The Physical Fundamentals of Welding

The physical essence of welding lies in formation of strong connections between atoms of parts to be welded. The parts should be squeezed together so that surface imperfections are crushed and atomic attractive forces start to act.

For metals which have high plasticity only plastic deformation is sufficient for activation of atoms. For metals of medium plasticity additional heating of parts is needed. Metals of poor plasticity must be heated over melting point. Hence, we can say that physical processes used for welding may be classified as mechanical, thermomechanical and thermal ones.

According to that all welding methods are divided into two groups:

-     pressure welding;

-     fusion, or non-pressure, welding.

The following scheme presents the classification of welding methods (Fig. 5.1).

 

Fig. 5.1. Classification of welding methods

 

 

Arc Welding

 

The method relies on the thermal effect of an electric arc sustained between the electrode and parts to be joined in order to provide the melting zone. The electric arc is a powerful stable arc discharge in ionized atmosphere of gases and metallic vapors (Fig. 5.2). The process possesses high efficiency (0.6...0.9), i.e. 60...90 % of evolved heat is consumed for melting of electrode and works.

Electric properties of an arc are described by a static voltage-current characteristic, representing the dependence between voltage and current intensity at stable arcing distance
(3...6 mm). The characteristic consists of three parts (Fig.5.3): dropping-I; stable (rigid)-II; increasing-III.

The stable part is more suitable for welding processes, because the voltage is independent of the amperage and arc burning is stable.

Fig. 5.2. Electric arc: 1-electrode; 2-part; 3-electron; 4-ion

Fig. 5.3. Static voltage-current characteristic of an arc: I – dropping; II – stable; III - increasing

 

Sources of welding current (power sources or welders) have the external characteristic, which is a dependence of voltage on external terminals upon amperage in electric circuit. They may be of four main types (Fig. 5.4): dropping - 1; gradually dropping - 2; rigid - 3; increasing - 4.

The intersection point of arc and power source voltage-current characteristics is supposed to accommodate the stable arcing (welding). Thus, if the process is conducted within rigid part of arc characteristic, the welder with dropping or abruptly dropping external characteristic should be employed (Fig. 5.5).

Power sources of both alternating current (electrical transformers) and direct current (electrical generators and rectifiers) are used for welding. Transformers are cheap, simple in design and more efficient power sources. But power sources of direct current offer stable welding process and give the possibility to use direct and reverse polarity if necessary.

Fig. 5.4. The main types of external characteristic of welding power sources:

1 – dropping; 2 – gradually dropping; 3 – rigid; 4 – increasing

Fig. 5.5. External characteristic of a power source (1) and stable characteristic of the arc (2):

A – point of open-circuit run; B – point of non-stable arcing; C – point of stable arcing;

D - point of short circuit; U - welding voltage; I_w- welding current

 

Welding current is adjusted by means of supply chokes (a-c welding) or variable resistances (d - c welding) (Fig. 5.6).

Fig. 5.6. Adjustment of welding current by means of a choke (a) and a variable resistance (b):

1 -transformer; 2-choke; 3 -windings of generator; 4-variable resistance; 5 – welding;

I, II, III – windings of transformer and choke; S- regulated clearance

 

The arc welding is performed by an arc arcing between electrode and works. It is possible to use either consumable or non-consumable electrodes.

Figure 5.7a represents arc welding scheme when the transformer is used as a power source. Consumable electrode 1 is melted in the electric arc 2 and its drops form liquid bath, which connects works 3 after solidification.

Figure 5.7b represents similar process, but the electrode 1 is non-consumable. It is made of carbon (graphite) or tungsten. A filler metal 4 is used for formation of a weld.

Fig. 5.7. Schemes of the arc welding: a - by N.Benardos; b - by N.Slavjanov;

1 – electrode; 2 – electric arc; 3 – works be welded; 4 – filled metal

 

Direct and reverse polarity may be used when direct current welding process takes place (Fig. 5.8).

Fig. 5.8. Direct (a) and reverse (b) polarity in welding process: 1 – core of electrode (wire); 2 – luting;

3 – gas protection; 4 – weld bath; 5 – slag crust; 6 – weld; 7 – base metal; 8 – drops of liquid metal

 

5.2.1. Manual Arc Welding

 

Manual arc welding is carried out by a welder, who keeps an electrode holder with an electrode and manually regulates the distance between the electrode and works to be welded. The metallurgical processes (oxidation of alloying elements, saturation with gases) take place in the welding bath. To prevent metal oxidation and alloying elements losses, consumable electrodes is covered by luting. The consumable electrodes represent the metallic rods from 1.6 to 12 mm in diameter coated with luting (Fig. 5.8).

