Super complete welding knowledge (II)

1、 Arc welding

Arc: a strong and lasting gas discharge phenomenon. There is a certain voltage between the positive and negative electrodes, and the gas medium between the two electrodes should be in an ionized state. When the welding arc is ignited, the two electrodes (one pole is the workpiece, the other pole is the filler wire or electrode) are usually connected to the power supply, contacted briefly and separated quickly. When the two poles contact each other, a short circuit occurs, forming an arc. This method is called contact arc striking. After the arc is formed, as long as the power supply maintains a certain potential difference between the two poles, the arc combustion can be maintained.

Arc characteristics: low voltage, high current, high temperature, high energy density, good mobility, etc. Generally, a voltage of 20~30V can maintain the stable combustion of the arc, while the current in the arc can range from tens of amperes to thousands of amperes to meet the welding requirements of different workpieces. The temperature of the arc can reach more than 5000K, which can melt various metals.

Arc composition: cathode area, anode area and arc column area.

Arc welding power source: the power source used for welding arc is called arc welding power source, which can be generally divided into four categories: AC arc welding power source, DC arc welding power source, pulse arc welding power source and inverter arc welding power source.

DC positive connection: when the workpiece is connected to the anode by DC welding machine and the electrode is connected to the cathode, it is called DC positive connection. At this time, the workpiece is heated greatly, which is suitable for welding thick and large workpieces.

DC reverse connection: when the workpiece is connected to the cathode and the electrode is connected to the anode, it is called DC reverse connection. At this time, the workpiece is less heated and suitable for welding thin and small workpieces. When the AC welding machine is used for welding, there is no positive or negative connection problem because the polarity of the two poles changes continuously.

Welding metallurgical process:

In the process of arc welding, the interaction of liquid metal, slag and gas is a process of metal remelting. However, due to the particularity of welding conditions, the welding chemical metallurgy process has different characteristics from the general smelting process.

First of all, the welding metallurgy temperature is high, the phase boundary is large, and the reaction speed is fast. When there is air intrusion in the arc, the liquid metal will undergo strong oxidation and nitridation reactions, and a large number of metals will evaporate. The water in the air, as well as the hydrogen atoms decomposed from the oil, rust, and water in the workpiece and welding materials under the high temperature of the arc can dissolve into the liquid metal, resulting in reduced joint plasticity and toughness (hydrogen embrittlement), and even cracks.

Secondly, the welding molten pool is small and the cooling is fast, which makes it difficult for various metallurgical reactions to reach the equilibrium state. The chemical composition in the weld is uneven, and the gas and oxide in the molten pool can not float out in time. It is easy to form defects such as pores, slag inclusions, and even cracks.

The following measures are usually taken during arc welding:

 (1) During welding, the molten metal shall be mechanically protected to isolate it from the air. There are three protection methods: gas protection, slag protection and gas slag combined protection.

 (2) Metallurgical treatment of welding pool is mainly carried out by adding a certain amount of deoxidizer (mainly ferromanganese and ferrosilicon) and a certain amount of alloy elements to the welding materials (electrode coating, welding wire and flux) to eliminate FeO in the pool during welding and compensate for the burning loss of alloy elements.

Common arc welding methods:

1. Manual arc welding

Manual arc welding is one of the earliest and most widely used arc welding methods. It uses the coated electrode as the electrode and filler metal, and the electric arc burns between the end of the electrode and the surface of the workpiece to be welded. On the one hand, the coating can produce gas to protect the arc under the action of arc heat, on the other hand, it can produce slag to cover the surface of the molten pool to prevent the interaction between the molten metal and the surrounding gas.

The more important role of the slag is to produce physical and chemical reaction with the molten metal or add alloy elements to improve the weld metal energy. Manual arc welding equipment is simple, portable and flexible to operate. It can be used for welding short seams in maintenance and assembly, especially for welding parts that are difficult to reach. Manual arc welding with corresponding electrodes can be applied to most industrial carbon steel, stainless steel, cast iron, copper, aluminum, nickel and their alloys.

2. Submerged arc welding

Submerged arc welding (SAW) is a melting electrode welding method, in which the granular flux is used as the protective medium and the arc is buried under the flux layer. The welding process of submerged arc welding is composed of three links: 1. Apply sufficient granular flux evenly at the joints of weldments to be welded; 2. The conductive nozzle and the weldment are connected to two levels of welding power supply respectively to generate welding arc; 3 Automatically feed the welding wire and move the arc for welding.

The main characteristics of submerged arc welding are as follows:

① Unique arc performance

High weld quality The slag has good air protection effect. The main component of the arc zone is CO2. The nitrogen content and oxygen content in the weld metal are greatly reduced. The welding parameters are automatically adjusted, the arc travel is mechanized, the molten pool exists for a long time, the metallurgical reaction is sufficient, and the wind resistance is strong, so the weld composition is stable, and the mechanical properties are good;

Good working conditions, slag isolation arc light is conducive to welding operation; Mechanized walking, low labor intensity.

