1. Welding method code and basic symbols for welding seams
1.1 Welding method codes and their annotations commonly used in sheet metal fabrication
Arabic numerical codes are used to represent various metal welding methods. These numeric codes can be used on the diagram as a symbol for the welding method and should be marked at the end of the guide line.
For example, the following welding symbol indicates that a fillet weld is made by manual electric arc welding.
(O 
indicates a fillet weld and the Arabic numeral 111 at the end of the reference line indicates that manual electric arc welding is used.)
| Code | Welding method |
| 111 | Manual arc welding (consumable electrode arc welding with coated electrode) |
| 131 | MIG welding (consumable argon arc welding) |
| 135 | Protected welding with carbon dioxide gas |
| 141 | TIG welding (tungsten and argon arc welding) |
| 311 | Oxygen and acetylene welding |
| 21 | spot welding |
| 782 | Stud resistance welding (seed welding) |
The numerical codes in the table represent welding methods commonly used in thin plate welding.
1.2 Basic welding symbols commonly used in thin sheet metal fabrication.
| Welding way | Anchoring | Corner joint T-joint | Cutting | ||
| Basic symbols |
 Rolled Edge Weld |
 Type I welding |
 Weld bead |
 Plug or slot welding |
 Welding point |
2. Manual arc welding (manual welding)
Manual arc welding uses coated (flux-coated) welding rods and workpieces as electrodes, using the high heat (6000-7000℃) generated by arc discharge to melt the welding rod and workpiece, transforming them in one body.
The welding rod is operated manually. It is flexible, maneuverable and widely applicable, and can be welded in all positions. The equipment used is simple, durable and cheap. The quality of the weld depends on the technical level of the operator.
2.1 Welding specifications for manual arc welding
Welding specifications for manual arc welding refer to the diameter of the welding rod, the intensity of the welding current, the arc voltage, and the type of power source (AC or DC). In DC manual arc welding, it also includes polarity selection.
2.1.1 Welding rod diameter
The diameter of the welding rod has a significant impact on welding quality and is closely related to improving productivity.
Using too thick a welding rod will cause incomplete penetration and poor weld formation; Using too thin a welding rod will reduce productivity. The main basis for selecting the welding rod diameter is the thickness of the welded part and the welding position.
The recommended diameter values based on the thickness of the welded part are as follows (mm):
| Welding thickness | 0.5-1.0 | 1.5-2.0 | 2.5-3.0 | 3.5-4.5 | 5.0-7.0 |
| Welding rod diameter | 1.6 | 1.6-2.0 | 2.5 | 3.2 | 3.2-4.0 |
When selecting the diameter of the welding rod, different welding positions must also be considered. A larger diameter welding rod can be used for flat welding.
For vertical welding, horizontal welding and overhead welding, a smaller diameter welding rod should generally be chosen.
2.1.2 Welding current selection
The size of the welding current has a significant impact on the quality of the weld. When the welding current is too small, it not only makes arc initiation difficult and makes the arc unstable, but also causes defects such as incomplete penetration and slag inclusion.
When the welding current is too large, it is easy to cause burning and undercutting defects, and excessive burning of alloy elements will make the weld too hot, affecting the mechanical properties of the weld and causing slag inclusion due to peeling and failure of the weld. coating.
The selection of welding current is related to the type (coating composition), diameter of the welding rod, welding position and formation of the welded joint.
The relationship between the intensity of the welding current and the diameter of the welding rod is:
| Welding rod diameter (mm) |
1.6 | 2.0 | 2.5 | 3.2 | 4.0 | 5.0 |
| Current intensity | 25-40 | 40-70 | 70-90 | 80-130 | 140-200 | 190-280 |
| The relationship between welding current and welding rod diameter is generally expressed as:
I = K * D Where: |
||||||
| Welding rod diameter (mm) | 1.6-2.0 | 2.0-4.0 | 4.0-6.0 | |||
| Experience coefficient K | 15-30 | 30-40 | 40-60 | |||
When using the calculated current value in practical applications, it is necessary to consider different welding positions.
For flat welding, a higher welding current can be used; for vertical welding, the current used must be reduced to 85-90% of the current used for flat welding; for horizontal and overhead welding, the current should be reduced to 80-85% of that used for flat welding.
When welding stainless steel parts in a flat position, a lower welding current should be selected because the welding core has high resistance and is prone to turning red.
When selecting the welding current, the following points must be observed:
(1) Is the welding current adequate?
a) It can be determined by observing spatter (large spatter when the current is very large, small spatter when the current is very small and iron and slag are not easily separated);
b) Observe the formation of the weld: (if the current is too large, there will be excessive height difference, large fusion depth and easy cutting; if the current is too small, there will be large height difference in the weld and poor fusion with the metal base);
c) Observe the welding electrode: (if the current is too large, the electrode turns red and the coating peels off; if the current is too small, the arc becomes unstable and the electrode becomes easily stuck).
(2) The selection of welding current should also consider the thickness of the workpiece, the shape of the joint, the welding position and site conditions. For thick parts, narrow spaces, low ambient temperatures but good ventilation conditions, a higher welding current can be used.
(3) In summary, while ensuring welding quality, large-diameter welding rods and high welding currents should be used as much as possible to improve welding productivity.
