shop procedures take into account the common physical properties of the metal. The factors that must be considered are not complex, but they are critical to successfully machining titanium. The different grades of titanium, namely commercially pure titanium and the various alloys, do not have the same machining characteristics, just as all steels or all aluminum grades have the same characteristics. As with stainless steel, titanium’s low thermal conductivity inhibits heat dissipation from the workpiece itself, so proper use of coolants is required. Good tool life and successful machining of titanium alloys can be ensured if the following guidelines are followed:
• Maintain sharp tools to minimize heat buildup and wear
• Use a rigid setup between tool and workpiece to counteract workpiece flexing
• Use plenty of cutting fluid to maximize heat removal
• Utilize lower cutting speeds
• Maintain high feed rates
• Avoid feed interruptions (positive feed)
• Remove turned parts from the machine regularly
Experienced shop workers have compared the machinability of commercially pure titanium to that of 18-8 stainless steel, with alloy grades being slightly less machinable. Specific information on machining, grinding and cutting titanium and its alloys can be found in RMI’s comprehensive brochure “Machining”.
Turning
Commercially pure and alloyed titanium can be turned with ease. Carbide tools should be used for turning and boring whenever possible, as they offer greater productivity and longer tool life. When using high-speed steels, ultra-high speeds are recommended. Tool deflection should be avoided, and a large and continuous stream of cutting fluid should be applied to the cutting surface. Live centers must be used, as titanium will bind in dead centers.
Milling
Milling of titanium is more difficult than turning. The tool mills only a portion of each revolution, and the chips tend to adhere to the teeth during that portion of the revolution that each tooth is not cutting. On the next contact, when the chips are knocked off, the teeth may be damaged. This problem can be largely alleviated by employing climb milling rather than conventional milling. In this type of milling, the tool contacts the thinnest part of the chip as it leaves the cut, minimizing chip “welding”. For slab milling, the workpiece should move in the same direction as the cutting teeth; for face milling, the teeth should emerge from the cut in the same direction as the workpiece feed. When milling titanium, cutting edge failure is usually due to chipping. Therefore, the results with carbide tools are generally less satisfactory than with HSS tools. Cutting speeds can be increased by 20-30% with carbide tools compared to HSS tools, but this does not fully compensate for the additional tool grinding costs. Therefore, it is recommended to try both HSS and carbide tools in each milling operation to determine which is better. Water-based coolant is recommended. Successful drilling is achieved by using a sharp drill with appropriate geometry and maintaining maximum drilling force to ensure continuous cutting. It is important to avoid letting the drill slip across the titanium surface, as the resulting work hardening makes it difficult to re-cut.
Another important factor in drilling titanium is the length of the unsupported section of the drill. This part 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 hole drilling. Tapping Percentage depth of thread has a definite influence on success in tapping titanium and best results in terms of tool life has been obtained with a 65% thread. Chip removal is a problem which makes tapping one of the more difficult machining operations. However, in tapping through-holes, this problem can be simplified by using a gun-type tap with which chips are pushed ahead of the tap. Another The problem is the smear of titanium on the land of the tap, which can result in the tap freezing, or binding in the hole. An activated cutting oil such as a sulfurized and chlorinated oil is helpful in avoid this problem. Grinding Titanium is successfully grounded 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 sulfochlorinated oils, but these can present a fire hazard, and it is important to flood the work when using these oil based coolants.
Sawing
The two common methods for sawing titanium are band sawing and power sawing. As with titanium machining operations, standard practices for sawing titanium are well established. Rigid, high-quality equipment should be used, in conjunction with an automatic positive feed. High-speed steel blades are effective, but the blade manufacturer should be consulted for specific blade recommendations and cutting practices. Cutting fluids are required. Abrasive sawing is also common with titanium. Rubber-bonded silicon carbide cut-off wheels have been used successfully with water-based coolants to flood the cutting area.
Waterjet
Waterjet cutting is a recent innovation in cutting titanium. A high-velocity jet containing entrained abrasive is very effective for high cutting speeds and producing smooth, burr-free edges. Sections up to three inches can be cut, and the process is relatively unaffected by differences in hardness of the titanium workpiece.
Electrospark Machining
Although uncommon, complex titanium parts with fine detail can be produced by electrospark machining. The dielectric fluid is typically composed of various hydrocarbons (various oils) or even polar compounds such as deionized water. Care must be taken to avoid or remove any fine surface contamination in fatigue-sensitive parts.
Chemical Milling
Chemical milling has been used extensively to shape, machine or stamp fairly complex titanium parts, particularly for aerospace applications (e.g. jet engine casings). These aqueous etching solutions typically consist of HNO3-HF or dilute HF acid, with the HNO3 content adjusted to the specific alloy to minimize hydrogen absorption.
Forming of Titanium
Titanium and its alloys can be cold-formed and hot-formed on standard equipment using similar techniques to stainless steel. However, titanium has certain unique properties that affect formability and must be considered when performing titanium forming operations. The room temperature ductility of titanium and its alloys is generally lower than that of common structural metals, including stainless steel. This requires larger bend radii and less stretch forming allowance.
