Billet Selection and the Specification Route
A forging starts with bar or billet to ASTM B348 or AMS 4928 for Ti-6Al-4V, or to commercially pure grades where formability and corrosion resistance matter more than strength. Ti-6Al-4V carries UNS R56400 and is by far the most widely forged titanium alloy: it combines about 895 MPa minimum tensile strength with useful toughness and weldability, and it holds most of that strength to roughly 400 °C. Near-alpha and beta alloys are chosen when service temperature rises further or when a higher strength-to-weight ratio is required.
Finished forgings are certified to ASTM B381, which covers chemistry, tolerances and mechanical properties, and to AMS 4928 for aerospace bar and forging stock. Medical forgings follow ISO 5832-3 or ASTM F1472, and the extra-low-interstitial Grade 23 material adds an oxygen limit so that fracture toughness and fatigue behaviour are preserved.
Heating Windows and the Beta Transus
The forging window is set by the beta transus, which for Ti-6Al-4V is close to 995 °C. Alpha plus beta alloys are forged in the two-phase field, typically 900 – 980 °C, some 30 to 60 °C below the transus, which recrystallises the structure into fine equiaxed alpha in a transformed beta matrix. Working above the transus produces coarse Widmanstatten plates with poor ductility at room temperature, while working too cold raises flow stress and risks surface cracking.
Because titanium absorbs oxygen, nitrogen and hydrogen aggressively above about 500 °C, furnace atmosphere and time at temperature are controlled by procedure, and any oxygen-enriched surface layer that forms is removed in later machining or pickling. Billets are cleaned of scale, oil and contamination by blasting, machining or pickling before heating, so surface defects are not driven into the part.
Forging Operations
Open-die forging forms large simple shapes between flat dies with a press or hammer, which is flexible and inexpensive but limited in dimensional precision. Closed-die forging shapes the billet in a die cavity that controls the fibre flow and the final contour, and it is the normal route for components such as discs, blades and structural fittings. Isothermal and hot-die forging hold the die at or near billet temperature to limit die chill and allow thinner sections to be filled, which suits intricate near-net parts. Each operation is defined by a die sequence with intermediate cleaning and, where needed, reheating.
Heat Treatment and Microstructure Control
Two treatments dominate. Mill annealing is a recrystallisation treatment in the two-phase field followed by air cooling, which relieves stress and gives a stable alpha plus beta structure for general service. Solution treatment and ageing, usually called STA, heats into the two-phase field near 955 °C, quenches in water and then ages around 480 – 595 °C for several hours; this raises strength at some cost in ductility. Thermal processing is recorded on the certificate along with the resulting tensile, yield and elongation values, and hardness is checked as a lot control.
Inspection and Quality Control
Ultrasonic inspection of the forged and heat treated part detects internal voids, laps and alpha inclusions.
Alpha case depth is measured on a section, and the layer is removed by machining or pickling before the part is released.
Macro-etch and micro-structure review confirms grain flow, absence of beta flecks and the required equiaxed condition.
Mechanical testing per lot covers tensile strength, yield strength, elongation and reduction of area, with hardness on each piece.
Dimensional inspection is performed against the drawing, with coordinate measurement of critical interfaces.
A common misconception is that a hotter or longer soak improves forging quality; in practice it drives grain growth and raises the oxygen uptake, so the acceptable window is deliberately narrow. Forging to a specification's minimum properties while ignoring structure also fails in service, because fatigue life is controlled by grain morphology rather than by tensile numbers alone.
Frequently Asked Questions
Q: Which standard covers titanium forgings for general engineering?
A: ASTM B381 covers wrought titanium forgings including Ti-6Al-4V, and aerospace work is normally also released to AMS 4928.
Q: Why must forging stay below the beta transus?
A: Forging above it produces coarse Widmanstatten structures with reduced ductility and fatigue resistance, so two-phase field working is used instead.
Q: What is alpha case and why is it removed?
A: It is an oxygen and nitrogen enriched surface layer formed at high temperature. It is hard and brittle, so it is machined or pickled away before the part goes into service.
Q: What does STA heat treatment change?
A: Solution treatment and ageing raise tensile and yield strength, at the expense of some ductility, and are chosen where the design is strength driven.
Q: How are internal defects detected?
A: Ultrasonic inspection is performed after heat treatment, supplemented by macro and micro structural examination on a section from the lot.
