13. Special Processes

Size: px

Start display at page:

Download "13. Special Processes"

Pamela Cross
6 years ago
Views:

1 13. Special Processes

2 13. Special Processes 183 Apart from the welding processes explained earlier there is also a multitude of special welding processes. One of them is stud welding. Figure 13.1 depicts different stud shapes. Depending on the application, the studs are equipped with either internal or external screw threads; also studs with pointed tips or with corrugated shanks are used. rammed flange In arc stud welding, a distinction is basically made between three process variations. Figure depicts the three variations the differences lie in the kind of arc ignition and in the cycle of motions during the welding process. br-er13-01e.cdr The switching arrange- Figure 13.1 ment of an arc stud welding unit is shown in Figure Besides a power drawn-arc stud welding capacitordischarge stud welding with tip ignition drawn-arc stud welding with ferrule ignition source which produces high currents for a shorttime, a control as well as a lifting device are necessary. ceramic ferrule cold-upset tip ignition ignition ring br-er13-02e.cdr Figure 13.2

3 13. Special Processes 184 In drawn-arc stud welding the stud is first mounted onto the plate, Figure The arc is ignited by lifting the stud and melts the entire stud diameter in a short time. When stud and base plate are fused, the stud is dipped into the molten weld pool while the ceramic ferrule is forming the weld. After the solidification of the liquid weld pool the ceramic ferrule is knocked off. Figure 13.5 illustrates tip ignition stud welding. The tip melts away immediately after touching the plate and allows the arc to be ignited. The lifting of the stud is dispensed with. When the stud base is molten, the stud is positioned onto the partly molten workpiece. power source br-er13-03e.cdr control device welding time adjustment A V lifting device stud holding device stud ceramic ferrule workpiece Studs with diameters of up to 22 mm can be used. Figure 13.3 Welding currents of more than 1000 A are necessary. The arc stud welding process allows to join different stud movement L time P P L materials, see Figure Problematic are the different melting points and the current projection P start lifting dipping end L > (L + P) heat dissipation of the indi- time vidual materials. Aluminium br-er13-04e.cdr studs, for example, may not be welded onto steel. Figure 13.4

4 13. Special Processes 185 The relatively high welding currents in the arc stud welding process cause the somewhat troublesome side-effects of the arc blow. Figure 13.7 depicts different arrangements of current contact points and cable runs and illustrates the developing arc deflection (B,C,E). A, D and F show possible countermeasures. a b In high-frequency welding of pipes the energy input into the workpiece may be carried out via sliding contacts, as shown in Figure 13.8, or via rollers, as shown in Figure Only the high-frequency technique allows a safe current transfer in spite of the scale or oxide layers. Through the skin effect the current flows only conditionally at the surface. Therefore no thorough fusion of thickwall pipes may be achieved. br-er13-05e.cdr c Figure 13.5 Phases of Capacitor-Discharge Stud Welding With Tip Ignition d base meatl stud material unalloyed sructural steel S235J0 and/or comparable steels other unalloyed steels stainless steels acc. DIN EN heat resisting steels acc. SEW 470 aluminium and aluminium alloys unalloyed structural steel S235J0, S355J0 and/or comparable steels (acc. DIN EN ) other unalloyed steels stainless steels acc. DIN EN heat resisting steels acc. SEW aluminium and aluminium alloys explanation of the weldability classification numbers: 1 = well suitable (transmission of energy) 2 = suitable (transmission of energy possible with restriction) br-er13-06e.cdr 3 = suitable only up to a point (not for transmission of energy 0 = not possible Figure 13.6

5 13. Special Processes 186 A B Only welding of small wall thicknesses is profitable as the weld speed must be greatly reduced with increasing wall thicknesses, Figure br-er13-07e.cdr C D Figure 13.7 moving direction of the pipe rotary transformer ~ pressure rollers Isolation sliding contacts (fixed) interstage transformer HF-valve generator copper electrode wheel (water-cooled) slot pipe pressure rollers counterpressure rollers br-er13-08e.cdr br-er13-09e.cdr High-Frequency Welding of Pipes Rotary Transformer Resistance Welding Figure 13.8 Figure 13.9

6 13. Special Processes m/min In induction welding a process which is used frequently nowadays the energy input is received contactless, Figure Varying magnetic fields produce eddy currents inside 2 the workpiece, which again cause resistance welding speed heating in the slotted tube. A distinction is made between coil inductors (left) and line inductors (right) mm 16 wall thickness 1: 2: 3: 4: 5: 6: Br-er13-10e.cdr 36 ka; 57 ka; 75 ka; 125 ka; 150 ka; 200 ka; Figure kva; 200 kva; 300 kva; 500 kva; 1200 kva; 1850 kva; 60 Hz 60 Hz 60 Hz 60 Hz 120 Hz 120 Hz Welding Speeds in HF-Resistance Welding Also in case of induction welding flows the current flows only close to the surface areas of the pipe. Only the current part which reaches the joining zone and causes to fill the gap may be utilised. Figure illustrates two current paths. On the left side: the useful current path, on the right side: the useless current path which does not contribute to the fusion of the edges. Figure shows the effective depth during the inductive heating for different materials, in dependence on the frequency. As soon as the Curie temperature point is reached, the effective depth for ferritic steels increases. coil inductor moving direction of the pipe pressure rollers line inductor moving direction of the pipe pressure rollers br-er13-11e.cdr Figure 13.11

