Mastering Furnace Brazing: Brazing Consideration and Lapping

Brazing, a crucial process in metallurgy, demands meticulous considerations to ensure the integrity of the joints formed. One of the key factors is the Coefficient of Thermal Expansion (CTE) alignment between the materials to be joined. This alignment is critical during the cooling cycle, as any mismatch in CTE can lead to stress-induced cracks when the braze solidifies. The finish of the joint components also plays a pivotal role. For instance, the thickness of nickel plating can influence braze flow, grainy appearance, and even determine whether a joint will blister after the brazing strip and subsequent processes.

In cases where components with different CTEs need to be joined, special brazing alloys like Incusil are employed. Indium, known for its low compression strength, is used as a shock absorber in such scenarios. However, it's important to note that Indium brazed joints may exhibit voids due to the material's volume shrinkage upon cooling. Additionally, aluminum impurities in brazing can adversely affect adhesion and must be meticulously avoided.

When dealing with parts requiring tight alignment tolerances, a binder or glue is often used to hold leads in position. However, excessive use of glue can lead to the formation of voids in the joints.

Lead configuration is another critical factor in achieving optimal adhesion. T leads and leads with holes are preferred as they increase the surface area, facilitating better braze flow and adherence.

Lapping: Ensuring Flatness and Surface Quality

Due to the thermal mismatch between brazed materials, especially in flanges, warping is a common challenge. To address this, flatness is measured. Strict specifications dictate that flatness should not exceed a very thin amount, requiring some products to undergo lapping to ensure conformance to these specifications.

During the lapping process, parts are placed flange side down on a lapping wheel, and pressure is applied via a top plate. The flange surface, which is not flat, is in contact with the rotating wheel. A lubricating liquid, composed of a lapping medium (typically alumina spheres) suspended in an oil or water-based carrier, is used to achieve a smooth finish. Larger diameter medium can lead to surface pitting.

Lapping Issues in Composite Flanges

Two major types of flanges are commonly used: bulk metal (such as Copper and CMC) and composite (such as Cu/W and Cu/Mo). Lapping composite flanges poses unique challenges. These materials, often powder metallurgy products, contain hard refractory particles and porosity. During lapping, the media can become entrapped in the pores, leading to blister formation upon plating and sintering. A degreasing operation after lapping is crucial to ensure a clean microstructure of the lapped surface.

In contrast, lapping bulk metal flanges is relatively straightforward, as these materials are typically dense and do not exhibit porosity issues. Careful consideration of these factors ensures that the brazing process results in joints of the highest quality and reliability.

HSA Atmosphere Furnace

• 1150°C Maximum Temperature Rating
• Air/Nitrogen/Hydrogen Capable
• < 2PPM Gas Tight muffle
• ±1°C Dew point Control
• ±1.5°C Cross Belt Uniformity

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