10 Essential Welding Techniques: A Comprehensive Guide

1. Electrode arc welding

(1) Welding arc

Arcing is a persistent and intense gas discharge phenomenon that occurs between two charged conductors.

Arc Formation

(1) Short circuit between the welding rod and the workpiece

In the case of a short circuit, individual contact points with high current density are heated by the heat of the resistance, q = I^2Rt, where I is the current and R is the resistance. The electric field intensity in the small air gap is very high, which results in:

① A small number of electrons escaping

② The individual contact points being heated, melted, and even evaporated and vaporized

③ The presence of many metallic vapors with low ionization potential.

Related Reading: How to Choose the Right Welding Rod?

(2) Lifting the welding rod to an appropriate distance

Under the influence of thermal excitation and a strong electric field, the negative electrode emits electrons and moves at high speed, colliding with neutral molecules and atoms, exciting or ionizing them. This results in:

• Rapid ionization of gas in the air gap.
• During the collision, excitation and recombination of positive and negative charged particles, energy is converted and released as light and heat.

Arc Structure and Temperature Distribution

The arc consists of three parts: the cathode area (usually a bright white dot at the end of the electrode), the anode area (a thin, bright area in the bath corresponding to the end of the electrode on the workpiece), and the column area of the arc. (the air space between the two electrodes).

Conditions for stable arc combustion

(1) Appropriate power supply

There must be a power source that meets the electrical requirements of the welding arc.

a) If the current is too low, the ionization of the gas between the air gaps is insufficient, the arc resistance is high and a higher arc voltage is required to maintain the required ionization level.

b) As the current increases, the ionization level of the gas increases, the conductivity improves, the arc resistance decreases, and the arc voltage decreases. However, the voltage must not decrease beyond a certain point in order to maintain the required electric field intensity and ensure the emission of electrons and the kinetic energy of charged particles.

(2) Proper electrode selection and cleaning

It is important to use clean electrodes with a suitable coating.

(3) Partial Blowing Prevention

Measures must be taken to avoid partial blowing.

(4) Electrode polarity

In welding, when using a DC welding machine, there are two methods: positive connection and reverse connection.

AC Arc Welding Equipment

AC arc welding equipment is widely used, and the polarity of the electrode changes frequently, so there are no problems with polarity.

1. Positive Connection

The workpiece is connected to the positive pole of the power supply, and the electrode is connected to the negative pole. This is the normal connection method used for general welding operations.

2. Reverse connection

The workpiece is connected to the negative pole of the power supply, and the electrode is connected to the positive pole. This method is generally used to weld thin plates to avoid burns.

(2) Electrode arc welding welding process

1). Welding process

two). Heating characteristics of arc welding with welding rod

• Arc welding with a welding rod results in high local heating. The metal near the weld is heated unevenly, which can cause deformation of the part, residual stress, uneven microstructural transformations and changes in material properties.
• The heating speed is fast (1500 ℃/s), leading to an uneven temperature distribution and the appearance of microstructural defects and changes that should not occur during heat treatment.
• The heat source is moving, causing constant changes in the heating and cooling areas.

(3) Metallurgical characteristics of arc welding

• The high temperature in the reaction zone causes strong evaporation of alloying elements and oxidation.
• The pool of molten metal is small in volume and remains in a liquid state for a short period of time, resulting in a uniform chemical composition. However, the limited time does not allow for the removal of gases and impurities, making it prone to the formation of defects such as pores and slag inclusions.

(4) Welding rod

Composition of welding rod for manual arc welding

The welding rod for manual arc welding is composed of a welding core and a cladding.

1. Welding Core

① As an electrode for arc welding, it conducts electricity with the workpiece to form an arc.

② During the welding process, it continuously melts and is transferred to the moving weld pool, where it crystallizes with the molten base metal to form a weld.

2. Electrode Coating

① Coating Paper

The coating provides effective protection for the molten pool and slag joint, deoxidizes and desulfurizes the molten metal in the pool, and infiltrates the alloy into the molten pool metal to improve the mechanical properties of the weld. It also stabilizes the arc to improve the welding process.

