Page 3 - HIWT Winter 2012 World of Welding
P. 3                                       HOBART INSTITUTE OF WELDING TECHNOLOGY

            Weldability of Metals Creates KnoWledge


            By André A. Odermatt                               by drawing heat away from the device and dissipating the heat
                                                               into    the    surrounding   air   either    through    fan    cooling or
                                                               convective heat transfer.) Aluminum conducts heat six times
                                                               faster than steel. This is another reason why welding aluminum
            Many years ago, earlier  in                        can be more problematic than welding steel. This difference in
            my  career, a  request  was                        thermal conductivity requires higher heat input.  As aluminum
            made   to  design  lighter                         is heated to its melting point of about 1,200 F, there is no color
            welding machines; one person                       change because it is highly reflective to visible light and heat.
            suggested using aluminum                           This makes it difficult to judge when aluminum is approaching
            instead of steel for the                           its melting point.
            transformer and reactor cores.
            Materials that are not affected                    Many industries are using stainless steel.  As  with other
            by   magnetic   fields   are                       materials,  there are several categories of stainless steels
            considered   non-magnetic.                         with  different  characteristics.  As  an  example,  the  thermal
            Aluminum is non-magnetic; steel on the other hand is affected  conductivity of stainless steel is about 1/3 the rate of plain
            by magnetic fields, a characteristic required in transforming  carbon  steel  and about  seventeen times  slower than  that  of
            electrical power through induction.  For welding, this is not  aluminum. To understand controlling heat input and interpass
            the  most  important  difference  in  the  properties  of  the  two  temperatures is very important when welding all materials to
            metals but it is one of many important differences between  avoid costly repairs or rejects.
            steel and aluminum. Both materials are now more widely used
            than ever and there is a good possibility that a welder will need
            to weld on both of these materials during his or her career.
            Welding of aluminum is different than welding of steel and
            it is important for the welder to understand the “weldability”
            of both.

            Just two examples to illustrate the difference in weldability
            between  the  two metals,  ferrous and nonferrous, are  the
            corrosion resistance and the thermal conductivity.  We know
            that aluminum, as opposed to steel, is more corrosion resistant,
            resisting weathering through the thin layer of aluminum oxide
            on the surface. This layer is formed in a matter of Pico seconds
            after  exposure  of  aluminum  metal  to  the  atmosphere,  since
            aluminum metal is very reactive with oxygen. (A picosecond
            is 10  of a second. That is one trillionth -- or one millionth
            of one millionth of a second, or 0.000 000 000 001 seconds.  teMperature  ManageMent  in   Welding
            A pico second is to one second as one second is to 31,700  Means Money gained or Money lost
            years)  Aluminum oxide is an electrical insulator, has a much
            higher melting point than the aluminum itself and hence, in  To learn about properties and uses of ferrous and nonferrous
            welding aluminum, it is required that the welding arc breaks  metals  gives  the  Hobart  Institute  diploma  student  a  unique
            through this oxide film, and cleans the oxide.  Cleaning of  body  of  knowledge  for  their  future  careers.  Decades  ago,
            the surface immediately before welding helps. This is one of  HIWT created  its  Weldability  of  Ferrous and Nonferrous
            several important differences between welding aluminum and  Metals Course and made it part of the diploma programs in
            welding steel.                                     response to the ever-increasing requirements of industry and
                                                               to make HIWT students even more employable.  Most of our
            Another  important  difference  is  that  aluminum  has  a  high  students appreciate this course and see the value in it for their
            thermal conductivity and this characteristic is often used in  future careers.
            welding  equipment  and  other  electronic  systems  as  a  heat
            sink.  (A  heat  sink is a passive component that cools a device     (Continued on page 5)

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