Thermal expansion is the material property of a volume change in response to changes in temperature. The net volume growth over a given temperature is dependant on the initial volume of the component. So, for most common braze materials (copper, stainless steel, ceramics, etc), this expansion varies from four to twenty thousandths of an inch of growth per inch of material in a given direction. With this in mind, brazed assemblies with different materials must be designed and toleranced such that the joint has the proper clearance at braze temperature taking into account the thermal expansion of each individual material.
To accomplish this, braze joint fit-up at room temperature may need to be made with “excessive” clearance or even interference such that at braze temperature, proper joint clearance exists to allow capillary action to work.
When dealing with materials with vastly different thermal expansion coefficients (CTE), brazing copper or stainless steel to ceramic for instance, fixturing becomes essential in creating the correct joint clearance. Fixtures work by using thermal expansion relative to one part in order to “re-size” (plastically deform) it to create the proper clearance between its mating part at temperature. What usually determines the type of fixturing needed (restraint vs. pusher) is the component that is the easiest to deform at braze temperature and its position relative to the stronger component. A combination of yield strength and geometry will determine ease of deformation. Fixture material must have higher yield strength at temperature than the material that is being deformed. Therefore, molybdenum and other refractory metals are commonly used for fixturing due to their high strength at high temperature properties.
After brazing is complete and cooling is occurring, stress will be induced into assemblies from differential shrinkage. The greater the CTE difference, the greater the stresses will be in the final assembly. Be mindful of joint location relative to geometry as joints can see multiple shear, tension, and/or compressive forces acting towards stress concentrations. When dealing with ceramics, it is especially important to design joints as to not create tension and shear stresses since cracking the ceramic or de-lamination of metallization can occur with minimal force. Additionally, furnace process controls become increasingly important since the temperature ramp rate is ultimately what controls the instantaneous differential expansion between components that heat differently.
On the surface, brazing may seem like a simple process. Although some aspects are fundamental, the reality is that as materials and configurations become more complex, so does the underlying process. This is where Altair’s expertise in joint design, experience with fixturing, and precise process controls make the difference between a successful braze and scrap metal.
We have years of experience bonding dissimilar materials with varying coefficients of thermal expansion. Our next blog entry will detail a sample CTE calculation depicting the approach of determining the nominal room-temperature dimensions to achieve an ideal braze gap distance at braze temperature. Stay tuned.