The luting may be thin, of medium- thickness, thick and very thick. It consists of following components:

- slag forming (chalk, marble, Mn-ore, SiO₂ sand);

- gas forming (wood powder, starch, etc.);

- deoxidizing (Mn, Si, Al, Ti, etc.);

- alloying (Mn, Si, Cr, Ni, Mo, V, Ti, etc.);

- stabilizing (CaCO₃, K₂O, Na₂O, etc).

The thin and medium-thickness lutings are used mainly for stabilization of electric arc. The thick and very thick lutings are used for stabilization of arc, protection of molten metal by slag and gases, deoxidizing and alloying it.

The electrode diameter depends on thickness of works to be welded. The amperage I_w depends on the electrode diameter d_e, mm:

                                                      (5.1)

So, welding regime is completely characterized by welding current (I_w) and electrode diameter (d_e). Electrodes with the diameter of 3...6 mm are mainly used in manual arc welding. Voltage ranges from 16 to 30 V and welding current is from 120 to 350 A.

The basic types of welds or joints produced by fusion welding are represented in figure 5.9.

 The main advantages of manual arc welding are

- simplicity of the process;

- ability to carry out welding in different space positions (Fig. 5.10).

Fig. 5.9. Basic types of welds produced by fusion welding (the figures are thickness of the parts be welded)

Fig. 5.10. Possible space positions of welding: a-down (flat); b-vertical;

c-horizontal; d-inverted (overhead); e-in-corner

 

The disadvantages of the process are

- manual labor;

- low productivity;

- the welder must have high qualification level (proficiency).

- non - stable quality of the weld.

 

5.2.2. Automatic Arc Welding

The automatic submerged-arc welding differs from manual arc process by the following peculiarities:

- the process is conducted by bare electrode, that is a wire without luting;

- arc and metal bath are submerged or covered by flux layer;

- all processes, such as electrode wire and flux feed, movement of welding head, are completely automated.

Flux or gases may be used as shielding substances.

Correspondingly, two types of automatic arc welding are distinguished:

- automatic welding under flux shield;

- automatic welding in shielding gases.

In both cases the welding tractor is used. Figure 5.11 illustrates the process of automatic welding.

Fig. 5.11. Automatic welding by a welding tractor: 1-tank for a flux; 2-wire; 3-cassete;

4-rolls for wire supply; 5-guide sleeve; 6-flux layer; 7-weld; 8-arc; 9-base metal

 

The arcing is carried out under flux, which shields metal fromoxidation and splashing, providing therefore ability to increase amperage in 15 to 20 times as compared with manual arc welding and correspondingly to raise quantity of overlaying metal and welding productivity.

Main advantages of automatic submerged-arc welding as compared with manual one are:

- high productivity;

- welders of high qualification aren't required;

- high quality and homogeneity of the weld;

- thickness of the weld reaches 20 mm.

Disadvantages of the method:

- only flat welds may be produced;

- the difficulties with producing the welds of small radiuses.

Instead of flux shielding gases, such as carbon dioxide CO₂, nitrogen N₂, hydrogen H₂, argon Ar, helium He are used. The distinction is made between: inert gases (Ar and He) and active gases (CO₂, N₂, H₂).

The automatic electric-gas welding has lower capacity than welding under flux shield, because of metal splashing.

There are several schemes of electric-gas welding: with tungsten or consumable electrode (Fig. 5.12) on direct, reverse polarity and alternating current welding.

Fig. 5.12. Semi-automatic shielding gas welding with non-consumable (a) and consumable (b) electrode:

1 -wire; 2-nozzle; 3-guide sleeve; 4- torch; 5-W-eletrode; 7-shielding atmosphere; 8-arc;

9-welding bath; 10-cassette; 11 – rollers for moving of wire

 

Ar and He are expensive gases, but provide high quality of welding.

CO₂, N₂ and H₂ are cheap gases, but CO₂ is decomposed at high temperature and oxidizes metal:

                                                   (5.2)

                                                       (5.3)

                                        (5.4)

 

Saturation of metal with N₂ and H₂ decreases its mechanical properties. CO₂ -shield is used with alternating current power sources, which decreases arc stability, but allows removing oxides from liquid metal.

Mixtures of different gases are used very often to decrease welding expenses and to raise quality of welding joints.

