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Jan 07, 2019

  High quality brazed joints require close fitting of the parts and the base metal is exceptionally clean and free of oxides. In most cases, it is recommended to use a joint gap of 0.03 to 0.08 mm (0.0012 to 0.0031 in) for optimum capillary action and joint strength. [2] However, in some brazing operations, it is not uncommon for the joint gap to be approximately 0.6 mm (0.024 inch). The cleanliness of the brazed surface is also important because any contamination can result in poor wetting (flow). The two main methods of cleaning a part prior to brazing are chemical cleaning and grinding or mechanical cleaning. In the case of mechanical cleaning, it is important to maintain proper surface roughness because wetting on rough surfaces is more likely to occur on smooth surfaces of the same geometry.

  Another consideration is the effect of temperature and time on the quality of the brazed joint. As the temperature of the brazing alloy increases, the alloying and wetting of the filler metal also increases. Generally, the brazing temperature selected must be higher than the melting point of the filler metal. However, there are several factors that affect the temperature choice of the joint designer. Usually choose the best temperature:

Be the lowest possible braze temperature

Minimize any heat effects on the assembly

Minimize filler metal/base metal interaction

Maximize the life of any fixtures or jigs used[2]

  In some cases, workers may choose a higher temperature to accommodate other factors in the design (eg, allowing the use of different filler metals, or controlling metallurgical effects, or adequately removing surface contamination). The effect of time on the brazed joint primarily affects the extent to which these effects exist. However, in general, most production processes are selected to minimize brazing time and associated costs. However, this is not always the case, because in some non-production settings, time and cost are secondary factors to other federated attributes (eg, strength, appearance).

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