② The electric field strength of arc column is high, which has the following characteristics compared with MIG welding

The equipment has good adjustment performance. Because of the high electric field strength and the high sensitivity of the automatic adjustment system, the stability of the welding process is improved;

The lower limit of welding current is high.

③ High production efficiency Because the conductive length of the welding wire is shortened, the current and current density are significantly improved, so that the penetration ability of the arc and the deposition rate of the welding wire are greatly improved; Because of the heat insulation effect of flux and slag, the overall thermal efficiency is greatly increased, which greatly improves the welding speed.

Scope of application:

Because of the large penetration, high productivity and high degree of mechanical operation, submerged arc welding is suitable for welding long welds of medium and thick plate structures. It is widely used in shipbuilding, boilers and pressure vessels, bridges, overweight machinery, nuclear power plant structures, marine structures, weapons and other manufacturing sectors, and is one of the most commonly used welding methods in today’s welding production.

In addition to the connection of components in metal structures, submerged arc welding can also overlay wear-resistant or corrosion-resistant alloy layers on the surface of base metal.

With the development of welding metallurgy technology and welding material production technology, the materials that can be welded by submerged arc welding have developed from carbon structural steel to low alloy structural steel, stainless steel, heat-resistant steel, and some non-ferrous metals, such as nickel base alloy, titanium alloy, copper alloy, etc.

Due to its own characteristics, its application also has certain limitations, mainly including:

① For the limitation of welding position, due to the reason of flux maintenance, if no special measures are taken, submerged arc welding is mainly used for welding the horizontal and downward position welds, but not for horizontal, vertical and overhead welding;

② Due to the limitation of welding materials, it is not allowed to weld aluminum, titanium and other highly oxidizing metals and their alloys, which are mainly used for welding ferrous metals;

③ It is only suitable for welding and cutting of long welds, and cannot weld welds with limited space;

④ The electric arc cannot be observed directly;

⑤ It is not applicable to thin plate and low current welding.

3. Gas tungsten arc welding

This is a kind of non melting electrode gas shielded arc welding, which uses the arc between the tungsten electrode and the workpiece to melt the metal and form the weld. The tungsten electrode does not melt during welding and only acts as an electrode. At the same time, argon or helium is sent into the nozzle of the welding torch for protection. Additional metals can also be added as needed. It is generally called TIG welding in the world.

Gas tungsten arc welding is an excellent method for connecting sheet metal and backing welding because it can control heat input well. This method can be used for joining almost all metals, especially for welding aluminum, magnesium, which can form refractory oxides, and active metals such as titanium and zirconium. The welding quality of this welding method is high, but compared with other electric arc welding, the welding speed is slow.

4. GMAW

GMAW belongs to the fusion welding method using the arc as the heat source. Its arc is based on the molten pool formed by mixing the molten wire metal and base metal between the continuously fed welding wire and the molten pool. After the arc heat source is removed, the weld is formed by crystallization and the separated base metal is connected by metallurgy.

Features of CO2 welding:

① CO2 will be decomposed into CO, O2 and O under the high temperature of welding arc, which has a strong compression effect on the arc. As a result, the arc shape of this welding method has the characteristics of small arc column diameter, small arc heel area, and it is often difficult to cover all the droplets at the end of the welding wire. Therefore, the transfer resistance (spot force) of the droplet is large, making the droplet coarser, the axial property of the transition path becomes worse, and the spatter rate is large;

② The welding area is well protected. The density of CO2 is the largest among common shielding gases. In addition, the volume of CO2 gas increases after thermal decomposition, so the protection is good;

③ The energy is relatively concentrated, and the penetration capacity is large;

④ Low production cost and energy saving;

⑤ In terms of process and technology, the welding area has good visibility, which is convenient for observation and operation; The welding heat affected zone and welding deformation are small; The volume of molten pool is small, the crystallization speed is fast, and the all position welding performance is good; The advantage of low sensitivity to rust.

Metallurgical characteristics:

① Oxidation of alloy elements During CO2 welding, under the action of high arc temperature, CO2 will be decomposed into CO, O2 and O. Under welding conditions, CO is insoluble in metals and does not participate in the reaction. CO2 and O have strong oxidation, which makes Fe and other alloy elements oxidized.

② Deoxidation and alloying of weld metal? Generally, a certain amount of deoxidizer is added to the welding wire for deoxidization. In addition, the remaining deoxidizer is left in the weld as an alloy element to compensate for the loss of oxidation and burning and ensure the chemical composition requirements of the weld.