7 13. Special Processes 188 The application of the in- l duction welding method δ 2 allows high welding speeds b b of more than 100m/min, δ 1 d δ 1 Figure s b s δ 1 width of the heating inductor wall thickness of the pipe current penetration depth on pipe backside br-er13-12e.cdr δ 2 d l current penetration depth at the strip edges outside diameter of the pipe distance inductor- welding point Figure effective depth δ mm ,0 0,8 0,6 0,4 0,2 0,10 0,08 0,06 0,04 0, m/min weld speed corrective factor 100 % mm 200 pipe diameter high frequency kw 600 kw khz 1000 frequency f kw br-er13-13e.cdr steel (ferritic steel (austenitic) brass aluminium copper brass copper aluminium steel (ferritic) 800 C C 800 C 600 C 850 C 20 C 20 C 20 C 20 C 300 kw kw 60 kw 100 kw 150 kw mm 20 wall thickness br-er13-14e.cdr Standard Values of the Effective Depths During Inductive Heating Welding Speeds in Induction Welding Figure Figure 13.14

8 13. Special Processes 189 3FeO + 2Al Al O + 3Fe kj 2 3 Fe O + 2Al Al O + 2Fe kj Fe O + 8Al br-er13-15e.cdr Figure Aluminothermic fusion welding or cast welding is mainly used for joining railway tracks on site. A crucible is filled with a mixture consisting of aluminium powder and iron oxide. An exothermal reaction is initiated by an igniter the aluminium oxidises and the iron oxide is reduced to iron, Figure The molten iron flows into a ceramic mould which matches the contour of the track. After the melt has cooled, the mould is knocked off. Figure shows the process assembly. 4Al O + 9Fe kj Explosion welding or explosion cladding is frequently used for joining dissimilar materials, as, for example, unalloyed steel/alloyed steel, copper/aluminium or steel/aluminium. The materials which are to be joined are pressed together by a riser workpiece thermit crucible slag mould riser thermit bulge weld cross-section thickness of the cast b br-er13-16e.cdr b A B preheating runner gate gas fuel air thermit slag thermit steel channel between riser and runner gate runner gate blow-hole orifice iron or sand plug foundry sand cut A-B c b slag mould mould riser workpiece blow-hole orifice riser runner gate workpiece cast-around bulge shock wave. Wavy transitions develop in the joining area, Figures and Figure

9 13. Special Processes 190 The determined cladding speed must be strictly adhered to during the welding process. If the welding speed is too low, lack of fusion is the result. If the welding speed is exceeded, the development of the waves in the joining zone is erratic. Figure shows the critical cladding speeds for different material combinations. a) explosive charge b) igniter buffer flyer plate A' A K Amboß anvil β br-er13-17e.cdr Figure α K v F v F v K β v K v P parent plate B v d B' B 90 - β + α/2 v P 90 - α/2 B' t d A' A igniter K anvil K v F v K v F β v K = vd β B B' explosive charge buffer flyer plate v d v P parent plate B v P B' t Figure shows a diagrammatic representation of a diffusion welding unit. Diffusion welding, like ultrasonic welding, is welding in the solid state. The surfaces which are to be joined are cleaned, polished and then joined in a vacuum with pressure and temperature. After a certain time (minutes, right up to several days) joining is achieved by diffusion processes. Br-er13-18e.cdr The advantage of this costly welding method lies in the possibility of joining dissimilar materials without taking the risk of structural transformation due to the heat input. Figure shows several possible material combinations. The joining of two extremely different materials, as, e.g. austenite and a zirconium alloy, may be obtained by several intermediate layers. Figure 13.18

10 13. Special Processes 191 materials flyer plate/ parent plate aluminium/ aluminium -1 critical speed [m s ] v k1 v k2 v k >4000 working pressure 1,33 mpa P measuring amplifier hydraulic aggregate unit copper/ copper steel/ steel >3600 >3900 HFgenerator workpieces copper/ aluminium aluminium/ steel pumping station cooper/ steel aluminium/ zinc recorder p,t = f(t) copper/ zinc loading device br-er13-19e.cdr br-er13-20e.cdr Critical Cladding Speeds in Explosive Cladding Schematic Representation of a Diffusion Welding Unit Figure Figure material tantalum niobium zirconium tungsten molybdenum titanium cast iron structural steel tool steel stainless steel aluminium copper nickel titanium molybdenum tungsten zirconium niobium tantalum Figure shows the structure of a joint where nickel, copper and vanadium had been used as intermediate layers. As the diffusion of the individual components takes place only in the region close to the surface, very thin layers may be realised. nickel copper aluminium stainless steel tool steel structural steel cast iron very good weld quality good weld quality bad weld quality not tested/ results not reported br-er13-21e.cdr Possible Material Combinations for Diffusion Welding Figure 13.21

13. Special Processes 192 In cold pressure welding - in contrast to diffusion welding - a deformation is produced by the high contact pressure in the bonding

Through transposition processes as well as through adhesion forces can joining of similar and dissimilar materials be realised. Figure 13.

