Summer 2009 (contents):

1. Cross Sections - The Soul of Weld Analysis

2. Ultrasonic Welding - The Good Vibrations

3. Chromium - The "Stainless" in stainless steel

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Cross Sections: Soul of Weld Analysis

 

Analyzing welds is important in trying to predict their behavior in use.  Analysis can include mechanical testing, corrosion resistance, fatigue life, and other requirements such as visual appearance.  Such testing evaluates performance based on functionality of the weld but does not provide enough information to understand the fundamental nature of the weld that produces such performance.  Additionally, there could be hidden problems with the weld that could lead to failure over a period of time.  Cracks in the weld are perhaps of greatest concern.  Other factors such as increase in hardness of the weld or the HAZ (Heat Affected Zone) can also give clues to longevity of the weld.  The only way to get a deeper understanding of the weld and to gain appreciation for long-term performance is to analyze weld cross-sections.  Cross sections can reveal nature of the bond (solid-state, fusion, or solder/braze), phase separations, solidification behavior, and weld defects including porosities and cracks.  Without such information and its analysis, it is impossible to gain a complete understanding of the weld.
 
Cross-sectioning starts with good equipment that makes sure the weld itself is not damaged during the sectioning process.  A thin diamond saw is often the best tool unless your samples are big chunks of steel.  The samples should be mounted and polished flat down to a mirror finish such that no scratches are evident when observed at 400X magnification.  Welds should be first evaluated with an optical microscope without etching; that is important especially when evaluating dissimilar metal bonds where it is difficult to etch all metals equally.  Samples are best observed by varying the light intensity so as to bring out the best contrast.  A 400X optical microscope with a good high resolution digital camera is an essential tool for analysis.  Unetched samples are also well suited to identify fine cracks that could otherwise be confused with etched grain boundaries or segregated second phase.  Etching brings out other aspects of the weld such as phase separation, grain growth, and size of the HAZ (Heat Affected Zone).  If presence of second phase happens to be in the vicinity of a weld defect such as a crack, the sample can be further analyzed with a SEM (Scanning Electron Microscope) for an elemental analysis.  An SEM is also a good tool to observe the weld at very high magnifications and in back-scattered mode to identify finely distributed second phase particles.
 
Results of other analysis techniques have to be consistent with those from the weld section analysis to gain a complete understanding of the weld.  Any weld analysis should be considered only partially complete till you get a chance to peer into the soul of the weld.

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Ultrasonic Welding - The Good Vibrations
 
Ultrasonic welding (UW) is a solid-state welding process where ultrasonic energy is used to soften the parts to be welded as they are squeezed together to form a solid-state bond.  When thought of in this context, it is quite similar to resistance welding where parts are heated to make them soft for solid-state bonding.  Contrary to popular belief, the bond is not created by friction between the two parts to be welded.  The popularity of the notion that weld heat is generated by friction at the weld interface probably comes from convenience of explaining the process and possibly due to convoluted explanations about the process found in established texts and then disseminated by equipment manufacturers.  During ultrasonic welding, ultrasonic energy shakes up the bonds between atoms thus temporarily reducing the bond strength and softening the parts to be welded.  The softened parts are pushed together by the welding force applied on the parts by the horn (vibrating tool) and supported by the anvil (stationary base).  The parts do get warm but that is due to internal friction as the atoms slip and slide past each other and not due to friction at the weld interface.
 
UW works well for soft metals such as copper, aluminum, and gold which can be easily gripped by the knurl patterns on the horn and anvil or held securely in a formed horn as in aluminum wire welding or gold ball bonding.  UW is also widely used for welding thermoplastics.  Factors such as surface cleanliness and a secure anvil are very important.  UW frequency is fixed for a given machine and typically ranges from 15 to 40 kHz.  Power delivered is set to a safe limit to ensure that the vibrating stack is not damaged.  Welding parameters include vibration amplitude, welding force, weld time, and weld energy.  Either time or energy can be used as a control parameter while the other is measured as the output.  With the right choice of design, materials, and process parameters, it can be quite a robust process.

 

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Chromium - The "Stainless" in Stainless Steel
 
Chromium has greater affinity for oxygen than iron and is added to steels to form a protective oxide layer on the surface that prevents the iron from rusting (oxidizing).  A drop of water allowed to dry on stainless steel surface will not "stain" the surface due to oxidation and hence the name - stainless steel.  At levels above 10.5 wt%, steel is considered stainless under ambient conditions; higher levels may be required for protection under more aggressive conditions.  Chromium is a ferrite stabilizer and at levels above 12 wt%, the steel will be fully ferritic in the absence of austenite stabilizers such as Nickel.
 
Unfortunately, Chromium also has strong affinity for carbon and forms complex carbides of the type M23C6 where M is predominantly Cr but could also include Fe or Mo.  During welding of stainless steels, Cr reacts and bonds with C and then is no longer available to form the protective oxide layer.  These carbides are more likely to form along grain boundaries in the HAZ (Heat Affected Zone).  If the local Cr content drops below threshold, the steel is no longer stainless and is susceptible to corrosion along the Cr depleted grain boundaries.  An option is to use stainless steels with reduced C content such as 304L.

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