2.1.3 Arc voltage
Arc voltage refers to the voltage drop between the two ends (two electrodes) of the arc. When the welding rod and base material are fixed, the arc voltage is high when the arc length is long and low when the arc length is short.
During welding, the distance between the end of the welding rod and the workpiece is called the arc length. Arc length has a significant impact on weld quality.
Generally, the following empirical formula can be used to determine arc length:
me = D
Where:
L – arc length (mm)
D – welding rod diameter (mm)
k – empirical coefficient
When the arc length is greater than the diameter of the welding rod, it is called long arc; when the length of the arc is less than the diameter of the welding rod, it is called a short arc.
When using acid electrodes, long arc welding must be used so that the arc can burn stably and obtain a good weld joint. When using alkaline electrodes, short arc welding must be used.
During welding, the arc should not be too long, otherwise the arc combustion will be unstable, resulting in poor weld quality and uneven flaking on the weld surface.
2.1.4 Selection of power supply type and polarity
The main basis for selecting the type of power supply is the type of welding rod. Generally, acidic electrodes can use AC or DC power supplies, while alkaline electrodes require DC power supplies to ensure welding quality.
(When AC and DC can be used, AC power supply should be used as much as possible, because AC power supply has simple structure, low cost and convenient maintenance.)
If a DC welding machine is used, there is a polarity selection problem. When the positive electrode of the welding machine is connected to the workpiece and the negative electrode is connected to the welding rod, this connection method is called positive connection or positive polarity; When the negative electrode of the welding machine is connected to the workpiece and the positive electrode is connected to the welding rod, it is called reverse connection or reverse polarity.
When using a DC welding machine for welding, polarity selection mainly depends on the properties of the welding rod and the heat required for welding. The selection principles are as follows:
When welding important structures, low hydrogen alkaline electrodes such as E4315 (J417), E5015 (J507) can be used, and DC reverse polarity welding is specified to reduce porosity generation.
When using titanium-calcium acid electrodes such as 4303 (J422), AC or DC welding can be used. When welding thin steel plates, aluminum and aluminum alloys, brass and other welded parts, DC reverse polarity must be used.
2.2 Common defects in manual analysis of arc welded joints.
| Defect | Defect characteristics | Cause of occurrence | preventive measure |
| Dimensional deviation | Weld density, reinforcement, weld leg size, etc. are too big or too small | Inadequate selection of electrode diameter and welding specifications Inadequate groove design and inappropriate strip handling gestures | Correct selection of electrode diameter and welding parameters can improve the level of operation technology. |
| Undercut | Dents in the base metal of the weld seam 
|
Inadequate welding specifications, excessive current, excessively long arc and excessively fast welding speed. The angle of the welding rod is incorrect, the operation gesture is poor, and the position of the arc blowing joint is incorrect | Reduce the welding current, do not draw the arc for too long, and the speed of the edge conveyor can be a little slower, while the middle conveyor can be a little faster. The welding rod inclination angle is appropriate |
| Stomach | There are pores sandwiched in the weld seam 
|
The oxide, rust and oil stains on the welding surface are not cleaned, the welding rod absorbs moisture, the welding current is too small, the arc is too long, the welding speed is too fast, the protective effect of the coating is bad and the operation gesture is bad | Clean the welding groove, dry the welding rod according to regulations, increase the welding current appropriately, reduce the welding speed, and prevent gas from escaping |
| Lack of penetration | Incomplete bond between welding rod and base metal 
|
Improper design of slots and gaps, incorrect welding rod angle, inappropriate operation gestures, insufficient heat input, low current, fast welding speed, and incomplete removal of slot welding slag oxides | Choose the appropriate slot size, choose a higher welding current, or slow down the welding speed to improve operating technology |
| Burn | When welding thin plates, holes are burned into the base metal 
|
Incorrect welding specifications (excessive current), incorrect welding methods | Select a lower welding current to accelerate the welding speed appropriately |
3. Gas metal arc welding with consumable electrode and CO2 shielding gas (CO2 gas welding, MIG welding, MAG welding)
CO2 shield welding uses CO2 gas as shielding gas and wire as electrode in metal arc welding with consumable electrode. Its characteristics are as follows:
a) CO2 gas is widely available and cost-effective, with costs equivalent to 40-50% of manual arc welding;
b) High deposition rate, great depth of penetration, absence of slag and concentrated heat source, resulting in high productivity;
c) Full position welding can be carried out using thin wires and short circuit transition methods;
d) Thin sheets of 1-3mm can be welded with thin wires, with minimal deformation after welding;
e) The hydrogen content in the weld is low and has strong corrosion resistance and good crack resistance;
f) Welding with CO2 shielding is easy to observe the arc and the molten pool due to its bright arc welding, allowing the timely detection and adjustment of problems, thus guaranteeing the quality of the weld;
g) Due to the strong oxidizing effect of CO2 gas in the arc space, spatter occurs easily and the weld is prone to porosity. CO2 shield welding is susceptible to airflow interference, which limits its use in outdoor construction.
3.1CO 2 gas shield welding specifications:
The main welding parameters for CO2 gas shielded welding are wire diameter, welding current, arc voltage, welding speed, gas flow rate, power polarity and wire extension length.