The surface layers of overlap arranged plates are destroyed by applying mechanical vibrator energy.

The joining members are welded under pressure, where one part small amplitudes (up to 50 µm) relative to the other is moved with specimen A specimen B

11 13. Special Processes 192 In cold pressure welding - in contrast to diffusion welding - a deformation is produced by the high contact pressure in the bonding plane, Figure The joint surfaces are moved very close towards each other, i.e., to the atomic distance. Through transposition processes as well as through adhesion forces can joining of similar and dissimilar materials be realised. Figure X10CrNiTi18 9 Ni Cu V Zr2Sn br-er13-22e.cdr Ultrasonic welding is used as a microwelding method. The process principle is shown in Figure The surface layers of overlap arranged plates are destroyed by applying mechanical vibrator energy. At this instance are joining surfaces deformed by very short localised warming up and point-interspersed connected. The joining members are welded under pressure, where one part small amplitudes (up to 50 µm) relative to the other is moved with specimen A specimen B br-er13-23e.cdr Figure dies guide and buffer d 1 d 2 with ultrasonic frequency. As far as metals are concerned, the vibratory vector is in the joining zone, in contrast to ultrasonic welding of plastics. The ultrasonics which have been produced by a magnetostrictive transducer and transmitted by a sonotrode lie in the frequency range of 20 up to 60 Hz.

12 13. Special Processes 193 Figure shows possible material combinations for ultrasonic welding. Further microwelding processes are methods which are also called heated element welding methods, as, for example, nailhead bonding and wedge bonding. These methods are applied in the electronics industry for joining very fine wires, as, for example, gold wires from microchips with aluminium strip conductors. In wedge bonding a wire is positioned onto the contact point via a feeding nozzle. The welding wedge is lowered and the wire is welded with the aluminium thin foil, Figure The wire is cut with a cutting tool. HFgenerator Figure pressure force ultrasonic vibrator br-er13-24e.cdr sonotrode process observation optics sonotrode tip workpiece anvil In nailhead bonding, the wire which emerges from the feeding nozzle may have diameters from 12 to 100 µm. By a reducing hydrogen flame its end is molten to a globule, Figure The nozzle then presses this globule onto the part aimed at and shapes it into a nail head. Figure depicts this type of weld. A further method related to welding is soldering. The process principle of soldering is briefly explained in Figure aluminium+alloy beryllium+alloy copper, Cu-Zn-alloy germanium gold iron magnesium+alloy molybdenum+alloy nickel+alloy palladium+alloy platin+alloy silicon silver+alloy tantalium+alloy tin titanium+alloy tungsten+alloy zirconium+alloy br-er13-25e.cdr aluminium+alloy beryllium+alloy copper, Cu-Zn-alloy germanium gold iron magnesium+alloy molybdenum+alloy nickel+alloy palladium+alloy platin+alloy silicon silver+alloy tantalium+alloy tin titanium+alloy tungsten+alloy zirconium+alloy Possible Material Combinations for Ultrasonic Welding Figure 13.25

13 13. Special Processes 194 heated wedge (tungsten-carbide) 5-50 µ m gold wire wedge bonding Al-strip conductor cutting tool br-er13-26e.cdr Figure heated wedge (tungsten-carbide) H2-flame 5-50 mm gold wire wedge bonding Al-strip conductor nailhead br-er13-27e.cdr Figure br-er13-28e.cdr Figure 13.28

14 13. Special Processes 195 The individual soldering methods are classified into different mechanisms depending on the type of heating, Figure There are two basic distinctions: soft soldering (melting temperature of the solder is approx. up to 450 C) and brazing (melting temperature of the brazing solder is approx. up to 1100 C. For high-temperature soldering solders with high melting points (melting temperature is approx. up to 1200 C) are used. This process is frequently subject to automation. In soldering, atomar forces of attraction are effective. Similar and dissimilar metals are joined by addition of a solder with a low melting point. In the boundary area transposition processes occur between solder and base metal. This is called a two-dimensional diffusion. In the subsequent diffusion glowing phase (high-temperature soldering) the solder may be completely absorbed by the base metal. A distinction is made between soft soldering (melting temperature of the solder is below 450 C) and brazing (melting temperature of the solder is 450 C up to 1100 C) as well as high-temperature soldering (melting temperature of the solder is up to 1200 C). Heating of the component for melting the solder may be effected in various ways. classification according to the type of heating: - flame brazing - iron soldering - block brazing - furnace soldering - salt bath brazing - dip soldering - wave soldering - resistance soldering - induction brazing br-er13-29e.cdr br-er13-30e.cdr Soldering - Definition and Process Principle Classification of Soldering Methods Figure Figure 13.30

WELDING Topic and Contents Hours Marks

WELDING Topic and Contents Hours Marks Topic and Contents Hours Marks 3.1 Introduction 04 Marks Classification and selection of welding process. Working principle of Gas welding and types of flames. 3.2 Arc welding process 08 Marks Metal arc,