Manufacturing Methods for Titanium
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Why Titanium-Steel Composite Plate Is Specified
A titanium-steel composite plate places a thin titanium layer on a structural steel backing, so the corrosion resistance of titanium is paid for only where it is needed. Compared with a solid titanium plate the saving in material cost is substantial, and the steel backing supplies the stiffness and strength that a pressure or vessel design requires. The product is covered as reactive and refractory metal clad plate, and the titanium layer is ordered to a recognised wrought grade such as ASTM B265/B265M Grade 2, which carries a minimum tensile strength of 345 MPa and a minimum yield strength of 275 MPa.
Process Routes in Commercial Use
Four families of processes are used to join the two metals: ingot or pack rolling with an inserted metal, explosive cladding, roll bonding with crimped and welded edges, and weld overlay. Titanium is an active metal that forms a stable oxide and absorbs interstitial elements at temperature, so commercial production concentrates on two of them, explosive cladding at ambient temperature and roll bonding with a controlled atmosphere or vacuum between the mating surfaces.
| Route | Bonding temperature | Typical cladding ratio | Notes |
|---|---|---|---|
| Explosive cladding | Ambient | 2-14 mm titanium on thick steel | Wavy interface, large plate sizes, no interlayer for many grades |
| Hot roll bonding | 850-1,000 C | 0.5-3 mm titanium | Assembly crimped and welded, rolled in one pass sequence |
| Explosive plus rolling | Combined | Thin cladding, tight tolerance | Explosive pre-bond then hot rolled for flatness |
| Weld overlay | Arc process | Local coverage | Used for repairs and nozzle areas |
Explosive Cladding in Practice
The titanium flyer plate is positioned at a controlled stand-off above the steel base plate and a detonation is initiated at one edge. The collision front travels across the plate at several kilometres per second and the surfaces are cleaned, pressurised and joined within microseconds, producing a wavy metallurgical bond without melting the bulk material. Because the process is carried out at ambient temperature, no titanium oxide forms ahead of the bond front and no interlayer is normally required. Detonation velocity, stand-off distance and charge density must be matched to the plate thickness so that the bond is continuous but the interface does not over-melt.
Roll Bonding and Roll Forming Sequences
In the roll bonding route the steel backing and the titanium sheet are stacked, the edges are crimped and welded to exclude air, and the pack is heated and rolled. Two variants are common: thick plate rolling, where the bonded pack is rolled to the finished thickness in stages, and continuous hot rolling, where a long assembly is processed on a strip mill. The pack must be flat and the rolling schedule must avoid excessive reduction per pass, otherwise edge cracking appears in the titanium layer. Heat treatment after bonding relieves residual stress and restores the corrosion properties of the titanium surface.
Bond Quality, Testing and Acceptance
Acceptance is written around bond integrity rather than around a single mechanical property. Ultrasonic examination of the full plate area is performed in accordance with ASTM A578/A578M practice for rolled plates, and a shear test is taken from a coupon attached to the plate. Procurement documents normally state a minimum shear strength for the bond, frequently 140 MPa for titanium on carbon steel, together with a maximum permissible unbonded area at the edges. A bend test confirms ductility of the joint, and a limited number of specimens are sectioned to reveal the interface profile. Where the plate will be formed into a vessel, forming trials are carried out before the full order is released.
Dimensional Control and Handling
Titanium has roughly half the elastic modulus of steel and reacts differently to rolling, so flatness and thickness tolerance in the titanium layer are controlled separately from the backing plate. Cladding thickness is usually specified as a positive tolerance only, since a thin area reduces service life, while overall plate thickness follows conventional rolled plate tolerance. Edges are trimmed to sound metal and the exposed steel edge is protected before shipment because galvanic coupling between titanium and steel accelerates corrosion of the steel when both are wetted.
Frequently Asked Questions
Q: Which process should be chosen for a 3 mm titanium layer on 30 mm steel?
A: Explosive cladding is the usual answer for that combination, because roll bonding at that cladding thickness demands very large reductions and gives less reliable coverage at the plate edges.
Q: Is an interlayer needed between titanium and carbon steel?
A: For most commercial grades no interlayer is needed if the bond is made in one stage and the interface is protected from oxygen. Interlayers are used where a specific metallurgical constraint exists or where the titanium grade is difficult to bond directly.
Q: What shear strength should the purchase order require?
A: Fix the value in the order from the design calculation, then verify it on a coupon from the same plate. A minimum of 140 MPa is a common commercial baseline for titanium on carbon steel, and it must be confirmed by testing rather than assumed.
Q: How is an unbonded area detected?
A: By straight-beam ultrasonic examination over the whole plate surface. Indications are mapped, compared with the acceptance limit in the order, and repaired by welding only where the specification permits repair.
Q: Can the composite plate be welded in the fabricator's shop?
A: Yes, with procedures written for dissimilar joints. The titanium side is welded with inert gas shielding and the steel side with conventional methods, and the transition is normally placed away from high-stress locations.