3.1.1 Wire diameter selection:
| Welding wire diameter (mm) |
Droplet Transfer Form | Board thickness (mm) |
Welding position |
| 0.5-0.8 | short circuit | 1.0-2.5 | Full position |
| grain | 2.5-4.0 | level | |
| 1.0-1.4 | short circuit | 2.0-8.0 | Full position |
| grain | 2.0-12 | level |
The wire diameter used for CO2 gas shielded welding has a wide range. Thin wires can be used for thin plate welding, flat welding and all-position welding (short circuit transition). Thick wires are only suitable for welding thick plates and welding in a horizontal position (globular transition).
3.1.2 Wire material:
For welding low carbon steel and low alloy structures, Ho8Mn2SiA solid core wire is commonly used.
The mechanical properties of the wire include σb ≥ 490MPa and σ ≥ 392MPa.
3.1.3 Selection of welding current and arc voltage:
| Welding wire diameter (mm) |
Short circuit transition | Granular transition | ||
| Current (A) |
Voltage (V) |
Current (A) |
Voltage (V) |
|
| 0.5 | 30-60 | 16-18 | ||
| 0.6 | 30-70 | 17-19 | ||
| 0.8 | 50-100 | 18-21 | ||
| 1.0 | 70-120 | 18-22 | ||
| 1.2 | 90-150 | 19-23 | 160-400 | 25-38 |
| 1.6 | 140-200 | 20-24 | 200-500 | 26-40 |
3.1.4 Welding speed:
Suitable welding speed is controlled at 30-60 cm/min.
3.1.5 CO 2 gas flow rate:
The gas flow rate is generally related to the welding current. When welding thin plates with small currents, the gas flow rate may be lower. When welding thick plates with large currents, the gas flow rate must be increased accordingly.
For thin wire welding, the CO2 gas flow rate is 5-15L/min, and for thick wire welding of thick plates, the CO2 gas flow rate is 15-25L/min.
3.1.6 Power polarity:
When welding low carbon steel and low alloy structural steel using CO2 gas shield welding, direct current reverse connection (the negative pole of the
The DC welding machine is connected to the workpiece, and the positive pole is connected to the electrode, which is called the reverse connection method).
3.1.7 Wire extension length:
Wire extension length refers to the distance from the end of the wire to the conductive nozzle of the nozzle. Generally, it is about 10 times the diameter of the wire.
3.2 CO 2 gas shielded welding specifications example
Specifications for welding thin plates using thin wire welding protected by CO2 gas.
| Welding thickness (mm) |
Joint form | Assembly release (mm) |
Welding wire diameter (mm) |
Arc voltage (V) |
Welding current (A) |
gas flow rate (L/min) |
| ≤ |
|
≤ | 18-1919-20 | 30-5060-80 | 6-7 | |
|
≤ | 20-21 | 80-100 | 7-8 | ||
| ≤ | ||||||
|
||||||
Causes of defects in CO 2 gas shielded welding and preventive measures
| Defect name | Causes | Prevention measures |
| Crack | The depth/width ratio of the weld is too large. | Increase arc voltage or decrease welding current to widen the weld and reduce penetration. |
| The weld size is very small (especially for fillet welds and root passes). | Reduce the travel speed to increase the cross-sectional area of the weld. | |
| The arc crater at the end of the weld cools very quickly. | Use mitigation measures to reduce the cooling rate and adequately fill the arc crater. | |
| Slag inclusion | The use of multipass short-circuit arc welding results in the presence of slag-type inclusions. | Clean the shiny slag shell from the weld bead before welding the next pass. |
| The high displacement speed results in the presence of oxide film-type inclusions. | Reduce travel speed, use welding wire (flux-cored, solid) with higher deoxidizer content and increase arc voltage. | |
| Stomach | Insufficient gas protection | Increase the shielding gas flow rate to remove all air from the welding area. Clean up spatter inside the gas nozzle to prevent airflow (caused by fans, door opening, etc.) from entering the welding area. Use a slower walking speed to reduce the distance between the nozzle and the weldment. The welding gun must be held at the edge of the weld seam until the arc crater solidifies |
| Contaminated welding wire | Use clean, dry welding wire to remove any oil stains adhering to the wire in the wire feed device or wire guide tube | |
| The workpiece is contaminated | Before welding, remove oil, rust, paint and dust from the groove and use high deoxidizing welding wire | |
| Arc voltage too high | Reduce arc voltage | |
| The distance between the nozzle and the workpiece is too large | Reduce the extension length of the welding wire | |
| Not fused | There is a film of oxide or rust in the welding area | Remove the oxide film and impurities from the groove and part surface before welding |
| Insufficient linear power | Increase wire feeding speed and arc voltage, reduce walking speed | |
| Inadequate welding technology | Using the rotating operation to obtain an instantaneous stop of sensitivity along the groove and keeping the direction of the welding wire in front of the welding puddle | |
| Unreasonable joint design | The included angle of the bevel joint must be kept large enough to achieve the groove grade using the appropriate welding wire extension length and arc characteristics. Change the V-shaped groove to a U-shaped groove | |
| Lack of penetration | Inadequate slot size | The groove listening design should be reasonable, so that the melting depth can reach the bottom of the groove listening, while maintaining a suitable distance between the nozzle and the workpiece to reduce blunt edges. Set or increase butt joint root clearance |
| Improper welding operation | Position the welding wire at an appropriate offset angle for maximum penetration while keeping the arc in front of the weld puddle | |
| Inadequate linear power | Increase the wire feed speed to obtain a higher welding current and maintain an adequate distance between the nozzle and the workpiece. | |
| Large fusion penetration | Excessive linear energy | Reduce wire feed speed and arc voltage to increase travel speed |
| Incorrect groove processing | Reduce excessive root gaps and increase blunt edges. |
4. Gas shielded welding with non-melting electrode (TIG)
Non-melting electrode welding, also known as tungsten inert gas (TIG) welding, is an arc welding method that uses inert gas (argon) as the shielding gas and tungsten electrode as the non-melting electrode. The heat source for fusion is produced by the arc between the tungsten electrode and the base metal (workpiece).
