How to Weld Kovar Alloy Effectively

Welding Kovar alloy requires extreme precision. This material is vital for hermetic seals. Our guide covers how to weld Kovar alloy effectively for high-integrity joints.

We will explore key challenges, preparation steps, and the best techniques. You will also learn about filler materials, post-weld treatments, and common defect avoidance.

What is Kovar Alloy?

Kovar is a specific iron-nickel-cobalt alloy. It is formally known as ASTM F-15.

It contains roughly 29% nickel and 17% cobalt. The rest is primarily iron.

Kovar's most critical feature is its thermal expansion. Its coefficient of thermal expansion (CTE) is very low. It matches the CTE of borosilicate (Pyrex) glasses.

This unique property allows Kovar to form strong, hermetic (airtight) seals. You find these glass-to-metal seals in electronics, vacuum tubes, and medical devices.

Key Challenges in Welding Kovar

Welding Kovar is not like welding steel. It presents unique difficulties.

The alloy is extremely sensitive to contamination. Sulfur, lead, and phosphorus can cause severe weld cracking. These elements lead to embrittlement.

Kovar also oxidizes readily when heated. This oxide layer must be managed.

The material has low thermal conductivity. Heat concentrates at the weld. This can cause distortion or alter the material's properties.

Finally, most Kovar applications demand a perfect hermetic seal. There is zero tolerance for leaks.

Essential Pre-Weld Preparations

Your welding success depends almost entirely on preparation. You cannot skip these steps. A clean, dedicated environment is mandatory.

Kovar Surface Cleaning Protocols

Cleaning is the most critical step. Your Kovar parts must be metallurgically clean.

1. Degrease: Start with a solvent degreaser like acetone. This removes all oils, greases, and fingerprints.
2. Etch: Next, use a chemical etch or bright dip. This removes the surface oxides.
3. Rinse: Rinse the parts thoroughly with deionized water.
4. Dry: Dry the parts immediately. Use a clean, lint-free method.

After cleaning, handle parts only with clean, powder-free gloves.

Preheating and Annealing Needs

Kovar is typically annealed before welding. This step relieves internal stresses from prior forming or machining. Annealing is done in a controlled atmosphere furnace to prevent oxidation.

Preheating is generally not recommended for Kovar.

Preheating can increase the risk of oxidation. It also widens the heat-affected zone (HAZ). Your focus should be on a low, controlled heat input during the weld itself.

Best Kovar Welding Techniques

Only high-precision, low-heat-input processes are suitable for Kovar. These methods protect the material's unique properties.

TIG (GTAW) Welding Kovar

Tungsten Inert Gas (TIG) welding is very common. It offers excellent operator control.

• Use a direct current, electrode negative (DCEN) setup.
• A sharp, 2% lanthanated or thoriated tungsten is best.
• Use 100% pure Argon as the shielding gas.
• Use low amperage and move quickly. This minimizes total heat input.
• Autogenous welds (no filler) are common for thin parts.

Laser Beam Welding (LBW) Kovar

Laser welding is ideal for Kovar. It provides extreme precision.

The heat-affected zone (HAZ) is incredibly small. This minimizes thermal distortion. It also protects the material's low-CTE properties.

LBW creates deep, narrow welds. This is perfect for high-reliability hermetic seals. It is a high-speed process well-suited for automation.

Resistance Welding Applications

Resistance spot welding is another option for joining Kovar.

It is typically used to join overlapping sheets. This process is fast and creates no fumes.

Precise control of current, time, and electrode pressure is essential. The electrodes must be kept clean and properly dressed for consistent results.

Choosing Kovar Filler Materials

Many Kovar joints are autogenous. This means they are fused without filler material.

When a filler is needed, you must match the base metal.

A Kovar-specific filler rod (ASTM F-15) is the first choice. This maintains the CTE properties across the entire joint.

In some cases, 308L stainless steel filler is used. This is only if a CTE match is not critical. Always consult your engineering specifications.

Critical Post-Weld Treatments

Your work is not finished after the weld cools. Post-weld steps are vital for performance and long-term reliability.

Post-Weld Stress Relieving

Welding introduces stress into the material. This stress can cause delayed cracking.

A post-weld stress relief, or anneal, is often necessary. This is done in a vacuum or hydrogen atmosphere furnace. The process restores ductility and stabilizes the material.

Inspection for Weld Integrity

Kovar welds demand 100% inspection.

1. Visual Inspection (VT): Check for cracks, undercut, or porosity under magnification.
2. Helium Leak Test: This is the standard for hermetic seals. It can detect microscopic leaks.
3. Dye Penetrant (PT): This test can reveal surface-breaking cracks.

5 Common Kovar Welding Defects

1. Hot Cracking: Occurs during cooling. Usually caused by contamination (sulfur, phosphorus).
2. Porosity: Gas bubbles trapped in the weld. Caused by a dirty surface or poor gas shielding.
3. Oxide Inclusions: Oxides trapped in the weld pool. Results from poor cleaning or shielding.
4. Lack of Fusion: The weld fails to join the base metal. Caused by low heat input or moving too fast.
5. Embrittlement: The weld becomes brittle. Caused by contamination or improper thermal cycles.

Industries Using Welded Kovar

Kovar's unique properties serve critical industries.

• Electronics: Hermetic packages for transistors, diodes, and microcircuits.
• Aerospace & Defense: Satellite components, avionics, and guidance systems.
• Telecommunications: Microwave tubes and high-frequency power feedthroughs.
• Medical: X-ray tubes, pacemakers, and implantable medical devices.
• Scientific: Vacuum chambers, sensors, and optical instruments.

Standard Machining’s Kovar Expertise

Welding Kovar is complex. Machining it beforehand is also a challenge.

Kovar is abrasive and builds up heat during cutting. This requires specialized tools and techniques to hold tight tolerances.

Standard Machining has deep expertise in Kovar. We provide precision CNC machining for Kovar components. Our processes manage heat and ensure dimensional accuracy.

We deliver parts ready for your critical sealing and welding operations.

Conclusion

Welding Kovar is a precise science. Success demands meticulous cleaning, controlled heat, and proper post-weld care. By following these steps, you can achieve the high-integrity, hermetic seals Kovar is known for.

For expert machining of your Kovar components before welding, trust Standard Machining. Contact our team today.

Kovar Welding FAQs

What Filler Rod is Used for Kovar?

The best choice is a filler rod made from Kovar (ASTM F-15) itself. This ensures the weld metal has the same low coefficient of thermal expansion (CTE) as the base parts. If a CTE match is not the primary concern, certain 300-series stainless fillers (like 308L) or nickel-based fillers are sometimes used, but this is application-specific.

Does Kovar Require Preheating Before Welding?

No, preheating is generally not recommended for Kovar. The material has low thermal conductivity. Preheating can increase the heat-affected zone (HAZ) and promote oxidation, which compromises weld quality. The focus should be on minimizing total heat input, not adding more.

Can You Weld Kovar to Stainless Steel?

Yes, Kovar can be welded to stainless steel (like 304L or 316L). This is a common requirement for creating transitions from a hermetic seal to a structural component. Laser welding is often preferred for this. A filler material (like 308L or a nickel-based alloy) is often used to accommodate the different expansion rates and prevent cracking.