② Coating Composition

• Arc stabilizer: mainly composed of potassium, sodium and calcium compounds that are easily ionized.
• Slag forming agent: forms slag to cover the surface of the weld pool, preventing the atmosphere from invading it and performing a metallurgical function.
• Gasifier: decomposes gases such as CO and H2 and surrounds the arc and the weld pool to isolate the atmosphere and protect the droplets and the weld pool.
• Deoxidizer: mainly composed of ferromanganese, ferrosilicon, ferrotitanium, ferroaluminum and graphite, used to remove oxygen from the molten pool.
• Alloying agent: mainly composed of ferroalloys such as ferromanganese, ferrosilicon, ferrochrome, ferromolybdenum, ferrovanadium and ferrotungsten.
• Binder: commonly composed of potassium and sodium silicate.

3. Types of Electrode Coating

• Acidic electrode: the coating contains a large amount of acidic oxides such as SiO2, TiO2 and Fe2O3.
• Alkaline Electrode: The coating contains a large amount of alkaline oxides, such as CaO, FeO, MnO, Na2O, MgO, etc.

Welding Rod Types

Welding rods are divided into ten categories:

1. Structural Steel Electrodes
2. Low Temperature Steel Electrodes
3. Heat Resistant Molybdenum Chromium Molybdenum Steel Electrodes
4. stainless steel electrodes
5. Surface electrodes
6. Cast Iron Electrodes
7. Nickel and Nickel Alloy Electrodes
8. Copper and copper alloy electrodes
9. Aluminum and aluminum alloy electrodes
10. Special purpose electrodes

Welding rod selection principle

When selecting a welding rod, the following principles must be considered:

1. Choose electrodes with the same or similar chemical composition as the base metal.
2. Select electrodes with the same resistance as the base metal.
3. The type of electrode coating must be chosen based on the service conditions of the structure.

(5) Changes in the metal structure and properties of welded joints

Temperature change and distribution in welding

The temperature of the metal in the weld zone begins to increase and reaches a steady state, and then gradually decreases to room temperature.

Changes in the microstructure and properties of welded joints (using low carbon steel as an example)

Main Defects of Welded Joints

1. Holes

Blowholes are holes formed when bubbles in the molten pool fail to escape during solidification.

Prevention Measures:

a) Dry the welding rod and thoroughly clean the welding surface and the surrounding area of ​​the workpiece.

b) Use an adequate welding current and operate correctly.

2. Slag Inclusion

Slag inclusion is the slag that remains in the weld after welding.

Precautions:

a) Carefully clean the welding surface.

b) Completely remove the slag between layers during multilayer welding.

c) Decrease the crystallization rate of the molten pool.

3. Welding crack

a) Hot Crack

Hot cracking is a crack in the welded joint that forms when the metal cools near the solidus during welding.

Preventive measures:

Reduce structural rigidity, preheat before welding, reduce alloying, choose electrodes with low hydrogen content and good crack resistance, etc.

b) Cold cracking

Cold cracking is a crack in the welded joint that occurs when it cools to a lower temperature.

Precautions:

a) Use a low hydrogen electrode, dry and remove oil and rust from the surface of the workpiece.

b) Preheat before welding and heat treat after welding.

4. Incomplete Penetration

Incomplete penetration is a phenomenon where the root of the welded joint is not fully penetrated.

Causes:

Groove angle or gap too small, blunt edge too thick, groove dirty, electrode too thick, welding speed too fast, welding current too small, and improper operation.

5. Incomplete Merger

Incomplete fusion is a phenomenon where the fusion between the weld and the base metal is not complete.

Causes:

Dirty groove, excessive electrode diameter and improper operation.

6. Undercut

Undercut is a groove or depression along the base metal portion of the weld tip.

Causes:

Excessive welding current, arc too long, inappropriate electrode angle, etc.

(6) Welding deformation

Causes of stress and strain in welding

Local heating during welding is the main cause of welding stress and deformation.

Basic Forms of Welding Deformation

Process Measures to Prevent and Reduce Welding Deformations

1. Inverse Deformation Method
2. Increased Margin Method
3. Rigid fixation method
4. Selecting a reasonable welding process

Process Measures to Reduce Welding Stress

1. Selecting a reasonable welding sequence
2. Preheating method
3. Post-Weld Annealing

two . Automatic submerged arc welding

The welding process where the arc burns under a layer of flux is known as Submerged Arc Welding (SAW).