Flat welds are commonly produced by automatic gas arc welding. But sometimes the welding tractor has a holder and can produce different welds, for example carry out a welding of pipes of great diameter.

Semiautomatic and manual gas-shielded arc welding are used, especially for welding in different space positions. In this case a welder holds arc torch, a wire and gas are fed automatically.

When the welder holds arc torch and feeds filler metal the manual gas-shielded arc welding takes place.

 

Gas Welding

 

Gas welding is one of the chemical welding processes in which the required heat energy is produced as a result of combustion of gases. The main materials available for gas welding are known to be acetylene C₂H₂, hydrogen H₂, natural gas C_mH_n, kerosene. Oxygen O₂ is used as an oxidizer.

Acetylene, possessing the highest heat-producing ability, provides in combustion the highest flame temperature (~3200°C) and gain wide application in gas welding. Calcium carbide CaC₂ interacts with water to produce acetylene in the acetylene generators:

                               (5.5)

 

Three types of generators are distinguished:

- contact-type (water recession);

- water-to-carbide;

- carbide-to-water.

Figure 5.13 represents the most widely applied generator of contact type.

Fig. 5.13. Acetylene generator of contact (water recession) type:

1 – tank; 2 – water; 3 – bell; 4 – calcium carbide; 5 – acetylene

 

A tank 1 contains water 2. Calcium carbide in net 4 is hang up in the bell 3 and reacts with water producing acetylene. When tap is closed, acetylene pressure increases, water is expelled and its level in the tank drops. The reaction is retarded. When acetylene is consumed, pressure is lowered, water returns to the bell, the reaction surface increases and acetylene yield correspondingly rises.

Hence, acetylene is usually produced in acetylene generators, or is taken from cylinders, charged at special station. Acetylene is explosive gas and requires careful treatment. Cylinders are filled with charcoal powder and acetone under a pressure of 1.6 MPa. Cylinders are white painted.

Oxygen is obtained from the air using a selective evaporation at special shops. Blue cylinder is filled by oxygen under a pressure of 15 MPa.

Equipment necessary for gas welding is as follows: protective water seals, acetylene and oxygen cylinders, pressure regulators, welding torch.

Welding torches are used to produce acetylene-oxygen mixture in subsequent combustion of which the welding flame is obtained. Welding torches of injector type are normally employed nowadays (Fig. 5.14). The oxygen pressure before an injector 4 is about 0.3...0.4 MPa. Running out with high speed into mixture chamber 3 it produces significant vacuum, by which acetylene is sucked into the chamber (its pressure within the hoseline may be rather low, from 0.001 to
0.015 MPa). There is replaced tip (head) 2 with calibrated orifice at the end of the gas torch. It serves for regulation of the flame power.

Fig. 5.14. Gas welding torch: 1 – fuel mixture; 2 – tip; 3 – mixture chamber; 4 – injector; 5 – valves

 

Pressure regulators are employed to reduce the gas pressure within the cylinder to the working value and to maintain the value at a constant level automatically.

The ratio between C₂H₂ and O₂ is adjusted by corresponding valves 5 on the torch.

Three particular zones are differed in welding flame (Fig. 5.15):

- core (I)

- welding zone (II)

- tongue, or jet (III)

The highest temperature is achieved in welding zone (3100...3200°C). Hence, welding process is carried out in this zone.

Three types of flame are distinguished according to the ratio between oxygen and acetylene in the mixture:

- balanced (normal) - O₂/C₂H₂»1

- oxidizing - O₂/C₂H₂ > 1

- reducing (carbonizing) - O₂/C₂H₂ < 1

Different flame types are employed in welding of various alloys. For instance, in welding the high carbon steel or cast iron reducing flame is needed; in welding the brasses oxidizing one is required. In the majority of cases balanced flame is employed.

Gas or oxy-acetylene welding provides gradual (smooth) and slow (regulated) heating to be achieved. It is the main peculiarity and advantage of the process. That is why gas welding is used for thin steel parts (0.2…0.5 mm in thickness), non-ferrous alloys, cast iron and a number of alloy steels inclined to cracking (crackness).

 

 

Fig. 5.15. Peculiar zones of welding flame: 1 – core; 2 – welding zone; 3 – tongue (jet)

 

Resistance Welding

 

Resistance welding is a group of pressure welding processes wherein coalescence is produced by the heat obtained from resistance of the work to the flow of electric current in welding circuit and by the application of pressure. There is no any external heat source. Heat is developed in the parts to be welded and pressure is applied by the welding machine through electrodes. No fluxes or filler metals are used.

