APRIL 2015 / WELDING JOURNAL 59 Because of its close connection to construction projects, many view welding as a primitive and dangerous art. At any sizable construction site, sparks fly, fumes spread their musty aroma, and large skull and crossbones warning posters speak volumes about welding’s hazards. However, the reality is very different from the perception. The welding industry has an excellent safety record, robots now perform many of the repetitious and difficult tasks, and a mature and sophisticated scientific knowledge base supports welding practices. Most exciting, since the 1970s, the expanding digital data processing capabilities have been combined with the well-established technological knowledge base of welding, totally transforming both its practice and the underlying analytical capability that supports it. Analytical capability is important for problem solving because problems in welding often affect life and property. This article focuses on uncovering a long-standing mystery in welding that remained elusive until the tools of the digital age were used. In a larger context, it shows how the renaissance brought about by the fusion of mature and new technologies has taught engineers powerful lessons while providing significant benefits to all people. A Welding Primer As you likely know, the purpose of welding is to combine two parts — metallic materials in most cases — into a strong joint. Several common terms used in describing different regions of the weld are shown in Fig. 2 (Ref. 2). In fusion welding processes, the joint forms by the melting and solidification of the metal parts. The region under the heat source melts forming a liquid metal puddle called the weld pool. A small solid region next to the weld pool, where the structure and properties of the workpiece are changed by heat, is called the heataffected zone (HAZ). The size and shape, or geometry, of the weld pool is affected by how much heat is absorbed and distributed within it. Temperatures vary within the weld pool, and heat flows by conduction from high to low temperatures and by convection from the motion of the hot liquid metal. The molten metal within the weld pool circulates under the action of several forces. The most important, Marangoni force, is named after Italian scientist Carlos Marangoni, who showed that liquids move from regions of low to high surface tension. The nature of this force can be easily understood from the motion of pepper in water — Fig. 3. When a cotton swab dipped in household soap is immersed in water, the pepper moves away from the swab (Ref. 3) (Fig. 3, right). Adding soap to water reduces its surface tension locally. Water flows away from the low surface tension region to where the surface tension is relatively high (Ref. 3). A similar effect causes weld metal to flow within the weld pool. Surface tension of the weld metal depends on temperature. So its value just under the heat source is different from that in other regions and this difference drives the flow of weld metal. The gravitational force tends to sink the colder, heavier liquid near the edge of the weld pool and raise the hotter, lighter liquid metal in the middle of the weld pool. In addition, during arc welding, an electromagnetic force is generated from the interaction between the current path in the weld pool and the magnetic field it generates. Of these three forces, the gravitational force is by far the weakest, and during arc welding, the electromagnetic force is comparable to the Marangoni force only at fairly high currents. In most cases, the Marangoni force provides the main driving force for the flow of weld metal within the weld pool. The rolling streams of weld met- Fig. 2 — A schematic of the fusion welding process is shown at left and a transverse cross section, perpendicular to the direction of welding, is shown at right (Ref. 2). Fig. 3 — The photo on the left shows pepper on the water’s surface and a cotton swab that was dipped in soap. The photo at right shows the pepper moves away from the center after the swab is dipped into the water (Ref. 3). Fig. 4 — Progression of computational hardware from mechanical to digital devices.
Welding Journal | April 2015
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