This method can be carried out with or without filler metal (welding wire), relying on the fusion of the base metal itself (generally used for welding structural components with a thickness of less than or equal to millimeters).
4.1 Tungsten Inert Gas Shielded Welding Process (hereinafter referred to as TIG Welding)
Tungsten inert gas shielded welding (TIG welding) is suitable for structural welding of thin plates of materials such as aluminum and aluminum alloys, stainless steel and common carbon structural steel.
During TIG welding, argon gas only serves as mechanical protection. It is very sensitive to oil, rust and other impurities on the surface of the part and the filler metal (welding wire). If not properly cleaned, defects such as porosity and slag inclusion may occur in the weld.
Therefore, before welding, the joint surface of the workpiece must be chemically cleaned or mechanically removed from oil stains and rust within a range of 30-50 millimeters (the welding wire must also be cleaned from oil stains and rust), so as to ensure reliable welding quality.
4.1.1 Welding Parameters
The main welding parameters of TIG welding include power supply and welding polarity, welding current, arc voltage, welding speed, tungsten electrode end diameter and shape, nozzle diameter and gas flow rate, distance from the nozzle to the surface of the part and the inclination of the welding torch. angle.
① Power supply and polarity selection
| Metal materials | DC power supply | AC power supply | |
| Direct connection | Reverse connection | ||
| aluminum alloy Stainless steel Carbon steel Light alloy steel |
× ×Good Good good |
Available Available × × × |
Good Good Available Available Available |
② Welding current
Welding current is the most important welding parameter that determines weld penetration. The welding current is selected based on the required welding depth and the current that the tungsten electrode can withstand.
Various manual TIG welding currents for different joints:
| Plate thickness (mm) | Joint form | Welding current (A) | ||
| Flat welding | Vertical welding | Aerial welding | ||
| 1.5 | Anchoring | 800-100 | 70-90 | 70-90 |
| Cutting | 100-120 | 80-100 | 80-100 | |
| Corner joint | 80-100 | 70-90 | 70-90 | |
| 2.5 | Anchoring | 100-120 | 90-110 | 90-110 |
| Cutting | 110-130 | 100-120 | 100-120 | |
| Corner joint | 100-120 | 90-110 | 90-110 | |
| 3.2 | Anchoring | 120-140 | 110-130 | 105-125 |
| Cutting | 130-150 | 120-140 | 120-140 | |
| Corner joint | 120-140 | 110-130 | 115-135 | |
Note: When the plate thickness is less than millimeters, millimeters and millimeters, the welding current can be obtained from the lower limit values listed in this table.
③ Arc Voltage
Arc voltage is the main parameter that determines the weld width. In TIG welding, a lower arc voltage is typically used to obtain good protection for the weld pool. The commonly used range of arc voltage is 10-20V.
④ Diameter and shape of tungsten electrode end
The choice of tungsten electrode diameter depends on the type of welding source to be used, as well as the polarity and magnitude of the current.
At the same time, the sharpness of the tungsten electrode end also has a certain impact on the depth, width and stability of the weld. The recommended parameters in the table below are available for selection.
Allowable welding current range for various tungsten electrode diameters:
| Tungsten electrode diameter (mm) | Direct current (A) | AC Power (A) | ||||
| Direct connection | Reverse connection | |||||
| pure tungsten | Tungsten Thorium Tungsten Cerium Tungsten | pure tungsten | Tungsten Thorium Tungsten Cerium Tungsten | pure tungsten | Tungsten Thorium Tungsten Cerium Tungsten | |
| 1.6 | 40-130 | 60-150 | 10-20 | 10-20 | 45-90 | 60-120 |
| 2.0 | 75-180 | 100-200 | 15-25 | 15-25 | 65-125 | 85-160 |
| 2.5 | 130-230 | 170-250 | 17-30 | 17-30 | 80-140 | 120-210 |
Before using the tungsten electrode, it is necessary to ensure that its surface is free from burrs and other metallic or non-metallic inclusions, and that there are no scars, cracks or other impurities.
Otherwise, an arc may occur at the welding torch clamp and contaminate the weld pool.
The extension length of the tungsten electrode is generally selected 1-2 times the diameter of the tungsten electrode.
Tungsten electrode tip shape and current range:
| Tungsten electrode diameter (mm) |
Tip diameter (mm) |
Tip angle (°) |
DC direct connection | |
| Constant DC (A) |
Pulse current (A) |
|||
| 12 | 2-15 | 2-25 | ||
| 20 | 5-30 | 5-60 | ||
| 25 | 8-50 | 8-100 | ||
| 30 | 10-70 | 10-140 | ||
| 35 | 12-90 | 12-180 | ||
| 45 | 15-150 | 15-250 | ||
⑤ Welding speed
TIG welding speed depends on the thickness of the part and the welding current. Due to the lower current that the tungsten electrode can withstand, the welding speed is generally less than 20m/h (controlled between 15-18m/h).
