ZTC4 Cast Titanium Alloy: Properties, HIP Treatment and Applications
Apr 07, 2024
What ZTC4 Cast Titanium Alloy Is
ZTC4 is a medium-strength alpha-beta titanium casting alloy developed for structural castings that must combine low density, corrosion resistance and reliable weldability. It is produced by investment casting and is normally supplied in the annealed condition or after hot isostatic pressing, a treatment that closes internal porosity and raises fatigue performance. The alloy is designed for long-term service below 350 °C, which places it in the same working window as wrought Ti-6Al-4V components while offering the design freedom of near-net-shape casting.
Because ZTC4 retains useful strength at moderate temperature and stays chemically inert in marine and most industrial atmospheres, it has become one of the most widely applied cast titanium materials in aerospace and general engineering.
Grade Equivalence and Chemical Composition
ZTC4 is the cast counterpart of the Ti-6Al-4V alpha-beta grade. The nearest castings designation in the ASTM system is the Ti-6Al-4V casting grade of ASTM B367, while the wrought references for the same chemistry are ASTM B348, ASTM B338 and AMS 4928. In the Chinese system ZTC4 is specified within the titanium alloy castings family of GB/T 6614.
| Element | Typical content, mass % |
|---|---|
| Aluminium, Al | 5.5 - 6.8 |
| Vanadium, V | 3.5 - 4.5 |
| Iron, Fe | 0.30 max |
| Silicon, Si | 0.15 max |
| Carbon, C | 0.10 max |
| Nitrogen, N | 0.05 max |
| Oxygen, O | 0.20 max |
| Hydrogen, H | 0.015 max |
| Titanium, Ti | Balance |
The limits finally applied to a delivered casting are taken from the governing castings standard and the approved drawing, because the oxygen and iron ranges also set the strength and ductility balance of the finished part. Interstitial control is the single most important metallurgical lever for this grade, and oxygen levels above the specified maximum raise strength at the cost of notch toughness.
Mechanical and Physical Properties
| Property | Typical value, annealed or HIP condition |
|---|---|
| Density | 4.43 g/cm3 |
| Tensile strength | 860 - 965 MPa |
| 0.2 % proof stress | 758 - 830 MPa |
| Elongation | 8 - 10 % |
| Reduction of area | 14 - 25 % |
| Elastic modulus | 110 - 114 GPa |
| Hardness | 33 - 36 HRC |
| Beta transus | approx. 995 °C |
| Maximum long-term service temperature | 350 °C |
| Thermal conductivity at 20 °C | approx. 6.7 W/(m·K) |
Castings are anisotropic, and the properties of a test coupon do not automatically represent the casting itself. Mechanical acceptance is therefore demonstrated on separately cast test bars that accompany the production lot, or on specimens cut from designated locations on the part when the drawing requires it.
Casting Route, Hot Isostatic Pressing and Heat Treatment
Production starts with a wax or resin pattern assembly, which is coated with successive ceramic slurry layers to build a refractory shell. After dewaxing and shell firing, the mould is filled with molten alloy melted under vacuum by consumable electrode or induction skull melting. The good fluidity of ZTC4 allows thin walls and fine surface detail, and its castability and weldability make it one of the more forgiving titanium casting grades for complex geometry.
After knock-out, gate removal and grinding, castings are usually hot isostatically pressed at approximately 900 - 950 °C and 100 - 105 MPa for 2 - 4 hours. The treatment closes shrinkage and gas porosity, improves fatigue life and equalises properties through the section. Annealing is then carried out at 700 - 800 °C in vacuum or inert atmosphere to relieve residual stress and stabilise the alpha-beta structure before final machining. Surface contamination such as alpha case is removed mechanically or by chemical milling so that the specified surface chemistry is met.
Where ZTC4 Castings Are Used
The alloy is chosen for static aerostructural parts such as magazines, brackets, shells and frames, where stiffness-to-weight ratio and corrosion resistance matter more than absolute strength. From the mid-1980s, the wider availability of hot isostatic pressing raised casting quality sufficiently for ZTC4 to be used in lightly loaded rotating components as well, a well-known example being supercharger impellers.
Static airframe and engine brackets
Housings, shells and frames
Magazine and support structures
Supercharger and compressor impellers in the HIP condition
Corrosion-resistant industrial and marine hardware
Designers normally base allowable stresses on the annealed plus HIP condition, and specify the casting factor required by the applicable airworthiness or pressure equipment code, since cast material carries a larger scatter band than wrought product.
Inspection, Testing and Documentation
Acceptance testing for castings of this grade typically combines chemical analysis of a representative sample, room-temperature tensile testing, liquid penetrant inspection, radiographic inspection of critical sections, ultrasonic inspection where specified, and full dimensional verification. Weld repair, when permitted, is performed under inert gas with qualified procedures and is followed by repeat inspection of the repaired area. The following points are commonly checked before a casting is released:
Heat identity, melt chemistry and interstitial analysis
Tensile properties of lot test bars
Internal soundness by radiography or ultrasonic inspection
Surface quality by penetrant inspection and visual examination
Dimensional conformity by coordinate measuring machine
Traceability records linking heat number, casting serial number and test results
Frequently Asked Questions
Q: Is ZTC4 the same material as Ti-6Al-4V?
ZTC4 has the same nominal chemistry as the Ti-6Al-4V alpha-beta grade but is a casting alloy, so its allowable properties, test sampling and defect acceptance rules follow castings standards rather than wrought product standards.
Q: Why is hot isostatic pressing specified so often for ZTC4?
HIP closes internal shrinkage and gas porosity. Fatigue strength and ductility scatter are both improved, which allows the alloy to be used in rotating or dynamically loaded parts that would otherwise be restricted to static applications.
Q: What is the maximum service temperature of ZTC4 castings?
The alloy is intended for long-term service below 350 °C. Above that level creep and oxidation behaviour degrade, and a higher-temperature titanium alloy should be selected instead.
Q: Can ZTC4 castings be welded?
Yes. The alloy has good welding characteristics, and repairs or fabrication joints are made by inert gas tungsten arc welding with fully protected, high purity argon shielding followed by stress relief when the drawing requires it.
Q: How is oxygen content controlled in a finished casting?
Oxygen enters mainly through the melt and through shell interaction, so it is controlled by raw material selection, vacuum melting practice and by removing the oxygen-enriched alpha case from the casting surface before the final chemistry sample is taken.
Q: Which documents are normally supplied with a casting lot?
A chemistry certificate, mechanical test report for the lot test bars, non-destructive test reports, dimensional inspection report and heat number traceability documents are the normal minimum package for industrial and aerospace orders.