Current for resistance welding is usually supplied through welding transformers, which transform the high-voltage, low-amperage power supply to usable high amperage at low-voltage.

Pressure, or more properly, the electrode force, is supplied either by air or oil pressure through a cylinder, mechanically by cams, manually by foot or hand levers through linkages or some other means.

 

5.4.1 Heating Fundamentals in Resistance Welding

 

Any current flow in an electrical conductor creates heat. The amount of heat generated depends on three factors:

- the current intensity;

- the resistance of the conductor;

- the time of current flow;

                                                                     (5.6)

where Q is heat generated in Joules;

I is current in Amps;

R is resistance of the work in Ohms;

t is time of current flow in seconds.

 

The formula shows that the heat generated is directly proportional to the resistance and square welding current. The total heat generated is partly used to make the weld and partly lost to the surrounding metal, mainly, by thermal conductivity.

Figure 5.16 represents spot welding. In making a weld, the current passes from one electrode through the base metal to the other electrode. During this flow it encounters seven separate resistance zones as shown in figure 5.16.

Points 1 and 7. The electrical resistance of the electrode material (copper, or bronze) is of low value.

Points 2and 6 are corresponding to the contact resistance between the electrode and the base metal. The magnitude of this resistance depends on the surface condition of the base metal and electrode, the size and contour of the electrode face and the electrode force P. This is a point of high heat generation, but due to thermal conductivity of the electrode material and the fact, that it is usually water cooled the base metal does not reach the fusion temperature during the current passage.

Points 3 and 5. The total resistance of the base metal itself is low, but it is higher than electrode material resistance.

Point 4. This is a point of highest resistance and therefore of the greatest heat generation. Due to hot spots 2 and 6, the heat generated at this interface is not readily lost to the colder electrodes. That effects formation of the weld interface contact.

Fig. 5.16. Scheme of spot welding: 1…7 – resistance and temperature zones; 8 – electrode;

A, B – parts to be welded; T – electrical transformer; P – compressing forces

 

The following types of resistance welding are distinguished: spot, seam, projection, flash, upset and percussion welding.

 

5.4.2 Spot Welding

 

Spot welding is a resistance welding wherein coalescence is produced by heat obtained from resistance to the flow of electric current through the work parts held together under pressure of electrodes. The size and shape of the individually formed welds are limited primarily by the size and contour of the electrodes. In the simple single spot weld shown in figure 5.17 the passage of current and electrode force application must be through the electrodes 2, the overlapped work pieces 1 and the weld.

The are four definite stages of time in the spot-welding cycle (Fig.5.18):

- squeeze time is the time between the first application of the electrode force P and welding current turn-on;

- weld time is the time, during which welding current flows:

- time, during which the electrode force either is still applied or is increased (for better deformation of weld) after the welding circuit has being deenergized;

- off time is the time when the electrodes are off the work.

Fig. 5.17. Spot welding: 1 – work pieces; 2 – electrodes; 3 – transformer; P – pressure

The thickness of welding works is from 0.5 to 12 mm. There are two main regimes of welding:

- soft (easy):

j=I/F=80...160 A/mm² (F-area of a weld);

P=15...40MPa;

t=0.5...3.0 sec;

- rigid:

j=120...360 A/mm²;

P=40...150MPa;

t=0.001...0.010 sec.

Fig. 5.18. Spot welding cycle: I-current; P-pressure; t-time

 

5.4.3. Seam Welding

 

Seam welding (Fig.5.19, 5.20) is a resistance-welding process wherein coalescence is produced by the heat obtained from resistance to the flow of electric current through the work parts held together under pressure by circular electrodes. The resulting weld in seam welding is series of overlapping spot welds (practically continuous or persistent) made progressively along a joint by rotating circular electrodes with automatic current cut-off.

Fig. 5.19. Seam welding process: 1-works; 2-electrodes; 3-transformer

Fig. 5.20. Seam welding cycles: I – current; P – compressing force;

S-distance of works’ movement; t – time

 

Seam welding has much in common with spot welding. Welds may be single or multiple, that is a single seam or more parallel seams may be produced simultaneously. Welds may be direct, or indirect, similar to spot welding.

Seam welding is used usually when persistent leak-free weld is required.

5.4.4. Projection Welding

Projection welding is schematically very similar to spot welding, but has some differences:

- a work undergoes stamping before welding for formation of projections (ledges);

- electrodes have large sizes and welding machine has high capacity; Types of projection welds are shown in figure 5.21.