⑥ Gas flow rate and nozzle diameter
The nozzle diameter depends on the thickness of the workpiece and the shape of the joint, and the gas flow rate needs to be increased correspondingly as the nozzle diameter increases.
When the nozzle opening is 8 to 12 millimeters, the shielding gas flow rate is 5 to 15 L/min; when the nozzle increases to 14-22 millimeters, the gas flow rate is 10-20 L/min. Gas flow rate is also related to the welding environment.
In case of strong air flow, the gas flow must be increased.
Experienced welders can evaluate the effect of argon shielding by observing the surface color of the weld metal during the process.
If the shielding effect is not ideal, the argon flow rate must be carefully adjusted, the nozzle diameter must be increased, the area must be increased, and, if necessary, the argon back shield must be increased.
4.2 Typical process parameters for manual tungsten inert gas (TIG) welding of aluminum alloy and stainless steel thin plates:
| Materials science | Board thickness (mm) |
Welding position | Welding current (A) |
Welding speed (M/MIN) |
Tungsten electrode diameter (MM) |
Filler Wire Diameter (MM) |
Argon flow rate (L/MIN) | Nozzle diameter (MM) |
| aluminum alloy | 1.2 | Horizontal and vertical | 65-80 50-70 |
5-8 | ||||
| two | Horizontal and horizontal tilt | 110-140 90-120 |
5-85-10 | |||||
| 3 | Horizontal and horizontal tilt | 150-180 130-160 |
7-11 | |||||
| 4 | Horizontal and vertical | 200-230 180-210 |
||||||
| stainless steel | 1 | Flat position | 50-80 50-80 |
|||||
| Flat position | 80-120 80-120 |
|||||||
| Flat position | 105-150 | |||||||
| Flat position | 150-200 |
Defects in the welding process with tungsten inert gas.
| Defect | Production reasons | Preventive measure |
| Tungsten inclusion | (1) Contact arc ignition (2) Melting of tungsten electrode | (1) Use a high-frequency oscillator or high-voltage pulse generator to start the arc (2) Reduce the welding current or increase the diameter of the tungsten electrode, tighten the tungsten electrode clamp and reduce the extension length of the tungsten electrode (3)Fit cracked or torn tungsten electrode |
| Weak gas shielding effect | Unnecessary components such as hydrogen, nitrogen, air and water vapor are mixed into the gas path | (1) Using argon gas with % purity (2) have sufficient advance gas supply and delayed gas stop time (3) correctly connect water and gas pipes, avoiding confusion (4) do a good job of pre-welding cleaning (5) correctly select the shielding gas flow rate, nozzle size, electrode extension length, etc. |
| Arch instability | (1) There are oil stains on the welding part. (2) The size of the joint groove is too narrow. (3) The tungsten electrode is contaminated. (4) The diameter of the tungsten electrode is too large. (5) The arc is too long |
(1) Do a good job of pre-welding cleaning (2) Enlarge the groove, reduce the arc length (3) Remove the contaminated part (4) Choose the appropriate electrode size and chuck (5) Lower the nozzle distance |
| Excessive loss of tungsten electrode | (1) Poor gas protection, tungsten electrode oxidation (2) Reverse polarity connection (3) Cuff overheating (4) Tungsten electrode diameter too small (5) Oxidation of tungsten electrode during interruption of welding |
(1) Clean the nozzle, shorten the nozzle distance, and appropriately increase the large argon flow rate. (2) Change the polarity of the power supply. (3) Polish the electrode fixing end and replace it with a new one. (4) Increase the diameter of the tungsten electrode. (5) Extend the delayed gas supply time by at least 1S/10A |
Note: Except for the defects unique to TIG welding mentioned above, other defects are basically the same as those of manual arc welding.
5. Spot welding process
Resistance spot welding is a resistance welding method that assembles and overlaps the welded joint and presses it between two electrodes to melt the parent metal in a resistance heat weld.
The spot welding process can be divided into three stages: pre-charging the welding between the electrodes, heating the welding area to the required temperature and cooling the welding area under the pressure of the electrodes.
The quality of spot welded joints depends mainly on the size of the fusion zone (diameter and penetration rate).
At the same time, surface defects such as excessive indentation, surface cracks and adhesion damage will also reduce the fatigue strength of the joint.
Spot welding process characteristics: low voltage, high current, high production efficiency, small deformation, limited to overlap, no need to add welding materials such as welding rods, wires and flux, easy to achieve automation, mainly used for thin plate structures.
5.1 Electrode structure and material
Spot welding electrodes consist of four parts: the tip, the main body, the tail (cone or pipe thread) and the cooling hole.
There are five common electrode shapes.
Where 1 represents the tip, 2 represents the main body, 3 represents the tail and 4 represents the cooling water hole.
