A Perspective on Welding Science
- PDF / 6,230,232 Bytes
- 8 Pages / 604.8 x 806.4 pts Page_size
- 96 Downloads / 227 Views
32
Development of numerical welding models, using both finite element and finite difference methods, has been the key to developing a base of understanding heat and mass transfer during fusion welding processes. This is because, to the extent that they include the correct physics and physical properties, they can be accepted to be quantitatively accurate. Compared to analytical solutions, the numerical models permit incorporating temperature-dependent physical properties, the latent heat of phase transformations, both spatial and temporal distribution of the heat sources, and real boundary conditions. These factors are essential to a model's quantitative accuracy. Development of numerical fusion welding models often starts with a conduction-mode solution. 58 That is, heat transfer within the metal is considered to occur by conduction only, although radiative and convective heat losses may be included.6"8 Comparison of experimental results with conductionmode models can be very useful in identifying important missing physics, as will be illustrated.9 Convection within the weld pool has become recognized as one of the most important elements in fusion welding. Heat transfer by convection often determines weld geometry;10"14 surface ripples result from convection;15"16 and compositional mixing results from convection.17 An illustration of convection in a weld pool is given in Figure 1, which is a metallographic cross section of a singlepulse Nd:YAG laser weld of aluminum alloy 5456. Evaporation of magnesium and manganese from the weld has resulted in composition inhomogeneity, which is revealed by etching. Frozen convection patterns are clearly seen near the edge of the weld pool. On the right side, nearer the center of the weld, a weaker counterflow can also be seen. It is interesting to note that, although
considerable mass transfer has occurred, the laser pulse used was only 5 ms long.9 Weld pool convection is driven by electromagnetic, surface tension, and buoyancy forces. Much of the recent research on weld pool convection has concerned the role of surface-tensiondriven fluid flow, termed Marangoni convection. Heiple and Roper13 used cine photography to follow surface fluid flows in gas tungsten arc welds of stainless steels. They observed that very small additions of surface active elements such as S, O, and Se (all Group. 6A) dramatically alter surface flow as well as the weld pool geometry. Addition of these elements resulted in radially inward surface flow accompanied by depression of the central region of the weld, whereas surface flow was radially outward for welds with low levels of surface active elements. Because the surface tension temperature coefficient (8y/8T) of liquid iron alloys changes from negative to positive over significant temperature ranges when surface active elements are added, they concluded Marangoni convection is a dominant mass and heat transfer mechanism in the weld pool. This appears to be an important observation because convective flow that is radially inward at the surface enha
Data Loading...
