Titanium: an Overview
Titanium is a “super metal.” It’s lightweight, strong, has excellent corrosion resistance, can withstand high temperatures, is crack and fatigue resistant, and has a high tensile strength-to-density ratio. Not surprisingly, titanium is used in many aerospace applications, and is also commonly used in the medical industry for artificial joints and implanted medical devices—titanium does not react with the human body.
Relative to stainless steel, Titanium is more expensive and is not utilized typically in high-volume production, but for many critical applications, titanium is the only choice.
However, titanium is a difficult material to weld, and getting quality titanium welds requires knowledge and experience.
The Challenges of Welding Titanium
Fusion welding processes, such as MIG, TIG, Laser, and Electron Beam, generate intense heat to melt the material in the desired weld area. Controlling the amount of heat fed into this area is critical for successfully welding titanium. Manual welding processes, such as MIG and TIG, rely on operator skill and heat sinking to control this heat. Automated methods, such as Laser and Electron Beam, which use computers to control feed rate, power, and target weld location, offer precision and repeatability that result in higher-quality welds. Our laser hermetic sealing department provides this precision and repeatability, which helps to make using titanium on electronic packages a cost-effective design choice.
When exposed to oxygen, Titanium quickly forms a microscopic layer of oxides which inhibits reactions with other chemicals. As Titanium is heated toward its melting point (3,034˚F / 1,668 ˚C), the oxides form even faster. Oxide contamination of the weld pool results in poor-quality welds, so control of oxidation is paramount, hence titanium is extremely difficult to weld without a controlled environment and strict conditions. Pure welds in titanium require welding in a vacuum or the use of a cover gas.
Electron beam welding, which happens in a vacuum, is an excellent process for titanium. As our operators have said, “Titanium welds like butter under an electron beam.” EB Industries also welds titanium using lasers, and we achieve high-quality joins by ensuring uniform gas coverage via laser welding titanium in glovebox environments.
Contamination is an issue that affects all weldable materials and titanium is no exception. Proper cleaning procedures when fabricating parts are crucial, as residual contaminants may become lodged within weld joints during the assembly process that are next to impossible to remove. Any contaminant residuals can cause major issues during laser sealing that might lead to irreparable weld seams.
The storage and packaging of any machined parts before laser sealing is also critical, as proper storage and packing further reduces the risk of residual contaminants building up and lingering after pre-weld cleaning operations.
Parts are typically cleaned and inspected before welding to reduce the risk of welding parts that have visible FOD present. For titanium parts entering our laser hermetic sealing department, they may be precision cleaned prior to arrival at our facilities, eliminating the need to clear contaminates.
Laser Hermetic Sealing Titanium
The most difficult aspect of laser welding titanium is gas coverage. As stated earlier, poor gas coverage results in oxidation and weak welds. With inadequate coverage, the weld will appear heavily discolored, and that discoloration will be difficult to remove. Using our glovebox, within our laser hermetic sealing department, we can work with titanium in an environment that provides excellent gas coverage, resulting in strong welds with a consistent, aesthetically pleasing finish.
Pre-Weld Preparation and Joint Fabrication
Titanium does not require special procedures or processes when fabricating components. Traditional machining equipment and tools can be utilized as necessary, unlike parts made with Kovar. Tight tolerances should be held when fabricating joints, as with all welded parts, to ensure the best seal without issues.
Our pre-weld preparation includes removing oxides and hydrocarbon contamination before welding. This is usually accomplished with any combination of stainless-steel wire brushes, light-fiber brushes, acetone, or alcohol-based solvents. Parts can also be ultrasonically cleaned in an acetone bath, but this might introduce excess heat that may not be suitable for small components found inside parts. Titanium parts should be welded immediately after cleaning.
Our technicians handle titanium parts wearing ESD-appropriate gloves while using best practices to avoid contamination and electrostatic damage. All parts containing electrically sensitive components are prepped in our electrostatic discharge (ESD) safe room with protocols in place to minimize damage from stray electric charges.