Standard shapes of spot welding electrodes:
- a) Conical electrode,
- b) Fixing electrode
- c) Spherical electrode
- d) Eccentric electrode
- e) Flat electrode
Spot welding electrode material.
| Material name | Mass fraction of alloy composition % |
performance | To apply | |||
| Tensile strength MPa |
Toughness HB |
Conductivity IACSx10 -2 |
Softening temperature ℃ |
|||
| Cold, hard, pure T2 |
Impurities< | 250-360 | 75-100 | 98 | 150-250 | Rust Resistant Aluminum Spot Welding 5A02, 2A21 (LF2, LF21) |
| Cadmium green steel QCD |
Cd, the rest is Cu | 400 | 100-120 | 80-88 | 250-300 | Hardened aluminum 2A12CZ (LY12CZ) after spot welding and quenching |
| engraved bronze | The rest is Cu | 480-500 | 110-135 | 65-75 | 510 | Spot welding of low carbon steel Q235, 08, 10, 20 |
| Cobalt chrome steel HD1 |
Cr, the rest is Cu | 170-190 | 75 | ≥600 | Steel and stainless steel | |
Basic electrode dimensions.
| Electrode body diameter D (mm) |
Electrode final diameter d (mm) |
Rear tube thread Gin) |
||
| 5-10 | 20-75 | 100 | ||
| Electrode body diameter D (mm) |
Determine based on spot welding process parameters | 1/2 “1” | ||
| 12-16 | 20-35 | 35-50 | ||
5.2 Pre-Welding Surface Cleaning
Pre-welding surface cleaning is crucial for spot welding, which involves removing dirt, oxide film and other contaminants from the surface of the workpiece.
Mechanical cleaning methods such as sandblasting and polishing are commonly used, which include sanding with a grinding wheel, sanding belt or wire brush.
Chemical cleaning includes alkaline washing to remove oil stains and acid washing to remove rust, followed by passivation (note: chemical cleaning should not be used for parts with closed shapes or gaps that make it difficult for acidic or alkaline liquids to escape).
5.3 Spot Welding Working Parameters
The main welding parameters for spot welding include electrode pressure, welding time, welding current, switch and electrode working end face size.
Spot welding parameters are generally determined based on the material and type of the workpiece, the electrode pressure and welding time, and the required melt diameter welding current.
Spot welding parameters are mainly selected in the following two ways:
(1) Proper matching of welding current and welding time. This combination mainly reflects the heating speed of the welding zone. Large current and short time are the strict specifications; on the other hand, small current and appropriately prolonged welding time are the soft specifications.
(2) Proper correspondence between welding current and electrode pressure. This combination is based on the principle of no spatter during the welding process.
5.4 Typical Welding Parameters for Spot Welding Low Carbon Steel.
| Plate thickness (mm) | Electrode end diameter (mm) | Electrode diameter (mm) | Minimum point distance (mm) | Minimum overlap (mm) | Electrode pressure (KN) | Welding time (weeks) | Welding current (A) | Nugget diameter (m) |
| 0.4 | 3.2 | 12 | 8 | 10 | 1.15 | 4 | 5.2 | 4.0 |
| 0.5 | 4.8 | 12 | 9 | 11 | 1.35 | 5 | 6.0 | 4.3 |
| 0.6 | 4.8 | 12 | 10 | 11 | 1.50 | 6 | 6.6 | 4.7 |
| 0.8 | 4.8 | 12 | 12 | 11 | 1.90 | 7 | 7.8 | 5.3 |
| 1.0 | 6.4 | 13 | 18 | 12 | 2.25 | 8 | 8.8 | 5.8 |
| 1.2 | 6.4 | 13 | 20 | 14 | 2.70 | 10 | 9.8 | 6.2 |
| 1.6 | 6.4 | 13 | 27 | 16 | 3.60 | 13 | 11.5 | 6.9 |
| 1.8 | 8.0 | 16 | 31 | 17 | 4.10 | 15 | 12.5 | 7.4 |
| 2.0 | 8.0 | 16 | 35 | 18 | 4.70 | 17 | 13.3 | 7.9 |
| 2.3 | 8.0 | 16 | 40 | 20 | 5.80 | 20 | 15.0 | 8.6 |
| 3.2 | 9.6 | 16 | 40 | 22 | 8:20 am | 27 | 17.4 | 10.3 |
Note: This form is for 60 Hz AC power frequency. When using 50/60 Hz AC power, the frequency must be multiplied by 5/6 (see welding time table).
Plate thickness should be based on the thinnest plate thickness in the overlapping pieces.