SAW is characterized by automatic assembly for opening the arc and feeding the electrode, which is why it is also known as Automatic Submerged Arc Welding (SAAW).

(1) Automatic submerged arc welding welding process

(2) Main features of automatic submerged arc welding

Submerged arc welding (SAW) offers several benefits, including:

• High Productivity: SAW allows for high-speed welding and can increase the overall efficiency of a welding project.
• High and stable welding quality: SAW provides consistent and reliable results, ensuring a high quality weld.
• Cost savings on welding materials: SAW uses less filler material, which can result in cost savings for the welding project.
• Better working conditions: SAW produces less smoke and gases, making it a more pleasant and safe working environment for welders.

However, SAW is not suitable for all types of welding. It is most suitable for welding flat, long and straight seams and large diameter circumferential welds. For short welds, zigzag welds, narrow positions, and thin plate welding, SAW may not provide the desired results.

(3) W wire and elding flux

(4) Characteristics of automatic submerged arc welding process

• Strict requirements for preparation before welding
• Large welding penetration
• Arc impact plate and outlet plate are adopted.
• Use flow pad or steel pad.
• Guide installation is adopted.

3 . Gas shielded welding

(1) Argon arc welding

Gas shielded welding that uses argon as the shielding gas is known as tungsten inert gas (TIG) welding or argon arc welding.

Argon, being an inert gas, protects the electrode and molten metal from the harmful effects of air.

Based on the type of electrode used, argon arc welding can be categorized into two types:

• Argon arc welding with fused electrode
• Argon arc welding with unfused electrode.

Argon arc welding with non-melting electrode

Non-fused electrode argon arc welding is a type of argon arc welding where the electrode is used only to generate an electric arc and emit electrons. The filler metal is added separately.

Common electrodes used in this process are tungsten electrodes doped with thorium oxide or cerium oxide. These electrodes have high thermal electron emission capacity, high melting point and high boiling point (3700K and 5800K, respectively).

MIG welding

Tungsten inert gas (TIG) welding is known for its low current and surface penetration. Despite this, it is often used for welding medium to high thickness alloys, such as titanium, aluminum, copper and others. This is due to its ability to achieve high levels of productivity.

The following are the main features of argon arc welding (TIG welding):

• Versatile welding: Due to the protection provided by argon, TIG welding is suitable for welding various alloy steels, non-ferrous metals prone to oxidation and rare metals such as zirconium, tantalum and molybdenum.
• Stable and Efficient Welding: TIG welding is known for its stable arc, minimal spatter, clean welds with no surface slag, and reduced welding deformations.
• Easy to operate: The open arc is visible, making TIG welding easy to operate and can be easily automated for full position welding.
• Ability to weld thin plates: Tungsten pulsed argon arc welding (TPAW) can be used to weld thin plates below 0.8mm and some dissimilar metals.

(2) Protected welding with carbon dioxide gas

Gas shielded welding that uses carbon dioxide (CO2) as the shielding gas is called gas metal arc welding (GMAW) or metal inert gas (MIG) welding.

The main purpose of using CO2 as a shielding gas is to isolate the welding area from the air and avoid the harmful effects of nitrogen on the molten metal. This helps maintain weld integrity and produce high-quality results.

During welding:

2CO ₂ =2CO+O ₂ CO2=C+O ₂

Therefore, welding is carried out in CO ₂ CO and O ₂ oxidizing atmosphere.

Characteristics of protected welding with carbon dioxide gas:

• High welding speed, automatic welding and high productivity.
• It is open arc welding, which is easy to control the weld formation.
• It is less sensitive to rust and less slag after welding.
• The price is low.
• Welding spatter and bubbles are still difficulties in production.

4 . Electroslag welding

Electroslag welding (ESW) is a welding technique that uses heat generated by the resistance of an electric current passing through liquid slag to produce a weld.

