Even if you’ve heard that “titanium is light and strong,” do you ever find yourself unsure about which parts to use it for when comparing it to SS400 or aluminum? In this article, we’ll organize the properties of pure titanium and titanium alloys, JIS standards, casting, forging, and machining methods, as well as costs, and explain material selection in mechanical design in a way that even high school students can understand.
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Introduction | Differences Between Titanium, Titanium Alloys, and Other Metals
Titanium (chemical symbol Ti) is a metal that is lighter than iron and exhibits excellent corrosion resistance in environments such as seawater. Products referred to as “titanium” include pure titanium (industrial-grade pure titanium) and titanium alloys containing elements such as aluminum and vanadium. In mechanical components, Ti-6Al-4V (commonly known as 64 titanium) is a representative alloy.
Strength is not the only important factor when selecting materials. Comparisons must include weight, stiffness, corrosion resistance, wear resistance, formability, procurement costs, and even maintenance.
| Material | Advantages | Precautions |
|---|---|---|
| Pure Titanium | Lightweight, highly corrosion-resistant in seawater and other environments, proven track record in biocompatible applications | Cost, low modulus of elasticity, machinability and cutting performance |
| Ti-6Al-4V | High specific strength, lightweight, properties can be adjusted through heat treatment | Expensive, generates heat during machining, performance depends on microstructure and fatigue |
| Mild Steel/Carbon Steel | Excellent price, rigidity, and availability | Heavy; corrosion protection required |
| SUS304 | Corrosion resistance; proven track record in distribution and machining | Heavy; susceptible to pitting corrosion in chloride environments |
| Aluminum Alloys | Lightweight, good thermal conductivity, variety of processing options | Relatively low rigidity, surface damage depending on application |
| Copper and copper alloys | Excellent thermal and electrical conductivity | Heavy, high cost |
Chemical and Physical Properties | Density, Thermal Conductivity, Electrical Conductivity, Corrosion Resistance
Density (Specific Gravity) | Approximately 57% the weight of steel
Typical densities near room temperature are approximately 4.51 g/cm³ for pure titanium, 4.43 g/cm³ for Ti-6Al-4V, 7.85 g/cm³ for mild steel, 7.9 g/cm³ for SUS304, 2.7 g/cm³ for aluminum, and 8.96 g/cm³ for copper. For the same volume, titanium weighs about 64% as much as steel, but the component dimensions required to achieve the same strength and deflection differ.
Electrical and Thermal Conductivity | May Be Disadvantageous for Heat Dissipation Components
The thermal conductivity of pure titanium is approximately 17–22 W/(m·K), while that of 64 titanium is about 6–8 W/(m·K), which is lower than that of pure aluminum (approximately 220 W/(m·K)) and pure copper (approximately 390 W/(m·K)). Mild steel typically ranges from approximately 45 to 65 W/(m·K), while SUS304 is about 16 W/(m·K). Typical electrical resistivity values are approximately 0.4–0.6 μΩ·m for pure titanium, approximately 1.7 μΩ·m for 64 titanium, and approximately 0.017 μΩ·m for copper. Therefore, copper and aluminum are generally suitable for high-performance heat sinks and high-current terminals.
Thermal Expansion and Magnetic Properties | Also Suitable for Designs Requiring Dimensional Stability
The linear expansion coefficients are approximately 8.5–9 × 10⁻⁶/K for pure titanium, approximately 8.5–9 × 10⁻⁶/K for 64 titanium, approximately 12 × 10⁻⁶/K for mild steel, and approximately 23 × 10⁻⁶/K for aluminum. For pure titanium with a length of 1 m and a temperature difference of 50°C, assuming α = 8.6 × 10⁻⁶/K, the elongation is ΔL = αLΔT = 0.43 mm. When joining dissimilar materials in precision parts, be sure to verify the difference in thermal expansion. Titanium is not typically ferromagnetic like iron and is generally used as a non-magnetic material.
Chemical Properties and Corrosion | Passive Films Are Not Universal
A dense passive film of titanium oxide naturally forms on the surface of titanium, providing high corrosion resistance in many aqueous, seawater, and oxidizing environments. However, under certain conditions—such as high-temperature, concentrated chloride environments; reducing acids; fluorides; or oxygen deficiency in crevices—corrosion may occur. Be sure to verify the type, concentration, temperature, flow rate, and geometry of any gaps in the chemical solution.