5.5 Causes and Prevention of Spot Welding Defects.
| Defect | Cause of occurrence | Preventative methods | |
| Defect in nugget size | Lack of penetration or small nugget size | The welding current is too low, the switching time is too short, and the electrode pressure is too high | Adjusting welding parameters |
| Excessive electrode contact area | cutting electrodes | ||
| Poor surface cleaning | Clean the surface | ||
| Excessive penetration rate | Excessive welding current, prolonged switching time, insufficient electrode pressure | Adjusting welding parameters | |
| Poor electrode cooling conditions | Strengthen cooling and replace with electrode materials with good thermal conductivity | ||
| External defects | Excessive indentation of solder joints and surface overheating | The electrode contact surface is too small | cutting electrodes |
| Excessive welding current, prolonged switching time, insufficient electrode pressure | Adjusting welding parameters | ||
| Poor electrode cooling conditions | Strengthen cooling and replace with electrode materials with good thermal conductivity | ||
| Local burn and surface overflow, surface splash | The electrode is very sharp | Repair Welding Parameters | |
| Foreign objects on the surface of electrodes or welding components | Improved cooling | ||
| Insufficient electrode pressure or virtual contact between electrode and welding | cutting electrodes | ||
| Radial cracks on the surface of solder joints | Insufficient electrode pressure, insufficient forging force or premature addition | Clean the surface of electrodes and welding parts | |
| Poor electrode cooling effect | Increase electrode pressure and adjust stroke | ||
| Circular cracks on the surface of solder joints | Too long welding time | Adjusting welding parameters | |
| Surface adhesion and damage to solder joints | Inadequate selection of electrode materials | Change suitable board materials | |
| Tilt of electrode end face | cutting electrodes | ||
| The surface of the solder joint turns black and the coating layer is damaged | Poor cleaning of the surface of electrodes and welding parts | Clean the surface | |
| Excessive welding current, long welding time, insufficient electrode pressure | Adjusting welding parameters | ||
6. Gas welding and welding code
Gas welding parameters and welding code include flame energy efficiency selection, wire diameter selection, oxygen pressure selection according to welding distance model, nozzle inclination angle selection welding speed and welding speed selection.
6.1 Selection of Flame Energy Efficiency
The energy efficiency of the gas welding flame is expressed in terms of the hourly consumption of acetylene gas (L/H). It is selected based on the thickness of the welded parts, the material properties and the spatial position of the welded parts.
When welding low carbon steels and alloy steels, acetylene consumption can be calculated using the following empirical formula:
- Left-hand welding (for welding thin plates): V = (100 – 120) δ
- Right-hand welding (for welding thick plates): V = (120 – 150) δ
In the formula,
δ represents the thickness of the steel sheet in millimeters and V represents the energy efficiency of the flame (acetylene consumption) in liters per hour.
When welding copper with gas, acetylene consumption can be calculated by the following empirical formula:
V=(150-200)δ.
Choose the welding torch model and nozzle number based on the calculated acetylene consumption, or choose them directly based on the thickness of the welding plate.
Consult the table of injection and suction welding torch models and their main parameters.
| welding torch model | H01-2 | H01-6 | ||||||||
| Welding nozzle number | 1 | two | 3 | 4 | 5 | 1 | two | 3 | 4 | 5 |
| Welding nozzle opening (mm) | ||||||||||
| Welding thickness (mm) | ||||||||||
| Oxygen pressure (MPe) | ||||||||||
| Acetylene pressure (MP) | ||||||||||
| Oxygen consumption (m/h) | ||||||||||
| Acetylene consumption (L/h) | 40 | 55 | 80 | 120 | 170 | 170 | 240 | 280 | 330 | 430 |
6.2 Types and Applications of Oxygen-Acetylene Flames
| Welded metal material | The type of flame to be used | Welded metal material | The type of flame to be used |
| Low and medium carbon steel | Neutral flame | Aluminum and aluminum alloys | Neutral flame or slightly charred flame |
| light alloy steel | Neutral flame | Nickel chrome stainless steel | Neutral flame |
| High carbon steel | Soft charring flame | Ming stainless steel | Neutral flame or slightly charred flame |
| Cast iron | Neutral flame or slightly charred flame | Nickel | Soft charring flame |
| Purple copper | Neutral flame | Menggang | Soft charring flame |
| brass | Soft Oxidation Flame | Galvanized Iron Sheet | Soft charring flame |
| Tin bronze | Neutral flame | Hard league | Soft charring flame |
| Monel League | Soft Oxidation Flame | High speed steel | Soft charring flame |
| Aluminum, tin | Neutral flame | Tungsten Carbide | Soft charring flame |
6.3 Welding Wire Selection
6.3.1 The material of the welding wire must be similar to the alloy composition of the part.
The following welding wire table can be used for gas welding of steel, aluminum and aluminum alloys, as well as copper and copper alloys:
A) Welding wires for different types of steel used in gas welding
| Welding wire name | Welding Wire Class | Applicable steel grade |
| Low carbon steel, low alloy structural steel, medium carbon steel welding wire | H08 | Q235 |
| H08A | Q235、20、15g、20g | |
| H08Mn | Medium carbon steel | |
| H08MnA | Q235, 20, 15g, 20g16Mn, 16MnV, medium carbon steel | |
| H12CrMo | 20Medium carbon steel | |
| Austenitic Stainless Steel Welding Wire | HoCrl18Ni9 | 0Cr18Ni9 0Cr18Ni9Ti 1Cr18Ni9Ti |
| H1Cr18Ni10Nb | Cr18Ni11Nb | |
| HCr18Ni11Mo3 | Cr18Ni12MoTi |
B) Welding wires for aluminum and aluminum alloys used in gas welding.
| Welding Material | Welding Wire | Cut or wire the base material |
| L1 | S (wire) AL-2 | L1 |
| L2 | L1 L2 | |
| L3 | L2 L3 | |
| L4 | L3 L4 | |
| L5 | L4 L5 | |
| L6 | L5 L6 | |
| LF2 | SA1Mg-2 SA1Mg-3 | LF2 LF3 |
| LF3 | SA1Mg-3 SA1Mg-5 | LF3 LF5 |
| LF5 | SA1Mg-3 | LF5 LF6 |
| LF6 | SA1Mg-3 | LF6 |
| LF11 | 8A1Mg-5 | LF11 |
| LF21 | SA1Mn SA1Si-2 | LF12 |
C) Welding wires for copper and copper alloys used in gas welding.