Droplet transfer:

① Short circuit transfer (short arc, thin wire, small current) is suitable for all position welding of thin plates;

② Fine grain transition, thick wire, long arc, high current welding;

③ Submerged arc droplet transfer (rarely used).

Scope of application:

At present, CO2 gas shielded welding is widely used in locomotive manufacturing, ship manufacturing, automobile manufacturing, coal mining machinery manufacturing and other fields. It is suitable for welding low carbon steel, low alloy steel, low alloy high strength steel, but not suitable for welding nonferrous metals and stainless steel. Although some data show that CO2 gas shielded welding can be used for welding stainless steel, it is not the first choice for welding stainless steel.

5. Plasma arc welding

The water cooling nozzle and other measures can reduce the cross-sectional area of the arc column area, and significantly increase the arc temperature, energy density and plasma flow rate. This kind of arc that compresses the arc column with external constraints is called plasma arc.

Plasma arc is a special form of arc, which is a kind of arc with high energy density and is still a gas conductive phenomenon. Plasma arc welding is a method that uses the heat of plasma arc to heat and melt the workpiece and base metal to realize welding.

Plasma arc welding includes keyhole plasma arc welding and micro plasma arc welding.

Perforated plasma arc:

When the welding current is 100~300A, the joint does not need to be beveled and there is no gap left. During welding, the plasma arc can completely penetrate the weldment and form a small through hole, the molten metal is squeezed around the hole, the arc moves, the hole moves with it, and a weld is formed behind it, so as to realize one side welding and two sides forming at one time. The upper limit of plate thickness that can be welded by this method is: 7mm for carbon steel and 10mm for stainless steel.

Micro plasma arc:

The welding current is 0.1~30A, and the welding thickness is 0.025~2.5mm. In addition, there are also fusion plasma arc welding applicable to copper and copper alloy welding, which can be used for deep penetration welding of thick plates or high-speed welding of thin plates, as well as MIG arc welding of surfacing welding, which can solve the AC (variable polarity) plasma arc welding and other process methods of aluminum alloy plasma arc welding. The main process parameters of plasma arc welding include welding current, welding speed, shielding gas flow, ion gas flow, torch nozzle structure and aperture, etc.

Plasma arc cutting: It refers to the cutting method that uses the high temperature and high speed arc flow of the plasma arc to locally melt the metal of the notch so as to evaporate, and uses the high-speed air flow or water flow to blow the melted material away from the substrate to form the notch.

characteristic:

① The plasma arc has high energy density, high arc column temperature, and strong penetration capability. The steel with a thickness of 10~12mm can be formed on both sides without groove, and can be welded through once. The welding speed is fast, the productivity is high, and the stress deformation is small;

② The weld section is in the shape of wine cup, without finger penetration;

③ The arc straightness is good, and the fluctuation of the weld pool is small due to the fluctuation of the arc length;

④ The arc is stable 0.1A and still has a relatively flat static characteristic. With a constant current source, the thin plate can be well welded (0.1mm);

⑤ The tungsten electrode shrinks inward to prevent tungsten inclusion in the weld;

Small hole welding technology is adopted to realize single side welding and double side forming;

⑦ The equipment is complex and the gas consumption is large, so it is only suitable for indoor welding. The accessibility of welding gun is worse than TIG;

⑧ The arc diameter is small, so the axis of the welding gun needs to be more accurately aligned with the centerline of the weld.

Metallurgical reaction: single, only evaporation.

Power supply: steep drop power supply, DC positive connection; AC and steep drop power supply shall be used for aluminum magnesium welding, and arc striking and stabilizing measures shall be taken. Welding materials: shielding gas, tungsten electrode.

Scope of application: It is widely used in industrial production, especially in the welding of copper and copper alloys, titanium and titanium alloys, alloy steel, stainless steel, molybdenum and other metals used in military industry such as aerospace and cutting-edge industrial technology, such as titanium alloy missile shells, some thin-walled containers on aircraft, etc.

6. Tubular wire arc welding

Tubular wire arc welding also uses the burning arc between the continuously fed welding wire and the workpiece as the heat source, which can be considered as a type of MIG welding. The welding wire used is tubular welding wire, and the pipe is filled with various components of welding flux. During welding, apply shielding gas, mainly CO2. When heated, the flux decomposes or melts, playing the role of slagging to protect the molten pool, alloying and arc stabilization.

In addition to the above advantages of MIG welding, tubular wire arc welding has more advantages in metallurgy due to the role of flux in the pipe. Tubular wire arc welding can be applied to the welding of various joints of most ferrous metals. Tubular wire arc welding has been widely used in some industrial advanced countries. “Tubular welding wire” is now referred to as “flux cored wire”.