(1) Welding process

(2) Characteristics of electroslag welding

• It can be welded into very thick welds at one time.
• High productivity and low cost.
• The weld metal is relatively pure.
• Suitable for welding medium carbon structural steels and alloy steels.

5 . Plasma arc welding and cutting

(1) Plasma arc concept

Typically, a welding arc is a free arc, which means that only a part of the gas in the arc area is ionized and the temperature is not high enough.

However, when the free arc is compressed into an arc with high energy density, the gas in the arc column becomes fully ionized and transforms into plasma, a fourth state of matter consisting of positive and negative ions.

Plasma arcs have high temperatures (ranging from 15,000 to 30,000K), high energy densities (up to 480 kW/cm ² ) and fast-moving plasma streams (several times the speed of sound).

There are three compression effects in plasma arc welding:

1. Mechanical compression effect: The arc is mechanically compressed as it passes through a small nozzle orifice in the plasma gun after the high-frequency oscillating arc causes the gas to ionize.
2. Thermal compression effect: The cooling water in the nozzle causes a sharp reduction in gas temperature and ionization near the inner wall of the nozzle, forcing the arc current to pass only through the center of the arc column, resulting in a significant increase in density of current in the center of the arc column and a further decrease in the arc section.
3. Electromagnetic contraction effect: The increased current density of the arc column creates a strong electromagnetic contraction force that compresses the arc for a third time.

These three compression effects result in a plasma arc with a diameter of only about 3 mm, but with greatly improved energy density, temperature and air velocity.

(2) Characteristics of plasma arc welding

Following are the main features of plasma arc welding:

• High energy density and temperature gradient: Plasma arc welding has a high energy density and a large temperature gradient, which leads to a small heat-affected zone. This makes it suitable for welding heat-sensitive materials or creating bimetallic parts.
• Stable arc and high welding speed: Plasma arc welding has a stable arc and high welding speed, making it ideal for penetration welding to form welds on both sides at the same time with a clean surface and high productivity .
• Ability to weld thick parts: Plasma arc welding can be used to weld thick parts, such as cutting thick stainless steel, aluminum, copper, magnesium and other alloys.
• Stable arc with low current: The fully ionized arc in plasma arc welding can still work stably even when the current is below 0.1A, making it suitable for welding ultra-thin plates (0.01-2mm ) with microbeam plasma arc (0.2-30 A), such as for thermocouples and capsules.

6 . Vacuum electron beam welding

Vacuum electron beam welding (VEBW) is a welding process where a high-speed, directional electron beam is directed at the part, converting its kinetic energy into thermal energy and melting the part to form a weld.

Following are the main features of vacuum electron beam welding (VEBW):

• High quality welds: VEBW produces pure, smooth and mirror-like welds, free from oxidation and other defects due to the welding process that takes place in a vacuum.
• High energy density: The electron beam in the VEBW has an energy density of up to 108 W/cm ² that allows rapid welding heating to a very high temperature, enabling the fusion of any metal or refractory alloy.
• Deep penetration and fast welding speed: VEBW has deep penetration and fast welding speed, minimizing the heat affected zone, resulting in little impact on joint performance and minimal deformation.

7 . laser welding

Laser welding is a welding process that uses a focused laser beam to provide heat to the weld.

The following are the main features of laser welding:

• High energy density and minimal deformation: Laser welding has high energy density and a short action time, which results in a small heat-affected zone and minimal deformation. It can be carried out in an atmospheric environment without gas protection or in a vacuum environment.
• Versatile welding: The direction of the laser beam can be changed with a reflector and there is no need for an electrode to come into contact with the welding during the welding process, making it ideal for welding parts that are difficult to weld with electric welding traditional processes.
• Ability to weld dissimilar materials: Laser welding is capable of welding insulating materials, dissimilar metallic materials, and even metallic and non-metallic materials.
• Limitations: Laser welding requires a small energy input and is limited in terms of the thickness of materials it can weld.

8 . Resistance welding

Resistance welding is a welding process where pressure is applied through electrodes after parts are combined. The resistance heat generated by the current passing through the joint contact surface and the surrounding area is used to weld the parts.

There are several types of resistance welding, including spot welding, seam welding, and butt welding. Each of these methods has unique characteristics and is used for specific welding applications.