If titanium comes into contact with materials such as iron or aluminum in the presence of an electrolyte, there is a risk of dissimilar metal corrosion. During the design phase, evaluate not only the titanium itself but also whether the materials it comes into contact with are susceptible to corrosion.
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Mechanical Properties | Strength, Toughness, Elongation, Workability, Price
Tensile Strength and Specific Strength | Pure Titanium and 64 Titanium Are Different Materials
The strength of pure titanium varies depending on the JIS grade and product shape. Generally, strength increases from Grade 1 to Grade 4, and there are also differences in ductility requirements and formability. Ti-6Al-4V is a representative annealed grade with a tensile strength of approximately 900–1,000 MPa (depending on the standard, product, and heat treatment conditions) and is used in applications requiring high specific strength.Specific strength = strength ÷ density. When comparing with steel, ensure that the strength classifications and part geometry conditions are consistent.
Stiffness (Young’s Modulus) | More prone to deflection than steel
The modulus of elasticity for titanium and 64-titanium is generally 100–120 GPa, while that for steel is approximately 200–210 GPa and for aluminum alloys is approximately 69–72 GPa. When comparing axial deformation under the same shape and load, titanium may elongate approximately 1.8 times as much as steel. Axial elastic deformation is ΔL = FL/(AE). Even for high-strength alloys, if dimensional accuracy, deflection, and buckling are critical, be sure to include the cross-sectional shape in your design.
Toughness, Ductility, and Fatigue | Heat Treatment and Defects Determine Service Life
Toughness refers to the ability to absorb energy before fracture and resist cracking, while ductility refers to the capacity for plastic deformation, such as elongation and drawing. While some grades of pure titanium generally have good formability, Grade 64 titanium requires careful control of forming and welding processes in exchange for its high strength. In fatigue design, factors such as surface roughness, notches, internal defects, residual stress, the environment, and heat treatment must be evaluated; service life should not be estimated based solely on the material grade.
Machinability | Countermeasures for Heat Generation, Tool Wear, and Gouging
Titanium has low thermal conductivity, so cutting heat tends to concentrate around the cutting edge. Taking into account elastic recovery, reactivity during machining, tool adhesion, and chatter, select a rigid setup along with appropriate tools, cutting conditions, and coolant. Metal powders and fine chips may ignite depending on the conditions, so fire prevention measures and proper dust collection and storage are necessary. Do not make your own judgment regarding the suitability of water or general fire extinguishing agents; instead, follow your workplace’s safety procedures and metal fire prevention guidelines.
Seizing and galling may occur in sliding parts and on the threads of titanium bolts. Verify the combination with dissimilar materials, surface treatments, lubrication, and fastening conditions during the design phase.
Price, Availability, and Recyclability | Comparison Based on Total Cost
Titanium materials are more expensive than general-purpose steel, and the cost difference may widen when machining costs, tooling, and administrative expenses are included. However, their corrosion resistance may reduce the frequency of painting and replacement. While recycling is possible, it is crucial to control contamination from impurities such as oxygen, nitrogen, and iron, and to sort materials by alloy type. For applications with strict quality requirements, such as those in the aerospace industry, melting history and material traceability must be verified.
How to Read JIS Standards | Material Designations for Pure Titanium and Ti-6Al-4V
The Meaning of the “H” and Numbers in JIS H
JIS standards are organized as “JIS + Division Code + Standard Number,” and “H” denotes the non-ferrous metals division. For example, JIS H 4600 covers plates and strips of titanium and titanium alloys, while JIS H 4650 covers bars. The standard number does not indicate strength. It is necessary to verify the standard symbol, product shape, and edition (year of enactment/revision).