Joint configuration is critical as certain parts require a weld directly on the joint while other parts require a weld to be offset. This offset may involve welding .001″ – .004″ toward more material (usually a cover) to prevent the weld from rolling over an edge. This significantly reduces the risk of cracking (not typically an issue for titanium). Joint configurations that allow easy access to cover gas are very weld friendly and will produce the best results.
- Butt Joint:
- A fit-up tolerance of 15% of the material thickness is desirable.
- Sheared edges are acceptable provided they are straight and square.
- Misalignment and non-flat of parts should be less than 25% of the material thickness.
- Lap joint (burn-through or seam weld):
- Air gaps between pieces to be Lap Joint welded severely limit weld penetration and/or feed speed.
- For round welds in titanium, no gap can be tolerated unless inert gas coverage can be maintained over the entire weld area.
- Fillet Joint:
- Square edges and good fit-up are also necessary.
- This joint type will require a higher degree of heat to ensure fusion
Laser hermetic sealing requires a precise joint to maintain permissible gaps and reduce mismatch. This reduces the risk of weld nonconformities and damage to internal components. In conjunction with a CNC-guided laser, adequate fixturing is necessary to eliminate the risk of a moving part and ensure the beam is precise and accurate. Ensuring that components are square and run true is critical for all jobs in the hermetic sealing department.
Titanium parts that are welded in our glovebox environments may go through some additional steps when they’re to be hermetically sealed. This includes vacuum baking, which occurs in a computer-controlled vacuum oven and results in the parts containing less than 1 ppm of oxygen and moisture. Vacuum baking can be done at various temperatures and durations to meet a wide range of requirements. Generally, longer vacuum bakes are better for most applications.
Recommended Laser for Hermetically Sealing Kovar
We use fiber optic lasers to hermetically seal Titanium. This technology produces high-quality welds and is very controllable via computers and automation.
Our gloveboxes are fitted with computer-controlled fiber optic lasers as well as oxygen and moisture monitoring, the data of which can be recorded for gas analysis and reportage.
Integrated vacuum ovens have inert gas drying with purification and recycling capabilities.
Supporting equipment includes laminar flow work benches and exhaust hoods. The control of cover gas for welding titanium is critical, which is why the hermetic sealing department is a great fit for titanium components.
Laser Hermetic Sealing Modes of Operation
Laser beam energy can be applied to the workpiece either as a series of pulses or in a laser stir weld configuration. The decision to use a particular method is dependent on the application, the penetration requirements, or other customer requirements.
Pulsed Laser Hermetic Seal
Commonly, welds are accomplished with a pulsed laser, which utilizes a beam that is switched on and off at a very high rate (10-1000 Hz), such that the energy is applied to the workpiece in a series of separate bursts. Each pulse creates an area of melted material. The workpiece is then moved slightly, and another pulse is applied, resulting in a series of overlapping welds that create a continuous bead. Each weld area created by a pulse cools quickly and minimizes the amount of heat building up in the surrounding area. Limiting the transfer of heat within the part reduces the risk of damage to electrical components and the distortion of the part. When working with titanium, pulsed laser welding is better for shallow welds, where minimizing heat is an important factor.
Laser Stir Welding
Laser stir welding is a process in which a continuous beam laser oscillates at a relatively high frequency, which causes a stirring action within the molten weld pool – hence the term “stir welding.” The result is a manipulation of the weld pool which changes some key characteristics of the weld.
Stir welding creates a keyhole-type weld, in which the trailing joint cools as the leading joint is fusing. There is also more precise control of the weld pool for increased keyhole stability. To prevent deformation of the part, feed rates of at least 25 inches per minute to 100 inches per minute are required. These higher feed rates can be achieved with laser stir welding, whereas pulsed techniques are more limited in terms of feed rate. The continuous beam can also achieve a deeper weld penetration if required.
- Laser Stir Welding results in largely defect-free joints, with no hot cracking, porosity or solidification cracks.
- More precise control of the weld pool for increased keyhole stability.
- Improved control over the geometry of the weld – as an example, joints can be designed with more width at the root of the weld, which can be very useful for Lap/Thru-/Blind welds.
- Weld profiles can be manipulated into asymmetry, such as increasing the penetration on one side of the weld joint.