(1) S ladle welding

Spot welding is a resistance welding technique in which parts are joined in a lap joint and placed between two electrodes. The resistance heat generated by the current passing through the joint contact surface and surrounding area melts the base metal to form a weld point.

This method is mainly used for welding sheet metal and involves three steps: pre-charging to ensure good contact of the parts, turning on the power to form a nugget and plastic ring in the weld, and breaking the forging point which allows the nugget to cool. and crystallize under the continuous action of pressure, resulting in a welded joint with a dense structure and no shrinkage cavity or crack.

(2) S eam welding

Seam welding is a type of resistance welding in which the workpiece is arranged in a lap or butt joint and positioned between two roller electrodes. The rollers apply pressure to the workpiece as they rotate, and energy is applied continuously or intermittently to form a continuous weld. This welding method is commonly used for structures that require regular welds and have sealing requirements, with plate thicknesses typically less than 3mm.

(3) Butt welding

Butt welding is a resistance welding process that joins two parts along their entire contact surface.

Resistance butt welding

Resistance butt welding is a process in which two workpieces are joined end-to-end in a butt joint and then heated to a plastic state by resistance heat. Pressure is then applied to complete the welding process. This method is typically used to weld parts with simple shapes, small diameters or lengths less than 20 mm, and low strength requirements.

Flash butt welding

Flash butt welding is a process in which two workpieces are assembled into a butt joint and connected to a power source. The end faces of the parts are gradually brought into contact and heated with resistance heat until they reach a predefined temperature within a certain depth range. This results in the generation of a flash, which melts the final metal. The power is then cut off and a disturbing force is quickly applied to complete the weld.

The joint quality of flash butt welding is superior to that of resistance welding, and the mechanical properties of the weld are the same as those of the base metal. There is no need to clean the pre-welded joint surface before welding.

Butt-to-butt flash welding is commonly used to weld important parts and can be used to weld similar and dissimilar metals, as well as metal wires with a thickness as small as 0.01 mm and metal bars and profiles with a thickness as large as 20,000 mm.

9 . Friction welding

Friction welding is a pressure welding process that utilizes the heat generated by friction between the surfaces of workpieces to bring the end face to a thermoplastic state and then rapidly disturbing to complete the weld.

Main characteristics of friction welding:

Clean Surfaces: The friction generated during the welding process cleans the oxide film and impurities on the contact surface of the parts, resulting in a dense, defect-free structure in the welded joint.

Compatibility with different metals: Friction welding can be used to weld the same or dissimilar metals, making it suitable for a wide range of welding applications.

High Productivity: Friction welding is known for its high productivity, making it an efficient method for welding parts.

10 . Brazing

(1) Types of brazing

Brazing can be classified into two categories based on the melting point of the brazing filler metal: hard brazing and soft brazing.

Brazing

Brazing with a solder melting point greater than 450°C is known as hard brazing. Filler metals used for hard brazing include copper-based, silver-based, aluminum-based, and other alloys. Commonly used fluxes include borax, boric acid, fluorine, chloride, among others. Heating methods for hard brazing include flame heating, salt bath heating, resistance heating and high frequency induction heating. The strength of the welded joint can reach 490MPa, making it suitable for parts that experience high stress and are exposed to high working temperatures.

Soldering

Brazing with a solder melting point below 450°C is known as soft brazing. Tin-lead alloys are commonly used as soft solders. Rosin and ammonium chloride solutions are commonly used as fluxes, and soldering iron and other flame heating methods are commonly used for heating.

(2) Brazing characteristics

Following are the main features of brazing:

• Low welding temperature: The temperature at which parts are heated is relatively low, resulting in minimal changes to the metal structure and mechanical properties of the parts.
• Minimum Deformation: The welding process results in minimal deformation of the parts, resulting in a smooth and flat joint.
• Precise size: The process helps maintain the accuracy of the size of the parts being joined.
• Welding of Different Metals: Brazing allows the welding of similar and dissimilar metals.
• Complex Shapes: Brazing is capable of welding complex shapes made up of multiple welds.
• Simple Equipment: The equipment required for brazing is relatively simple.