Titanium has a relatively low modulus of elasticity, about half that of stainless steel. This results in greater springback during forming, which needs to be compensated for in bending or post-bending processing. Titanium in contact with itself or other metals is more susceptible to wear than stainless steel. Therefore, sliding contact with tool surfaces during forming requires the use of lubricants. Effective lubricants typically include greases, heavy oils, and/or waxy lubricants that may contain graphite or molybdenum disulfide additives for cold forming; and solid film lubricants (graphite, molybdenum disulfide) or glass coatings for high temperature forming. The following is basic information about titanium forming. There is a wealth of published information on titanium forming practices in common commercial forming processes.
Surface Preparation
Before forming titanium plate, it should be clean and free of surface defects such as nicks, scratches, or wear marks. All scratches deeper than those produced by 180 grit diamond should be removed by grinding. Burrs and sharp edges should be rounded to prevent edge cracking. Surface oxides can cause cracking during cold forming and should be removed mechanically or chemically. Plate products should be free of coarse stress concentrators, very rough, irregular surface finishes, visible scale, and brittle alpha layers (diffused oxygen layers) to allow reasonable cold or hot forming. Experience has shown that pickled plate generally exhibits enhanced formability (e.g., in bends and coil forming) compared to plate with a sandblasted and/or ground surface finish.
Cold Forming vs. Hot Forming
Commercially pure titanium, ductile, low alloy alpha, and unaged beta titanium alloys can be cold formed within certain limits. The amount of cold forming in bending or drawing depends on the tensile elongation of the material. Tensile elongation and bend data for various grades of titanium sheet and plate can be found in ASTM specification B265. Heating titanium improves its formability, reduces springback, and allows maximum deformation while minimizing annealing between forming operations. Mild and warm forming of most grades of titanium is performed at 204-316°C (400-600°F), while more aggressive forming is performed at 482-788°C (900-1450°F). Heated forming dies or radiant heaters are occasionally used for low-temperature forming, while electric furnaces in an air atmosphere are best suited for heating to higher temperatures. Gas furnaces may be used if flame impingement is avoided and the atmosphere is slightly oxidizing. Hot forming and/or annealing of titanium products in air at temperatures above about 590-620°C (1100-1150°F) will produce a visible surface oxide scale and diffused oxygen layer (alpha shell) that may need to be removed on fatigue and/or fracture critical components. Scale removal can be achieved by mechanical means (i.e., sandblasting or grinding) or chemical descaling treatments (i.e., molten hot alkaline salt descaling). This is usually followed by pickling in an HF-HNO3 acid solution, machining, or grinding to ensure complete removal of the alpha shell (if required). These pickling solutions are typically maintained at a 5:1 to 10:1 volume percent HNO3 to HF ratio (as a reserve acid) to minimize hydrogen absorption, depending on the alloy type.
Stress Relief and Hot Sizing
Cold forming and straightening operations can create residual stresses in titanium that sometimes need to be removed for reasons of dimensional stability and property recovery.
Stress relief can also serve as an intermediate heat treatment between The stages of cold forming. temperatures employed lie below the annealing ranges for titanium alloys. They generally fall within 482-649°C (900-1200°F) 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 springback and inaccuracies in shape and dimensions of preformed parts. The part is suitably fixtured such that controlled pressure is applied to certain areas of the part during heating. This fixtured 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 Operations
Following are descriptions of several typical forming operations performed on titanium. They are representative of operations in which bending and The stretching of titanium occurs. forming can be done cold, warm or hot. The choice is governed by a number of factors among which are workpieces section thickness, the intended degree of bending or stretching, the speed of forming (metal strain rate), and alloy/ product type.
Brake Forming
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
Stretch forming has been used on titanium sheet primarily to form contoured angles, hat sections, Zsections 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, 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 (e.g., cylinders, cones, hemispheres) using pressure on a rotating workpiece. Spinning produces only minor thickness changes in the sheet, whereas shear-forming involves significant plastic deformation and wall thinning.
Superplastic Forming (SPF)
SPF of titanium alloys is commonly used in aircraft part fabrication, allowing production of complex structural efficient, lightweight and cost-effective component configurations. This high temperature sheet forming process (typically 870-925C°(1660-1700°F)) is often performed simultaneously with diffusion bonding (solid-state joining) in argon gas-pressurized chambers, eliminating the need for welding, brazing, sizing or stress relief in complex parts. Titanium sheet alloys that are commonly superplastically-formed include the Ti- 6Al-4V and Ti SP-700 alpha-beta alloys.
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 (1000°F) max.) pressing operations can be less expensive than that utilizing conventional mating “hard die” tooling. Even explosive forming has been successfully employed to form complex titanium alloy sheet/plate components. The lower strength, more ductile titanium alloys can be roll-formed cold as sheet strip to produce long lengths of shaped 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.
Deep Drawing
This is a process involving titanium bending and stretching in which deep recessed parts, often closed cylindrical pieces or flanged hat-sections, are made by pulling a sheet blank over a radius and into a die. During this operation buckling and tensile tearing must be avoided. It is therefore necessary to consider the compressive and tensile yield strengths of the titanium when designing the part and the tooling. The sheet blank is often preheated as is the tooling. The softer, highly ductile grades of unalloyed titanium is often cold pressed or stamped in sheet strip form to produce heat exchanger plates, anodes or other complex components for industrial service.
WELDING TITANIUM
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 tungstenarc (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.
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 argonhelium 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.
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.
Weld Joint 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 be 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.
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 that proper weld interpass 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 rewelded. For the finished weld, non-destructive examination by liquid penetrant, 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.
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.
