Designing Weldments. Группа авторов

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СКАЧАТЬ the following paragraphs we discuss some of the physical properties of metals that have significant impact on the welding and weldments.

       2.1.6.1 Thermal Conductivity

      Thermal conductivity is one of the physical properties of the metal. For a welding engineer this is an important property to know and understand about the metal they intend to use for welding. Metals are a good heat conductors and they transmit (welding) heat away from the heat source, some metals conduct heat much faster than the other; for example, Aluminum and Copper conduct heat much faster than steel. This difference in thermal conductivity explains the varying degree of weldability and use of low heat welding process for Aluminum and Copper and their alloys.

       2.1.6.2 Coefficient of Thermal Expansion

      Metals when heated, expand in volume. The coefficient of thermal expansion is the liner measure of this expansion. It give a unit change in the liner dimension per unit increase in temperature. Note that the term used in the description above is change, because coefficient of thermal expansion also gives the change in dimension as contraction as the metal cools. But the expansion and contraction with the change in the temperature is not just liner, it is volumetric. Engineering data and table only give the Liner values as coefficient of thermal expansion.

      The volumetric expansion is of interest to welding engineers, and fabricators, because the expansion and subsequent cooling causes shrinkages and distortions in the structure. A welding engineer is concerned about these changes in dimension and provides solutions to avoid, or eliminate the effect of thermals effect on the dimension of the weldment.

       2.1.6.3 Melting Point

Welding is an activity significantly aligned with melting of metals by application of heat. Elemental metals have different melting points, and alloy metals have different range of melting temperature. Higher the melting temperature larger amount of heat is required to melt the metal being welded. This requires proper selection of welding process by the welding engineer.

      If however joining is planned by Brazing or soldering then too, selection of heat source assumes importance because the heat source should be able to melt the brazing rod to wet both joining surfaces.

      For welding two different metals that have significantly differing melting points or ranges becomes a challenge to a welding engineer, a well-considered heat source is necessary and welding procedure should be developed to regulate application and control of welding heat.

      2.1.7 Electrical Conductivity

      Electrical conductivity is an important property to consider if electric resistance welding is the process used for joining two metals. Metals are relatively good electrical conductors, however as the temperature is raised the electron flow is reduced, making them relatively less conductors. The cold work and alloying elements in the metal also affect metal’s conductivity.

      2.1.8 Corrosion Properties

      Corrosion is an electrochemical reaction by which the metal deteriorates to its natural state. The deterioration is initiated and affected by the environment the metal is placed, or the environment around it.

      Different metals have different resistance to corrosion, this resistance is further affected by the environment. The selection of specific metal to serve in specific environment requires that the weld also meets those corrosion resistant properties. Careful consideration is required in selection of welding electrode/welding wire, and impact of heat on the weld and HAZ of the base metal being welded.

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