Schlagwortarchiv für: Filler Metal

Filler Metal Management 2

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Case Study

A radiator core retrieved from service was examined for a suspected premature corrosion related failure.
Upon closer metallographic examination, no evidence of corrosion was found at the failed area.

33% tube core erosion in the failure area


Header: AA4343/ AA3005

Tube: AA4343/ AA3003
 
 
 
 
 
 
 
 
It was concluded that the cause of the failure was in fact a mechanical failure occuring in the thinned wall area.

The following sequence of events proposes a rational explanation for the eroded tube area:

In service radiators are subject to internal pressure fluctuations and expansion and contraction due to heating and cooling. Mechanical failure was imminent and occured in the weakest part of the tube, the thinned down tube wall area adjacent to the tube to header joint.

Conclusions

Erosion of the base metal is undesirable since it reduces the wall thickness of the brazed component.
In addition Si penetration in the grain boundaries is known to increase the susceptibility to intergranular corrosion. Therefore proper filler metal management practices should be observed to prevent undesirable effects. One such factor easily controlled by the brazer is maximum peak brazing temperature.

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Filler Metal Management 1

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Experimental

The effect of temperature on filler metal erosion was studied using an automotive radiator core.

610 ° for 2 minutes

610 °C for 2 minutes - no thinning of the tube core

625 °C for 2 minutes

625 °C for 2 minutes - significant erosion of the tube core

In this case, joining progresses initially as expected. The cladding layer on the tube melts and flows by capillary action to the fin to tube joint and a normal fillet forms. However, as the peak brazing temperature is allowed to rise beyond the recommended maximum (605 °C) the following occurs:

  • The fluidity of the filler metal at the tube to header joint is increased and some of the liquid filler metal is released and flows to the nearest tube to fin joints.
  • Excess filler metal at the tube to fin joints accelerates dissolution of the tube core adjacent to the fin, eroding the tubewall thickness.
  • The excess filler metal pool is then drawn by capillary action in between the fins, particularly where the finspacing is narrow. The fins are drawn together by the strong capillary forces, displacing the fin from its original fin to tube position.
  • As the fins move together, drawing the filler metal pool from its original position, the denuded area is significantly reduced in cross sectional thickness.
Catastrophic Failures

In some instances the extent of filler metal erosion is so severe that the entire thickness of the tube is consumed resulting in catastrophic failures.

More about this topic in our next issue.

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Sil Flux Brazing

NOCOLOK® Sil Flux brazing is a technique, which eliminates the need for clad brazing sheet or conventional Al-Si filler metal. Sil flux brazing uses filler metal generated in-situ to effect brazing. The mechanism for creating this filler metal in-situ is described below:

  1. One of the surfaces to be joined is coated with a mixture of NOCOLOK® Flux and metallic Si powder. The coated assembly is then heated in the same fashion as in conventional furnace or flame brazing techniques.
  2. As the temperature rises, the flux melts at 565°C, dissolving the oxides on both the Al substrates and the Si particles.
  3. The bare Al surface is now in contact with metallic Si, and in the absence of oxides, allows solid-state inter-diffusion of Al and Si. Very quickly the composition near a Si particle reaches that of the Al-Si eutectic (Al-12.6% Si).
  4. As the temperature increases beyond the eutectic reaction temperature of 577°C, the formation of a liquid pool is established. The formation of the liquid leads to rapid dissolution of the remaining Si through liquid diffusion. The pool of liquid continues to grow, consuming Al, until all of the Si is consumed in the melt. In the presence of a joint, the liquid pool is drawn to the joint by capillary action.
  5. Upon cooling, the liquid layer solidifies to form a metallurgical bond between the components.

Sil Flux Process

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