Welding Journal - May 2009 - American Welding S...
Each month the Welding Journal delivers news of the welding and metal fabricating industry. Stay informed on the latest products, trends, technology and events via in-depth articles, full-color photos and illustrations, and timely, cost-saving advice. Also featured are articles and supplements on related activities, such as testing and inspection, maintenance and repair, design, training, personal safety, and brazing and soldering.
Welding Journal - May 2009 - American Welding S...
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Working toward a welding badge Ken J. Kinley, an assistant professor of electronics, shares knowledge amassed through decades of industry and teaching experience. Scouts assess the world through the viewfinders on their digital cameras during a photography session with Mark W. Wilson, graphic design instructor. This year's Merit Badge College patch was created by Natalie K.
Dietrick (right foreground) enjoys a retirees' get-together in the Keystone Dining Room in December 2011. David C. Dietrick served as a welding faculty member from August 1979 until his retirement in May 2009. David C. Dietrick, a longtime member of the welding faculty at Penn College (and its immediate predecessor, Williamsport Area Community College), died Jan. 23 at age 75.
Since the lung deposition of welding particles depends on particle size and morphology (density, shape), which in turn depends on the welding methods, these factors must be considered when developing protective strategies. Inhaled particles may deposit throughout the respiratory system, but are also exhaled, and different mechanisms affect particle deposition, such as impaction, sedimentation, and diffusion. Generally, particle capability to penetrate deeper into the respiratory system is inversely related to particle size [3,14]. However, the smaller the size, the more complicated become their behavior. In fact, the mechanisms and the efficiency of particle deposition in the respiratory tract depend not only on aerodynamic but also thermodynamic diameter of the inhaled particles. Nanoparticle deposition occurs with high efficiency in the entire respiratory tract due to diffusion, which makes thermodynamic diameter more relevant than aerodynamic diameter because drag forces are absent .
Percentage of total deposited aerosol of metal particles arising from stainless steel welding based on aerodynamic diameter (calculated by particle optical diameter, dynamic shape factor, and density). Adapted by Cena et al . TF: total fume; Cr: chromium; Cr VI: hexavalent chromium; Mn: manganese; Ni: Nickel. For mild steel, percentage of total deposited aerosol are for TF 12, 6.3, 17, and 38 and for Mn 7.8, 2.0, 11, and 23, respectively, in upper, tracheobronchial, alveolar, and total regions.
Cohort and case-control studies indicate an excess of risk for lung cancer both in SS and MS welders. Some studies have reported a higher risk in SS with respect to MS welders and an exposure-response relationship only in SS welders. However, studies conducted on MS welders alone showed an excess of risk also in these workers. Although tobacco smoke and asbestos are confounder factors, studies of welders not exposed to asbestos and after adjustment for cigarette smoking confirmed the association between exposure to WFs and lung cancer. Further studies are needed to understand if specific components of the welding fumes or if the welding fumes themselves are responsible for the carcinogenicity. Cohort studies and largest case-control studies related to risk of lung cancer in welders are summarized in Table 4 and Table 5.
Most pulmonary function studies in welders show a significant decrease in FEV1 in smoking welders regardless of smoking status after 20/25 years of welding activity. In addition, a clear association between occupational exposure and COPD diagnosis has been reported. COPD, which represents a leading cause of premature death and disability in developed countries, is a relevantly susceptible condition for those workers who are occupationally exposed to WFs, making avoidance of direct and indirect exposure to tobacco smoke of primary importance. Therefore, it is crucial to stop smoking and to use collective and individual protection devices properly and, if necessary, to introduce control measures to reduce WFs exposure .
Welders work with different materials under diverse conditions and are exposed to respiratory tract health hazards. The number of welders exposed to WFs is growing constantly despite process mechanization and automation. Acute respiratory injuries related to WFs are preventable with strict adherence to appropriate safety procedures. Reduction in welding exposure through engineering and correct use of personal protection will lead to a reduction in traditional welding-related respiratory diseases. However, with the development of new welding technologies, new hazards are likely to be introduced into the workplace. In particular, welding exposure is associated with small particles, e.g., NP production whose characterization and relationship with the development of human diseases is still unknown. NPs in WFs are considered a risk factor because of their specific particle characteristics, such as small size, large surface area, high particle concentrations, and complex metal composition. Studies on WFs in the area of particle research may aid the understanding of cellular and molecular mechanisms involved in welding-related lung carcinogenesis.
However, this field is far from being completed, and more investigations are required since there is an increasing considerable interest in applying laser welding to aluminum alloys particularly those used for sheet metal processing industries. In other words, laser welding of aluminum alloys, though being investigated for decades, still deserves further investigation as some problems are yet to be solved. This is due to many advantages of laser welding including low-heat input, low distortion, high welding speed, and inherent flexibility of laser system.
Besides, it is believed that cleaning of the workpiece surface prior to welding using a stainless steel brush anda nitric acid solution has resulted in increasing surface roughness and subsequently decreasing surface reflectivity that means enhancing the laser energy coupling during welding. This could be a practical solution to reduce reflectivity and then improve efficiency of the laser, which is known to be lower than that of the Nd:YAG laser for the welding of aluminum alloys.
Generally, weld cracking in aluminum alloys can occur as a result of the aluminum relative high thermal expansion, large change in volume upon solidification, and wide solidification temperature range. The investigated AA5052, AA5083, and AA6061 alloys are known to be highly susceptible to weld cracks also in other conventional welding processes . The mechanism of crack occurrence in the laser welding is considered to be similar to that of the arc welding. However, severity of weld metal cracking in the case of laser welding is much less than that in the case of arc welding due to the lower heat input with laser welding . It is believed that the solidification cracking problem in this investigation is attributed to high stress concentration at the center of weld bead due to its concavity and its high depth/width ratio as a result of unacceptable excess penetration.
It is believed that this problem could be related to two main reasons. Firstly, the high depth/width ratio due to a remarkable excess penetration (Figure 8(a)), which could increase the welding residual stresses acting on the weld bead in a perpendicular direction. Secondly, the concave shape of the weld face (Figure 8(a)) that could result in high stress concentration at the center of the weld bead, in comparison with the convex weld shape (Figure 8(b)).
The other approach was concerned with solving the solidification cracking problem in autogeneous welding using a backing strip from the same base metal. Visual examination of welded joints produced using this technique revealed smooth and uniform weld beads where undercut and excess penetration were remarkably reduced to an acceptable limit that in turn eliminated solidification cracking. It should be reported that the same parameters were used for welds produced using either a backing strip or a filler wire. In other words, the welding speed or the productivity was not decreased with the addition of the filler fire.
A cross section taken from laser welded joint of AA5083 alloy produced using the optimum welding parameters and a backing strip is shown in Figure 10(a). It is obvious that using a backing strip resulted in remarkable improvement in weld bead shape, where it became convex, the recommended and acceptable shape concerning hot cracking. Besides, the root penetration was limited to the required and acceptable limit. Another important finding is the absence of weld metal solidification cracking in this case. This has been confirmed from optical microscopic examination of weld metal at higher magnification as shown in Figure 10(b). This is attributed to the decrease in stress concentration at the center of weld bead due to improving the weld profile, that is, obtaining a convex bead and decreasing penetration depth/width ratio. It should be reported that similar results regarding the effect of the backing strip on the weld profile and then on the weld solidification cracking were obtained for both AA5052 and AA6061 alloys. It can be deduced that using a backing strip could be a new and relatively inexpensive practical solution for preventing the solidification cracking of aluminum alloys welded joints. Other researchers have eliminated the solidification cracking problem in aluminum alloys using temporal pulse shaping . 041b061a72