| Welding Material | Welding wire name | Welding Wire Class |
| pure copper | Copper wire | HsCu |
| Brass | 1-4# brass wire | HsCuZn-1~4 |
| White copper | Zinc White Copper Wire | HsCuZnNi |
| Copper wire | HsCuNi | |
| Bronze | Silicon Blue Copper Wire | HsCuSi |
| Tin Blue Copper Wire | HsCuSn | |
| Aluminum Bronze Wire | HsCuAl | |
| Nickel Aluminum Bronze Wire | HsCuAlNi |
6.3.2 Welding Wire Diameter Selection
The selection of welding wire diameter is mainly based on the thickness of the workpiece material.
If the welding wire is too thin, it will melt too quickly and the melting point will fall at the weld seam, which can easily cause poor fusion and uneven weld seams.
If the welding wire is too thick, the melting time of the welding wire will be prolonged, the heat-affected zone will be enlarged, and the fabric may overheat, which will reduce the welding quality of the joint.
Relationship between part thickness and welding wire diameter:
| Part thickness (mm) |
1-2 | 2-3 | 3-5 | 5-10 | 10-15 |
| Welding wire diameter (mm) |
1-2 | 2-3 | 3-4 | 3-5 | 4-6 |
6.4 Welding nozzle inclination angle
The inclination angle of the welding nozzle is generally determined based on the thickness of the workpiece, the size of the welding nozzle and the welding position. A large angle of inclination of the welding nozzle results in a concentrated flame, minimal heat loss, high heat input and rapid heating of the workpiece.
On the other hand, a small angle of inclination of the welding nozzle results in a scattered flame, significant heat loss, low heat input and slow heating of the workpiece. The welding nozzle inclination angle is generally in the range of 20°-50°.
Gas welding nozzle inclination angle selection:
| Welding thickness (mm) |
≤1 | 1-3 | 3-5 | 5-7 | 7-10 | 10-15 |
| Welding nozzle inclination angle | 20° | 30° | 40° | 50° | 60° | 70° |
6.5 Principles for Selecting Gas Welding Specifications
| Parameter | Selection principles |
| Type of flame | Types of oxygen and acetylene flames, selected according to table |
| Acetylene consumption and oxygen working pressure | Based on factors such as the melting point of metals and alloys, the thickness and small size of the welded parts, the thermal conductivity and the shape of the joint, select the welding torque and nozzle with appropriate flame energy rate (consumption of acetylene) and adjust the oxygen working pressure appropriately according to the acetylene consumption. |
| Welding wire diameter | Table selection based on the relationship between part thickness and welding wire diameter |
| Welding nozzle number | Determine based on welding joint thickness, material and shape |
| Welding nozzle inclination angle | Determine according to the thickness of the welding part (see welding nozzle inclination angle selection) |
| Welding speed | Based on operational skills and the strength of the flame used, try to increase the welding speed as much as possible to ensure penetration |
6.6 Common Defects and Preventive Measures in Gas Welding
| Defect | Cause of occurrence | Preventive measure |
| To snap | The sulfur content in the weld metal is too high, the welding voltage is too high, the flame energy rate is low, and the weld fusion is poor | Control the sulfur content of weld metal, improve flame energy efficiency and reduce welding stress |
| Stomach | Poor cleaning of welding wires and parts, high sulfur content, incorrect flame composition and high welding speed | Strictly clean the surface of the part and control the metal composition of the welding wire; Reasonable selection of flame and welding speed |
| The weld size and welding switch do not meet the requirements | Improper welding groove angle, uneven mounting gap, improper selection of welding parameters, etc. | Reasonable slot angle processing, strict assembly clearance control, and correct selection of welding parameters |
| Undercut | Excessive adjustment of flame energy rate, incorrect tilt angle of welding nozzle, improper movement method of welding nozzle and welding wire | Correctly select the correct welding parameters and operating methods |
| Burn | Excessive heating of welding parts, inadequate operating process, slow welding speed and prolonged stay in a certain location | Reasonable heating work, adjusting welding speed and improving operating skills |
| Pit | Excessive flame energy rate, incomplete filling of the weld pool at the end | Pay attention to the basics of welding in the end and choose a reasonable flame energy rate |
| Slag inclusion | The welding edges and layers are not completely clean, the welding speed is too fast, the weld shape coefficient is too small, and the welding nozzle inclination angle is not appropriate | Strictly clean the welding edges and layers of welded parts, control the welding speed, and appropriately increase the shape coefficient of the welding seam |
| Lack of penetration | There are oxides on the welding surface, the groove angle is too small, the flame energy rate is insufficient, and the welding speed is too fast | Strictly clean the welding surface, select appropriate groove angles and gaps, control the welding speed and flame energy rate |
| Not fused | Flame energy rate is too low or leans towards the groove side | Choose the appropriate flame energy rate to ensure that the flame is not biased |
| Welding | Excessive flame energy rate, slow welding speed, large assembly gap of welding parts, incorrect movement method of welding gun, etc. | Select the appropriate welding speed and flame energy rate; Adjust the mounting clearance of welding parts and use the welding gun correctly |
