1. From Sponge to Ingot
Titanium melting begins with titanium sponge produced by the Kroll process, in which titanium tetrachloride is reduced with magnesium. The sponge is blended with alloying elements such as aluminium, vanadium, molybdenum, tin and zirconium, and with controlled amounts of scrap, then compacted into electrodes. The electrodes are welded together and melted under vacuum to form the ingot. The purity of the sponge and the accuracy of the alloy addition determine the composition of the final ingot.
2. Vacuum Arc Remelting
Vacuum arc remelting is the standard process for titanium ingots. The consumable electrode is melted by an electric arc in a water-cooled copper crucible under vacuum, and the metal solidifies progressively from the bottom. Volatile impurities are removed by the vacuum, and the controlled solidification reduces segregation. Most industrial ingots are melted twice, and aerospace rotating components are often melted three times to achieve the highest cleanliness and homogeneity.
3. Cold Hearth Melting
Cold hearth melting, performed with an electron beam or a plasma arc, is used to eliminate defects that can survive conventional remelting. The molten metal is held in a water-cooled hearth where high-density inclusions sink and low-density defects float, while the clean metal flows into the ingot mold. This process is specified for critical aerospace parts where a single defect could cause a failure. It also allows the recovery of machining scrap.
4. Alloy Composition Control
The alloying elements are added in precise quantities, and the interstitial elements oxygen, nitrogen, carbon and hydrogen are controlled to tight limits because they strongly affect strength and ductility. Samples are taken during melting and analysed to confirm the composition. Alpha-beta alloys such as Ti-6Al-4V must be melted and solidified without forming beta segregation, which appears as dark bands known as beta flecks in the microstructure.
5. Ingot Quality and Inspection
After cooling, the ingot surface is conditioned by peeling or machining to remove the contaminated outer layer. The ingot is examined by ultrasonic testing to detect internal porosity, cracks and inclusions, and macroetch samples are taken from the top and bottom to reveal the solidification structure. Chemical analysis confirms the composition at both ends. Ingots that meet the requirements are released to forging and rolling with full traceability.
6. Downstream Processing
The ingot is forged or rolled into billet, bar, plate and other forms, and homogenisation heat treatment is applied where needed to reduce segregation. Each downstream step is controlled so that the quality built into the ingot is preserved in the final product. For critical applications the entire chain from sponge to finished material is documented, and the heat number provides traceability back to the melting records.
Vacuum arc remelting is the standard process for titanium ingots.
Cold hearth melting removes high-density and low-density inclusions.
Triple melting is used for the most critical aerospace components.
Frequently Asked Questions
Q: Why is titanium melted in a vacuum?
Molten titanium reacts violently with air, so melting is done in vacuum to remove gases and volatile impurities and to prevent contamination.
Q: What is double melting?
Double melting means the electrode is remelted twice, which improves composition uniformity and removes residual defects compared with single melting.
Q: What is the difference between VAR and cold hearth melting?
VAR uses a consumable arc in vacuum, while cold hearth melting holds the metal in a water-cooled hearth to separate inclusions before solidification.
Q: What are beta flecks?
Beta flecks are localised regions of beta-stabiliser enrichment that can form during solidification of alpha-beta alloys and reduce fatigue performance.
Q: How is ingot quality checked?
By ultrasonic testing, macroetch examination, chemical analysis and surface conditioning, with records kept for traceability.