JIS Categories and Representative Materials
| Materials and Designations | How to Read Standards and Symbols | Examples of Characteristics and Applications |
|---|---|---|
| Pure Titanium Type 1 | Example: Pure Titanium Type 1 in JIS H 4600 (Sheets and Strips) | Relatively low strength and high ductility. Thin sheet parts where formability is a priority |
| Pure Titanium Type 2 | Example: Type 2 Pure Titanium per JIS H 4600 | Balance of corrosion resistance, strength, and machinability. Piping, vessels, etc. |
| Type 3 and Type 4 Pure Titanium | Example: Type 3 and Type 4 Pure Titanium per JIS H 4600 | Relatively high strength among pure titanium grades. Pay attention to machining conditions |
| Ti-6Al-4V (64 Titanium) | An α+β-type alloy with a nominal composition of 6% Al and 4% V. Check classifications such as JIS Grade 60 | High specific strength. Aviation, transportation, and high-load components |
| Ti-6Al-4V ELI | Extra Low Interstitial (ELI) specification. Check JIS specifications such as Grade 60E | For applications requiring consideration of toughness and other properties. For medical applications, verify separate, appropriate standards and certifications |
*JIS product designations vary depending on the shape (sheet, bar, tube, etc.), grade, and manufacturing method. Simply specifying “Grade 60” or “Grade 60E” does not satisfy material certification requirements. Please verify the chemical composition, mechanical properties, test methods, and delivery condition as stated in the latest version of the standard.
What are the α, β, and α+β phases?
Titanium has the characteristic that its crystal structure changes with temperature; at lower temperatures, the α phase (hexagonal close-packed structure) is stable, while at higher temperatures, the β phase (body-centered cubic structure) is stable. The stability of these phases varies depending on the alloying elements, and they are classified as α-type, β-type, and α+β-type. Ti-6Al-4V is a representative example of the α+β-type, and its strength and fatigue properties vary depending on heat treatment and microstructure.
Examples of Specifications in Drawings | Standards, Heat Treatment, and Inspection Included
In design drawings, simply using the common name “64 Titanium” is insufficient. You must specify the applicable JIS or contractually required material standard, the product form (such as sheet, bar, or forging), the supply condition, heat treatment, required hardness and strength guarantees, surface quality, traceability, and inspection methods. Even if symbols are similar across different standards, do not automatically assume that the mechanical properties are the same.
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Differences in Manufacturing Methods | Casting, Forging, Rolling, Machining, Additive Manufacturing
Casting | Allows for the production of complex shapes as a single piece, but molten metal management is critical
Casting is a method in which the material is melted, poured into a mold, and allowed to solidify. Since molten titanium readily reacts with oxygen, nitrogen, and the mold, a vacuum or inert atmosphere, appropriate molds, and treatment and inspection of the reaction layer are required. While effective for complex shapes, castings are evaluated for casting porosity, shrinkage cavities, reaction layers, dimensional accuracy, and defects.
Forging | Controls the microstructure and flow lines of high-load components
Forging is a method of shaping a heated solid by applying pressure. In titanium alloys, the microstructure, fatigue properties, and strength vary depending on the processing temperature and the state of the α/β phases. While this method is used for shafts, discs, and connecting parts, performance is not guaranteed by the forging process alone; it is necessary to verify the absence of forging defects through heat treatment and inspection.
Machining and Sheet Metal Processing from Rolled Materials
For prototyping or small-lot production, it is practical to purchase standard-compliant bar stock or sheet metal and machine it. When bending sheet metal, care must be taken to prevent springback and cracking. Compared to steel, machining time and tooling costs may increase; be sure to factor machining costs and yield rates into your estimate.
Welding and Additive Manufacturing (3D Printing)
When welding titanium, the molten pool and high-temperature areas are prone to absorbing oxygen, nitrogen, and hydrogen from the air; therefore, protection using shielding gas and a clean working environment are essential. Metal additive manufacturing can produce complex, lightweight shapes, but it is important to consider the build direction, porosity, residual stresses, heat treatment, HIP, and other post-processing steps in conjunction with inspection.
| Perspectives | Casting | Forging | Machining from bars and plates |
|---|---|---|---|
| Shape | Advantageous for complex and hollow shapes | Constrained by forging dies and plastic flow | Primarily limited to shapes accessible to tools |
| Quality Aspects | Caution required regarding solidification defects and reaction layers | Control of microstructure, forging flow lines, and cracks | Material Quality, Tool Wear, and Finishing |
| Cost | Varies depending on dies, equipment, and production volume | Die costs recouped through mass production | Suitable for prototyping and small-batch production |
| Application Examples | Complex monolithic shapes | High-load joints and shafts | Prototype brackets and fasteners |
How to Select? | When Should You Use Titanium or Titanium Alloys?
Step 1: Evaluate weight, strength, corrosion environment, and budget
First, clarify the following: ① Loads (static load / fatigue / impact), ② Allowable weight, ③ Rigidity and deflection, ④ Temperature, ⑤ Water, seawater, and chemicals, ⑥ Joints and sliding surfaces, and ⑦ Cost and procurement. The main reasons for choosing titanium are high specific strength, corrosion resistance, non-magnetic properties, and low thermal expansion, but be sure to verify the numerical requirements for each.
