Description of electric resistance welding and its place and importance in the industry
Problems experienced in welding of thin-section metal materials used in many fields such as automotive industry, aerospace and aircraft technology, steel constructions, steel products manufacturing, production of precision instruments, electro technic, and machine industry led development of different types of welding. When thin section materials remain under high temperature they cause permanent deformity. Therefore, welding process must be performed under minimum temperature and as quick as possible. Performing welded joints quickly with minimum deformation, where economic and weld resistance are required to be high, electric resistance welding becomes the first choice.
Electric resistance welding is connection made with the help of heat formed of resistance of parts against electrical current passing through the metal pieces. Parts are partially melted and welding environment necessary for welding is created. After the formation of welding environment electric current is stopped and pressure is applied to work pieces and thus a combination of the kind that cannot be dismantled is created through cooling under this pressure. The common name for the welding process performed by this method is referred to as electric resistance welding.
In our company PUKA Makine we produce resistance welding machines and gas metal welding machines, arc welding machines. As types of resistance welding machines we produce butt welding, multi-piston group spot welding, mechanical footed spot welding, servo or PLC controlled spot welding machines.
Electric Resistance Welding Types
Electric resistance welding is classified as follows according to the way of welding and maximum area of use in the sectors.
1 Spot (spot) welding
2. Projected spot welding
3. Seam welding
4. Resistance butt welding
1. SPOT (SPOT) RESISTANCE WELDING METHOD
Spot welding is the most commonly used one in resistance welding types. Parts to be welded are placed between two copper electrodes and electric current is applied through applying a certain pressure. In Figure 1.1, spot welding machine and basic components of welding area cross-section are given.
While current flows from one electrode to the other for a period specified by timer and switch on the machine, resistance to this current reaches to the highest point at contact point between the two sheets and the material starts to melt at this point. After automatic disconnection of electric current welding bath solidifies by cooling under applied pressure and two sheet materials are combined in a non-removable way. The basic components of a standard spot welding machine are mechanical system used to apply load to the copper electrodes, transformer that reduces the voltage from the mains, current control devices and a time counter in some machines.
Thermal properties of certain materials welded with Resistance Welding
1.2. Electrodes Used in spot Welding
Selection of electrode according to type and thickness of part in spot welding is an important factor that determines the quality of the welding. Types of electrode heads are shown in Figure 1.2.
2. PROJECTED SPOT WELDİNG METHOD
Projected spot welding is similar to spot welding in terms of method. In spot welding sheet to be welded is trapped between the electrodes by overlapping materials and an electric current passes. With this method while size and shape of the electrode tip limit flowing current in the projected spot welding, current is limited to projection on at least one material to be welded.
In projection welding method welding area is the area with projection in one of the sheets to be welded. When sheets are overlapped they contact with each other only at the point where projection is located. The electric current passing from this point concentrates on the projection and thus projection rapidly warms. Due to warming projection melts and collapse and a molten zone occurs between the two sheets. Electric current is stopped and pressure applications continue for some time more and welding is completed.
Projected spot welding machines are same with spot welding machines. But in this method flat electrodes to which equal pressure can be applied are used instead of electrode types used in spot welding. Projected spot welding is used for attachment of fittings to small sheet structures. This method is used in especially automotive industry in connection of fixed nuts to the frame, screw connection of home appliances, office furniture, in manufacture of machinery parts, threaded studs and many such areas. Projection welding method is used in reinforced concrete applications and production of wire mesh in construction sector.
2.1. Cooling and maintenance of Spot and Projected Welding Electrodes
Because projected and spot welding is used in series production usually, electrode is heated due to the resistance created by the current passing through the electrodes.
Because increase in electrode temperature may affect welding negatively it should be cooled. Typically water is used as cooling system in welding machines. This system allows constantly cooling of electrodes with an assembly within machine through water circulation. Simple cooling system of electrodes is schematically shown in Figure 2.2.
Welding electrodes wear out over time depending on usage. As a result of wearing tip of the electrode is disrupted and causes decrease in flow and pressure and this is one of the factors adversely affecting the welding quality? This requires periodic maintenance of the electrode tips. Maintenance operation is accomplished by elimination of elements blocking current transition and gaining former form to deteriorated tip. This process is made with apparatuses like sandpaper and rasp.
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