Titanium Alloy Forging Defects: Causes, Detection and Prevention

Why Titanium Forging Is Less Forgiving Than Steel

Titanium forges in a narrow temperature window and conducts heat poorly, so the differences between the surface and the centre of a billet are larger than they would be in steel. Alpha and alpha-beta alloys are especially sensitive to overheating, and the defects that follow from a bad heating cycle are often not recoverable. Understanding which defects can be reworked and which mean the part is scrap is the difference between a controllable process and a scrap rate nobody can explain.

Beta Brittleness from Overheating

Beta brittleness is caused by overheating. In alpha and alpha-beta alloys, and particularly in the alpha-beta grades, a forging temperature above the beta transus produces a coarse, equiaxed prior-beta grain structure with alpha precipitated along the grain boundaries and a striated intracrystalline structure. Room-temperature plasticity falls sharply. This is the classic overheating signature.

Overheating cannot be repaired by heat treatment. The structure has to be broken down by reworking the material below the beta transus, and only if the forging geometry and the remaining stock allow it. Prevention rests on furnace control: hold the furnace temperature within a qualified band, survey the chamber regularly, position the charge so that the workpieces are not too close to the heating elements, use baffles on both sides in resistance-heated furnaces to prevent localised overheating, and limit the charge size. Measuring the actual beta transus of the alloy heat in each furnace is a further effective safeguard, because the transus moves with interstitial content.

Localised Coarse Grain

In hammer and press die forging, the poor thermal conductivity of titanium means the billet surface cools markedly on contact with the die, and friction between the workpiece and the upper and lower dies concentrates deformation in the middle of the billet. The surface deforms less, its original structure survives, and a locally coarse grain band results.

Three measures address it. A pre-forging operation distributes deformation more uniformly in the finishing pass. Better lubrication reduces friction between billet and die. And thorough preheating of the dies cuts the temperature drop the billet suffers during transfer and contact.

Surface Cracks

Surface cracks appear mainly when the finishing temperature falls below the full recrystallisation temperature of the alloy. In die forging, an excessive contact time between billet and die is the usual cause: because titanium conducts heat poorly, the surface can drop below the permissible finishing temperature while the centre is still hot. Glass lubricant on press die forging, and shortened contact time between the blank and the lower die on hammer forging, are the standard remedies. Cracks are shallow and can sometimes be removed by blending, but only within the limits set by the drawing and by the applicable forging specification.

Retained Cast Structure

When an ingot is forged with insufficient reduction or with the wrong sequence, the cast structure survives into the finished part. The remedy is process-based rather than inspection-based: increase the forging ratio and use repeated upsetting and drawing so that the cast dendrites are broken down and the flow lines follow the part contour. A forging with retained cast structure shows characteristic macrostructure on a sectioned and etched test piece.

Bright Bands

Bright bands are visible in the low-magnification macrostructure as streaks of different brightness from the surrounding metal. Depending on the angle of illumination they can appear lighter or darker than the base material. On a cross-section they show as dots or flakes; on a longitudinal section they run as smooth streaks that may be anywhere from ten millimetres to several metres long.

There are two causes. The first is chemical segregation in the alloy, generally of elements that affect the response to etching. The second is deformation heating during forging, where the temperature varies too much from point to point in the workpiece. Bright bands degrade plasticity and high-temperature performance, so prevention means controlling segregation at the melting stage and choosing thermal forging parameters, heating temperature, degree of deformation and deformation rate, that keep the temperature spread across the forging under control.

Alpha Case

The alpha embrittlement layer forms when oxygen and nitrogen diffuse inward through the loose oxide scale at high temperature, raising the interstitial content of the surface metal. As the oxygen and nitrogen content of the surface rises, so does the proportion of alpha phase, until the surface layer may be entirely alpha. That layer is hard and brittle, and on a billet or a forging it can crack during subsequent deformation.

Thickness depends on the furnace type, the nature of the furnace gas, the heating temperature and the holding time. Thickness increases with temperature, with holding time and with the oxygen and nitrogen content of the furnace atmosphere. Alpha, beta and alpha-beta alloys can all form the layer, and alpha alloys are especially prone to it; beta alloys resist it until they are heated above roughly 980 °C. The layer must be removed by pickling or machining, and the depth allowed for on a forging drawing is a real dimension, not tolerancing slack.

Hydrogen Embrittlement

Two mechanisms operate. In strain-ageing type embrittlement, hydrogen atoms in interstitial lattice sites diffuse under stress and gather at stress concentrations over time, pinning dislocations so that they cannot move freely and making the matrix brittle. In hydride type embrittlement, hydrogen dissolved at high temperature precipitates as hydride as the temperature falls, embrittling the alloy. Both can occur in titanium and its alloys.

The root cause in both cases is excessive hydrogen content, and industrial titanium alloys therefore have to be held at or below 0.015 %. Prevention during forging and heat treatment means using a slightly oxidising furnace atmosphere, and where hydrogen already exceeds the limit, vacuum annealing is the route to remove it. Hydrogen content is measured on the finished product by inert gas fusion under ASTM E1447, and a forging certificate should state the result rather than the intention.

Detection and Acceptance

ASTM B381 covers titanium alloy forgings and sets the quality classes and the examination requirements that a purchase order can call up; AMS 4928 covers the Ti-6Al-4V bar, forging stock and forging route used in aerospace work. Mechanical acceptance is by tensile testing to ASTM E8/E8M, with elevated-temperature tensile and stress rupture testing for hot-section parts. Internal soundness is normally verified by ultrasonic examination, and surface condition by penetrant examination. Alpha case and microstructure are verified on sectioned coupons from a sacrificial or qualification forging.

Frequently Asked Questions

Q: Can an overheated titanium forging be salvaged by re-heat treatment?
A: No. The coarse transformed structure cannot be reversed by thermal treatment alone. Recovery requires plastic deformation below the beta transus, and only where the geometry and the remaining stock allow it.

Q: Why does titanium forge with a narrower window than steel?
A: Because the beta transus sets a hard upper limit and low thermal conductivity makes the surface-to-centre temperature difference large. The usable window is further reduced by the need to finish above the recrystallisation temperature.

Q: What is the maximum permitted hydrogen content?
A: 0.015 % for industrial titanium alloys. Above that level both strain-ageing and hydride type embrittlement become a real risk, and vacuum annealing is required to reduce it.

Q: Do beta alloys form an alpha case?
A: They resist it better than alpha and alpha-beta alloys and will not form the layer until heated above roughly 980 °C. Alpha alloys are the most sensitive and need the tightest atmosphere control.

Q: How deep is the alpha case on a typical forging?
A: Depth depends on temperature, time and furnace atmosphere. It must be removed by pickling or machining, and the drawing allowance for the affected depth is a process requirement rather than a tolerance.

Q: What causes bright bands in a forged titanium macrostructure?
A: Chemical segregation in the alloy, or excessive temperature variation from deformation heating during forging. Both degrade plasticity and high-temperature performance and are controlled at melting and at the press.