Why Titanium Layered Metal Composites Need a Bonding Route
Layered metal composites pair titanium with a second metal so that the assembly keeps titanium's corrosion resistance and low density while gaining the strength, thermal conductivity or wear behaviour of the partner metal. The bonding route decides bond integrity, interface microstructure and the shapes that can be produced. The established industrial routes are explosive welding, explosive welding plus hot rolling, direct rolling, extrusion compounding, overlay welding and brazing. Because titanium reacts readily with oxygen and nitrogen at elevated temperature and forms brittle reaction products with iron, nickel and copper, production of load-bearing composite plate, rod and tube concentrates on the first three routes.
Explosive Welding and Explosive Welding Plus Rolling
Explosive welding is performed at ambient temperature. A controlled detonation accelerates the flyer plate against the base plate at an angle and velocity high enough to create a wavy interface and a thin, continuous metallurgical bond, while the bulk of both metals stays cool and keeps its original structure.
Common plate combinations: titanium to steel, titanium to copper, titanium to nickel, titanium to zirconium.
Layer arrangements: double layer, multi-layer and graded stacks bonded in one or several passes.
The bonded assembly is then hot rolled to flatten the interface wave, close micro-voids and raise shear strength.
Composite plate is annealed after bonding to relieve residual stress, then straightened, trimmed and cut to size. Clad ratio, base plate thickness and final flatness are agreed per application.
Extrusion Compounding for Composite Rod and Tube
Extrusion compounding serves composite bars and tubes. Prepared layers are assembled or cast into solid or hollow billets, sealed under vacuum or inert gas, and pushed through a die at a controlled temperature and ram speed. Tooling and reduction ratio are selected so that the softer component flows around the harder one and the interface develops a continuous metallurgical bond along the full length.
| Parameter | Typical practice |
|---|---|
| Billet assembly | Machined, degreased sleeves, evacuated and sealed |
| Extrusion temperature | Held below the beta transus to limit grain growth |
| Reduction ratio | Set high enough to close voids and refine the bond zone |
| Post-processing | Straightening, pickling, ultrasonic inspection |
Rolling, Overlay Welding and Brazing
Direct roll bonding demands heavy reduction and tight atmosphere control: titanium oxide scale has to be removed before each pass and re-oxidation avoided. Overlay welding deposits titanium onto a steel substrate and suits localised protection rather than full-area cladding. Brazing produces a lower-temperature joint for parts that cannot tolerate a full thermal cycle, but the filler metal limits the service temperature of the joint.
Grades, Bond Quality and Inspection
Commercially pure layers: TA1, TA2, Gr1, Gr2 for chemical and marine duty.
Palladium-modified grades: TA9, TA10, Gr7, Gr12 for strongly reducing or chloride-bearing media.
Structural alloy layers: TC4 / Gr5 where higher strength is required.
Bond integrity is proved by full-area ultrasonic testing for unbonded zones, shear, tensile and bend tests on coupons from the same lot, and metallographic examination of the interface. The clad plate specification follows ASTM B898 for reactive-metal clad plate, the titanium layer follows ASTM B265 for plate and sheet, and seamless or welded titanium tube for the same service is ordered to ASTM B338 or ASTM B337 respectively.
Frequently Asked Questions
Q: Why is explosive welding so widely used for titanium composites?
It bonds at ambient temperature, so no brittle reaction layer grows at the interface and no vacuum furnace is required.
Q: Can explosive welding produce multi-layer plate?
Yes. Two, three and graded multi-layer stacks can be bonded in one or more shots, then rolled and stress relieved.
Q: How thick can the titanium layer be?
Clad ratio is fixed per project; thicker titanium layers need a higher explosive load and tighter control of the collision wave parameters.
Q: Which tests prove bond integrity?
Full-area ultrasonic testing, plus shear, tensile and bend tests on coupons taken from the same production lot as the delivered plate.
Q: Are extruded composite tubes leak tight?
Sealed-billet extrusion gives a continuous interface; tubes are pressure tested and ultrasonically checked before despatch.
Q: How are faying surfaces kept clean before bonding?
Plates are machined, degreased and assembled under clean conditions, and the titanium surface is protected until the joint is made.
Q: Where are titanium layered composites used?
Pressure vessels, heat exchangers, desalination plant, chemical reactors, electroplating cells and transition joints between titanium and steel pipework.