After welding the upsetting is produced to smooth down the work surface. Main advantage of the process is high productivity. Main disadvantage is high welding machine capacity.

Fig. 5.21. Examples of projection welds

 

5.4.5 Flash welding

 

Flash welding (Fig. 5.22) is a resistance-welding process wherein coalescence is produced simultaneously over the entire area of abutting surfaces by the heat obtained from resistance to the flow of electric current between the two surfaces, and by the application of pressure after heating is substantially completed. Flashing and upsetting are accompanied by expulsion of metal from the joint.

Fig. 5.22. Scheme of flash welding (a) and welding cycle (b):

1 – immovable plate; 2 – clamp (electrode); 3 – work; 4 – movable plate; 5 – transformer;

6 – contact (weld); I – current; S – replacement of movable plate; P – pressure

 

Flash welding is done by placing one of two work parts in the jaws of the machine. As the parts are brought together into very light contact, a voltage of sufficient magnitude is applied to form a flashing action between the parts. Flashing continues as the parts advance until the work pieces reach a forging temperature (sometimes a melting point). The weld is completed by the application of sufficient forging pressure and the interruption of current.

 

 

5.4.6 Upset welding

 

Upset welding is a resistance-welding process wherein coalescence is produced simultaneously over the entire area of abutting surfaces or progressively along a joint by the heat obtained from resistance to the flow of electric current through the area of contact of those surfaces. Pressure is applied before heating is started and maintained throughout the heating period (Fig. 5.23).

Fig. 5.23. Upset welding cycle

 

In upset welding the parts are brought into solid contact and current takes the path through the contact area until a sufficiently high temperature is generated to allow the forging of a weld. The heat is generated mainly by the contact resistance between the two pieces. The difference between upset welding and flash welding is that no flashing from the abutting surfaces occurs at any time and the heat is developed slowly by the resistance between the two parts.

 

5.4.7 Percussion Welding

 

Percussion welding (capacitor energy-storage, electrostatic percussive welding) is a resistance welding process wherein coalescence is produced simultaneously over the entire area of abutting surfaces by the heat obtained from an arc produced by a rapid discharge of stored electrical energy with pressure percussively applied during or immediately following the electrical discharge.

There are two main variations of this process:

- transformer welding (with transformer 5)

- transformerless welding (without transformer 5) (Fig. 5.24).

Fig. 5.24. Percussion welding: 1 – rectifier; 2, 5 – transformer; 3 – capacitor;

4 – switch; 6 – parts to be welded

 

Transformer 2 is the main transformer. It reduces voltage, for instance, from 16 kV to 220V; transformer 5 is reducing welding transformer.

When switch 4 is on the left position the charging of the capacitor 3 takes a place. When the switch is in the right position the discharge of the capacitor 3 and welding of works 6 takes a place.

The method allows:

- to remove overloads in electrical circuit (network), which is peculiar to resistance welding;

- to produce a welding by definite dose of electrical energy and eliminate overheat of welding parts.

 

Diffusion Welding

 

The method was found out in the USSR by N.F. Kozakov. It is one of methods of pressure welding. The works 1 (Fig. 5.25) are replaced into vacuum chamber 3, pressed and heated by heater 2. The welding occurs by the mutual diffusion of atoms in the surface layers of the pieces brought in contact.

The advantages of the method are

- the absence of electrodes and fluxes;

- the absence of weld when the same metals are welded;

- the possibility to weld materials which cannot be welded by other methods, for instance, metal and glass, steel and aluminium, etc.

The disadvantages of this method are the following:

- long time of welding (15…25 min);

- welding in vacuum.

 

Fig. 5.25. Diffusion welding: 1 – works to be welded; 2 – heater; 3 – vacuum chamber

 

METAL CUTTING OPERATIONS

Quite accurate workpieces and parts may be produced by the advanced methods of casting, metal forming and welding. But their dimensional accuracy and surface finish are not satisfactory to use them for assembling the machines and devices. Therefore machining still remains and will probably be for a long time the main technique of finishing mechanical treatment.

We can say that machine tool is a mother of all machines, devices and things used by people, because each of them is produced by machine tool. So, machine tool is the basis of human society.

In simple terms a machine tool is a power-driven machine designed to cut or shape metal or other material. From the machine tool flows every object of our industrialized word: automobiles, airplanes, atomic bombs and atomic power plants, washing machines, electric stoves, radio-sets, refrigerators, etc.