Spring 2008 (contents):
1. Definition of a Good Weld
2. Fiber Lasers
3. Weld Cracking in Liberty Ships
____________________________________________Definition of a Good Weld
One of the most frequently asked questions in welding is how does one define a good weld. The plain and simple answer is a weld is good if it satisfies all the requirements, both short and long term. Welding standards and codes are available but are typically applicable to select few applications such as building structures and pressure vessels. For majority of manufacturing applications which use unusual weld geometry or material combinations, the manufacturer (or the customer) has to establish weld specifications based on intended use. Specifications can include non-destructive testing such as x-ray analysis or eddy current testing. Destructive tests can include peel test, pull test, pressure test or any other suitable option. For joints that are visible to the end user, the bond also may have to be aesthetically pleasing. In some applications, requirements in actual usage may not be stringent, but the bond has to be strong enough for handling during follow-on manufacturing operations. Keep in mind that a weld that looks pretty may be not be good and a weld that looks ugly might be perfectly acceptable.
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Fiber Lasers - The New Frontier
As they say, overnight success takes about 15 years. And that is absolutely true for the "new" fiber lasers now being used in materials processing. Fiber lasers produce laser energy in the fiber itself with the photons being multiplied as they rush along the length of the fiber. For the last three decades, CO2 and Nd:YAG have been the workhorses of the laser welding industry with high power CW (continuous wave) applications skewed towards CO2 lasers and pulsed applications gravitating towards YAG lasers. However, neither of these power supplies could satisfy all the needs of manufacturing. Fiber lasers seem to have hit the mark with improvements in all categories including cost, efficiency, maintenance, power levels, and beam quality. Fiber lasers have wall plug efficiency of 30% (compared to 5% and 15% for YAG and CO2) with a much smaller footprint and significantly reduced cooling requirements; Fiber lasers are pumped with laser diodes that are tuned for efficient absorption. Maintenance is minimal since fiber lasers do not have many of the component associated with a typical laser resonator.
Fiber laser designs are scalable and can produce from few watts to multiple kW of power without any significant degradation in beam quality. High beam quality makes it possible for a single laser to be used for cutting and welding; high beam quality also makes it possible to have a long working distance (focal length) of the order of 10" or more. High power density (energy per unit area) also makes them capable of welding reflective materials such as Aluminum. They can be used in pulsed as well as CW mode and produce laser light at 1.064 micron; Fiber lasers are also available for marking applications.
The fact that practically all laser manufacturers are now offering fiber lasers is testimony to their potential and it is likely that fiber lasers will carve a wide niche and perhaps open up new opportunities. Fiber lasers have already shown potential to encroach into the territory of Electron Beam welding and could compete with plasma and waterjet for cutting applications. For fiber lasers, the future is indeed very bright.
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Weld Cracks in Liberty Ships during World War II
At the beginning of WWII, approximately 5000 ships of Liberty and Victory designs, were built to meet the needs of Allied Forces for transport ships. This was the first large scale use of welding technology for rapid construction of ship hulls and other structures. Of these ships about 20 percent developed cracks in the hulls and main decks in or near welds in steel plate. Cracks occurred at sea and sometimes even in ships standing in port; failures were often catastrophic and sudden. After thorough investigation, weld failures were attributed to three principle causes:
- Steel used had low toughness, especially in low temperatures.
- Hull and deck construction techniques had not been refined to avoid points of severe stress concentration.
- Questionable workmanship procedures followed under intense production pressures.
Source: Welding Metallurgy, Volume 1, George E. Linnert, AWS, Miami, Fl.
(It is interesting to note that even 70 years after the Liberty ship failures, most production weld failures on a shop-floor can be attributed to the same three principle causes: material selection, part design, and process control.)