When it comes to aerospace engineering, titanium alloys have long been a preferred choice due to their exceptional strength-to-density ratio and excellent corrosion resistance.
The use of titanium in various parts of aircraft is done keeping in view the criticality of its end application to ensure safety and performance.
The grades of titanium are applied in different use case scenarios. While both Grade 5 and Grade 23 have a similar ratio of chemical elements, the difference in interstitial content directly impacts strength, ductility, and applicability to certain types of load conditions.
Understanding the difference between the two types of titanium can serve as guidance for mechanical engineers, procurement departments, or design offices to make a better choice of material.
Titanium Grade 5: The Aerospace Workhorse
Grade 5 titanium, or Ti-6Al-4V, contains around 6% aluminium, 4% vanadium, and the balance titanium; it is defined by UNS R56400.
It is strong, with a high tensile strength and an excellent strength-to-weight ratio. It has good corrosion resistance and is widely used in aerospace manufacturing. It is frequently cited as contributing to around 50% of the titanium alloy market.
The grade can be found in applications including airframe structure, fastening solutions, structural and engine sections. Its well-defined manufacturing processes and wide availability make it a logical choice for aerospace applications.
Relevant standards include AMS 4911 for sheet, strip, and plate, and AMS 4967 for bar and forgings.
Titanium Grade 23: Understanding ELI
Grade 23, or Ti-6Al-4V ELI, is similar to grade 5, but the interstitial elements, especially oxygen and iron, are intentionally restricted to a lower amount. ELI means Extra Low Interstitials.
The lower interstitial levels are beneficial for fracture toughness and ductility, allowing grade 23 to be used in components subjected to high loading and in applications where fatigue needs to be minimized; however, it is necessary to qualify the results with those of a specific manufacturer’s process as well as surface finish, heat treatment, and design geometry.
Grade 23 is given under UNS R56401. The use in aerospace applications must be qualified with specific requirements of the manufacturer, and the individual components as implantable devices in the medical industry require specific qualifications.
Grade 5 vs Grade 23: Key Differences
| Property | Grade 5 | Grade 23 |
| Composition | Ti-6Al-4V | Ti-6Al-4V ELI |
| UNS | R56400 | R56401 |
| Interstitial content | Standard limits | Reduced limits |
| Strength | High | Generally comparable |
| Toughness and ductility | Good | Typically improved |
| Availability | Broad | More specialized |
Grade 23 may not inherently be stronger or superior for fatigue life in all scenarios. It will offer offsetting advantages and disadvantages relative to Grade 5 based on the material processing, geometry, loading, and the applicable specification.
Choosing the Right Grade for Aerospace
Typically, Grade 5 is a good tradeoff for general structural applications, where the emphasis is on strength, weight efficiency, and where the established supply base supports the use of this material.
In areas where the component is sensitive to fatigue and/or in higher-criticality applications potentially seeking the better tradeoff in terms of fracture toughness and ductility, Grade 23 can be a rational choice.
Before deciding which grade to use, engineers should also evaluate the ambient conditions, the expected loading conditions, the recommended inspection regimens, and the verified material condition properties, listed on the Qualified Producers List, for the selected products.
ASTM F136 and ISO 5832-3 are standards for Grade 23 in surgical implant applications and are not automatic aerospace qualifications.
Procurement should specify the exact product form, heat treatment, mechanical properties, and traceability documentation.
Making an Informed Selection
Grade 5 is the well-established trade-off between strength, weight, and availability. Grade 23 provides additional interstitial control, which can improve toughness and ductility for the corresponding application.
Choose based on component requirements, actual material data, and aerospace qualification caveats.