- Patterns can be programmed to compensate for large gaps in weld joints and other potentially problematic weld geometry problems.
- Higher feed rates can be achieved than with pulsed laser techniques.
- Filler material (wire or shim) is typically added manually before welding.
Cover Gas Requirements for Titanium Welding
Cover gas is necessary for all laser welded titanium parts. Our unique glovebox environments often provide excellent gas coverage even when hermeticity is not a strict requirement. Sealing titanium parts within our glovebox environments produces much better results in terms of surface finish and aesthetics than welds executed in an open air environment.
Cover gasses are usually selected on a per project basis, but a few general guidelines are as follows:
- Argon–Helium Mixtures: recommended for most general laser welded applications depending on laser power level. This mixture is the ‘go-to’ for our operators on most titanium applications as it produces the best finish and cosmetic appearance.
- Argon: commonly used and can minimize plasma generation, argon should not be used with C02 lasers exceeding 3kW of power to further reduce plasma generation. Argon is preferred for most titanium applications.
- Argon–Helium Mixtures: recommended for most general laser welded applications depending on laser power level.
- Nitrogen – C02 Mixtures: can produce acceptable welds although often the seam will be slightly oxidized. When welding titanium in these mixtures, the overall finish is not the same relative to the finish on parts made of aluminum or other materials.
90-10 Nitrogen–Helium: commonly used in housing assemblies, helium tends to suppress plasma generation, but since it is very lightweight it can require a high flow rate, which can cause weld pool turbulence, which is undesirable. Nitrogen environments are generally eschewed for welding titanium.
Generalized Process for Hermetic Sealing a Titanium Package
Titanium is used in many applications ranging from super lightweight aerospace parts to implanted medical devices. Welding titanium generally demands a strict environment, as gas coverage is critical. It is standard procedure for our qualified and highly trained operators to regularly ensure the appropriate calibration of all equipment found within our hermetic sealing department. Before handling parts, ESD precautions are followed per industry standard specifications and customer requirements. These are implemented as required; some titanium applications do not have stringent requirements but still benefit from welding in our glovebox environments.
When components are fully prepped, pre-welding inspections take place as needed, which may involve a face-down leak test. This test verifies the integrity of ports found on the unit, such as feed-throughs or connectors, to ensure the part is not leaking before welding. The parts are then cleaned using acetone, following proper procedure, before the covers are manually tack welded in place using a laser.
Depending on the requirements, the parts are then vacuum-baked at high temperatures to mitigate oxygen and moisture issues. While this step is time-consuming, it ensures the integrity of the finished part. Once the bake is done and any pertinent data is recorded, the parts move into the welding area of the glovebox.
Using rigid welding fixtures, the parts are squared off to ensure a true and repeatable weld of the covers to the housings and hermetically sealed with an automated laser routine. The parts are further cleaned if required and visually inspected per industry standard specifications.
Per customer requirements, different applications might need additional post-weld testing to ensure hermeticity and find parts that might require repair.
Fine Leak Testing: Our glovebox welding environments contain a standard 10% Helium that can be adjusted to a specific requirement. During the welding process, some of that helium is sealed into the part. Fine leak testing can then detect leakage of that helium by utilizing a helium mass spectrometer in a special testing environment.
Pressure Bombing: This step exposes the completed parts to helium for an extended period in a specialized test environment, which can locate larger leaks, again using a mass spectrometer.
Gross Leak Testing: Leaks that are too large to be detected using pressure bombing or fine leak testing can be subjected to gross leak testing. The parts in question are exposed to Fluorinert FC-72 under pressure. The parts are then submerged in a bath of FC-40, which is a similar liquid with a higher boiling point. The FC-40 is then heated, which causes the FC-72 to bubble out from any units with leaks, which is visible to our inspectors.
All testing is done in accordance with MIL-STD 883 Method 1014, MIL-STD 202 Method 112, and MIL-STD 750 Method 1071.6.
Finally, the parts are cleaned and packaged carefully with ESD precautions still in place to ensure parts are returned safely to our customers or sent to the next link in the supply chain.