Step 2: Specific Examples by Component
| Component and Application | Candidate Materials | Reasons for Selection and Verification Points |
|---|---|---|
| High-load, lightweight brackets for aviation and robotics | Ti-6Al-4V | Balances mass and strength. Verify deflection, fatigue, notch sensitivity, and machining costs |
| Heat exchangers and piping handling seawater | Pure titanium (appropriate grade) | Emphasis on corrosion resistance. Verify chemical composition, temperature, clearances, and corrosion of mating materials |
| High-strength, lightweight bolts | Ti-6Al-4V | Weight reduction. Verify thread galling, fatigue, clamping force, and mating materials |
| Non-magnetic components for precision measuring instruments | Pure titanium / titanium alloys | Evaluate based on magnetic properties and thermal expansion requirements. Compare rigidity and cost |
| General-purpose mounting bases and frames | Steel such as SS400 | Low cost and high rigidity. Consider weight, coating, and corrosion resistance |
| Heat sink fins and electrical terminals | Aluminum/Copper | Thermal and electrical conductivity are priorities. Titanium is generally at a disadvantage |
| Contact areas subject to high wear and sliding | Hardened steel or surface-treated materials | Beware of galling during direct sliding between titanium parts. Test surface treatments, etc. |
Step 3: Can rigidity be maintained even with weight reduction?
Replacing a steel bracket with a Ti-6Al-4V bracket of the same dimensions reduces mass, but also decreases the modulus of elasticity, resulting in increased deflection. If there is design flexibility to adjust the part’s shape, it is necessary to redesign the wall thickness, ribs, second moment of area, and joining methods. Compare not only material costs but also lifecycle costs, including machining, assembly, and maintenance.
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Frequently Asked Questions (FAQ)
Which is more resistant to rust, titanium or stainless steel?
Titanium has an advantage in many environments, such as seawater; however, corrosion resistance varies depending on the type, temperature, and concentration of the solution, as well as the structure of any gaps. Since stainless steel grades such as SUS304 and SUS316 also differ, please compare them based on the specific corrosion environment.
Which is lighter, titanium or aluminum?
For the same volume, aluminum (approx. 2.7 g/cm³) is lighter than titanium (approx. 4.5 g/cm³). However, for high-strength applications, the required cross-sectional area and design geometry differ, so it is necessary to compare the quality and weight of the finished parts.
What is the difference between 64 titanium and pure titanium?
64 titanium is a Ti-6Al-4V alloy that offers high strength and high specific strength. Pure titanium, depending on its composition and grade, is suitable for applications that require formability and corrosion resistance. If high strength is not required, pure titanium may be the appropriate choice.
Why is titanium difficult to machine?
Reasons include low thermal conductivity, tool sticking, elastic recovery, and the risk of combustion from fine chips. The design must account for machine rigidity, tools, machining conditions, and safety management.
Summary | Decisions Should Be Based Not Only on Strength but Also on “Rigidity, Corrosion Resistance, Manufacturing Methods, and Total Cost”
Titanium is lighter than steel, exhibits excellent corrosion resistance in certain environments, and offers high specific strength in the Ti-6Al-4V grade. On the other hand, it has a lower Young’s modulus than steel, presenting challenges in machining, sliding, joining, and material costs. Determine the required performance based on the application, and specify the entire set of requirements—including the grade of pure titanium, alloys such as Ti-6Al-4V, JIS product standards, and heat treatment and inspection conditions.
Reference Materials and Related Articles
- Japan Titanium Association (General Incorporated Association): Basic Knowledge of Titanium, Materials, and Applications
- Japan Industrial Standards Committee (JISC): Check current standards such as JIS H 4600 and JIS H 4650
- Mechanical Design 1 | Properties and Selection of Aluminum Alloys
- Mechanical Design 2 | Properties and Selection of Copper and Copper Alloys
- Mechanical Design 3 | Properties and Selection of Iron and Iron Alloys (Effective upon publication)
Note: The physical property values provided here are approximate values for introductory purposes and do not constitute guaranteed values for specific products or heat-treated conditions. For safety-critical components, please prioritize the original text of current standards, material manufacturers’ mill sheets and data sheets, and various design standards and test results.
