1. Can grade 5 titanium be hardened?
Solution treatment: The alloy is heated to a temperature range of 925–955°C (1700–1750°F), which dissolves most of the β-phase elements (e.g., vanadium) into a homogeneous α+β matrix. It is then rapidly cooled (quenched) in water or air to trap these elements in a supersaturated solid solution, forming a metastable phase called martensite (α').
Aging: The quenched material is reheated to a lower temperature (typically 480–650°C / 900–1200°F) for several hours. During aging, fine, uniform precipitates of the β-phase form within the martensite matrix. These precipitates impede the movement of dislocations in the metal, significantly increasing its strength and hardness.
2. Can you polish grade 5 titanium?
Surface preparation: First, any surface defects (e.g., scratches, burrs) are removed using abrasive methods-starting with coarse grits (e.g., 120–400 grit sandpaper or grinding wheels) for heavy material removal, then progressing to finer grits (e.g., 600–1200 grit) to refine the surface.
Mechanical polishing: For a smoother finish, mechanical polishing uses increasingly fine abrasives (e.g., 2000–5000 grit sandpaper, diamond pastes with particle sizes as small as 1–0.5 μm). This step is often performed with polishing wheels or buffing pads, using light pressure to avoid overheating (which can re-form the oxide layer and dull the surface).
Chemical polishing (optional): To enhance reflectivity (e.g., for decorative or optical applications), chemical polishing may follow mechanical polishing. This involves immersing the titanium in a corrosive solution (e.g., a mixture of hydrofluoric acid, nitric acid, and water) that selectively etches surface irregularities, creating a uniform, glossy finish without mechanical contact.
Post-polishing cleaning: After polishing, the surface must be thoroughly cleaned to remove residual abrasives or chemicals, as these can cause staining or corrosion. A final pass with a non-abrasive cleaner (e.g., isopropyl alcohol) ensures a pristine finish.
3. Can you weld different grades of titanium together?
Grade compatibility: Most industrial pure titanium (CP Ti) grades (e.g., Grade 1, 2, 3, 4) can be welded to each other, as they have similar phase structures (primarily α-phase) and only differ in impurity content (e.g., oxygen). Welding CP Ti to alloyed titanium (e.g., Grade 5 Ti-6Al-4V, Grade 9 Ti-3Al-2.5V) is also feasible but more challenging, as alloyed grades contain β-stabilizing elements (e.g., vanadium) that can alter the weld's microstructure.
Filler metal selection: The choice of filler metal is critical to match the mechanical properties and corrosion resistance of the base metals. For example:
When welding CP Ti grades (e.g., Grade 2 to Grade 3), a Grade 2 or Grade 3 filler metal (e.g., ERTi-2, ERTi-3) is typically used to maintain compatibility.
When welding CP Ti to Grade 5 titanium, a Grade 5 filler metal (ERTi-5) is preferred to ensure the weld has similar strength to the alloyed base metal; using a CP Ti filler would result in a weaker weld zone.
Welding process: Gas Tungsten Arc Welding (GTAW, also called TIG welding) is the most common method for titanium welding, as it provides precise heat control and allows for shielding the weld zone with inert gas (argon or helium). This shielding is critical to prevent titanium from reacting with oxygen, nitrogen, or hydrogen at high temperatures-reactions that cause severe brittleness.
Post-weld heat treatment (PWHT): Depending on the grades being welded, PWHT may be required to reduce residual stresses and refine the weld microstructure. For example, welding Grade 5 to Grade 2 may require annealing at 650–700°C (1200–1290°F) to soften the weld zone and improve ductility.
Quality control: Dissimilar titanium welds require rigorous testing (e.g., visual inspection, radiography, tensile testing, corrosion testing) to detect defects (e.g., porosity, cracks) and verify performance, especially in critical applications like aerospace or medical devices.
