The welding process of fusion welding is to use a heat source to locally heat the workpiece to a molten state to form a molten pool, and then as the heat source moves forward, the liquid metal in the molten pool cools and crystallizes to form a weld.

The welding process includes thermal process, metallurgical process and crystallization process.According to the different heat sources, it can be divided into: gas welding, arc welding, electroslag welding, laser welding, electron beam welding, plasma arc welding, etc. The following takes arc welding as an example to amusement equipment introduce.

1.Welding arc

1）Generation of welding arcs.The welding arc is an intense, long-lasting and stable gas discharge between the electrode and the workpiece. When the welding arc starts, the electrode and the workpiece contact instantaneously to form a short circuit, and the strong current generates strong resistance heat to melt or even evaporate the contact point.When the electrode is lifted, under the action of the electric field, the hot metal emits a large number of electrons, and the electrons collide with the gas to ionize them, and the positive and negative ions and electrons form an arc.

2）The structure of the welding arc

①cathode region：2400K。In the electron emission area, the heat accounts for about 36%, and the average temperature is 2400K.

②Anode area：2600K。The area bombarded by electrons accounts for about 43% of the heat, with an average temperature of 2600K.

③Arc column area：The area between the yin and yang poles is almost equal to the arc length, and the heat accounts for 21%.

2.The base metal and the electrode are melted by the high temperature of the arc to form a metal melt of 6000~8000K.

The pool will carry out physical and chemical processes such as melting, oxidation, reduction, slagging, refining and alloying.The effect of metal and oxygen has the greatest impact on welding quality, and oxygen reacts with various metals

Fe +0→FeO       Mn+O→MnO

Si+20 →SiO₂         2Cr+30 →Cr₂2O₃

2Al+30 →Al₂O₃

Oxides that can be dissolved in liquid metal (such as ferrous oxide) are precipitated due to the decrease in solubility during condensation, which seriously affects the quality of welds；However, most metal oxides (such as silicon and manganese compounds) are insoluble in liquid metal and can float with slag to purify the molten pool and improve the quality of the weld.Hydrogen easily dissolves into the molten pool, forms pores in the weld, or accumulates at weld defects to cause hydrogen embrittlement.

Secondly, nitrogen in the air dissolves in a large amount of liquid metal at high temperature, and the solubility of nitrogen decreases during cooling and crystallization.；The precipitated nitrogen forms pores in the weld, and some are also precipitated in the form of needle-like nitrides；The nitrogen content in the weld increases, so that the strength and hardness of the weld increase, and the plasticity and toughness decrease sharply.

Compared with the general metallurgical process, the metallurgical process of the weld has the following characteristics：

1）The metal molten pool is small in size, the molten pool is in liquid state for a short time, and the metallurgical reaction is insufficient.

2）The high temperature of the molten pool makes the metal elements burn and evaporate strongly, and the cooling speed is fast, and it is easy to generate stress and deformation, and even crack.

In order to ensure the quality of the weld, measures can be taken from two aspects:

1）To reduce harmful elements entering the molten pool, mechanical protection is mainly used, such as electrode coating, submerged arc welding flux and shielding gas (carbon dioxide, argon, etc.) for gas shielded welding.

2）Remove harmful elements that have entered the molten pool, and increase alloying elements, such as adding alloying elements to the electrode coating for deoxidation, dehydrogenation, desulphurization, and alloying.

3.Different points have different thermal cycles, that is, the maximum heating temperature, heating rate and cooling rate are different.The parameters that have an important influence on the welding quality are:

the maximum heating temperature, the dwell time above the overheating temperature of 1100°C, and the cooling rate.It is characterized by fast heating and cooling rates. For easily quenched steel, air-cooled quenching occurs after welding, and for other materials, welding deformation, stress and cracks are easy to occur.

4.The microstructure and properties of welded joints take low carbon steel as an example to illustrate the changes in metal microstructure and properties caused by the welding process.

①Affected by the welding thermal cycle, the area where the structure or properties of the base metal near the weld change is called the welding heat affected zone.

②The boundary line between the fusion weld and the base metal is called the fusion line.

③On both sides of the fusion line, there is a very narrow transition zone between the weld and the heat-affected zone, called the fusion zone.

1）Weld area. The crystallization starts from the unmelted half grain at the bottom of the molten pool, and grows perpendicular to the fusion line to the center of the molten pool,During the crystallization process, component segregation will occur at the last crystallization site.

The weld structure is the as-cast structure of liquid metal crystallization, with coarse grains, segregation of components, and non-dense structure.However, due to the small molten pool, fast cooling, strict control of chemical composition, the content of carbon, sulfur and phosphorus are lower, and with containing certain alloying elements, so the mechanical properties of the weld metal can be not lower than that of the base metal.

2）Fusion zone.The chemical composition is uneven and the structure is coarse, which is often a coarse overheated structure or a coarse hardened structure, which reduces the strength, the plasticity and toughness are extremely poor, cracks and brittle failure occur, and the performance is the worst in the welded joint.

3）Heat affected zone.The maximum heating temperature of each point in the heat-affected zone is different, and the structure changes are also different.

①Over heat zone：In the region heated to above 1100 °C, the grains are coarse and even overheated structure is generated.The plasticity and toughness decreased significantly, and it was the part with the worst mechanical properties in the heat-affected zone.

②Normalizing area:In the region where the maximum heating temperature is from Ac3 to 1100 °C, the normalized structure with finer grains can be obtained by air cooling after welding, and the mechanical properties are better.

③Partial phase transition region：In the region with the highest heating temperature from Ac1 to Ac3, only part of the structure undergoes phase transformation, the grains are not uniform, and the performance is poor.

④Re-crystallization zone：The temperature range of this zone is from Ac1 to 450~500℃. The weldment that has been cold-deformed before welding has lattice distortion and broken grain structure in the base metal. When heated to this temperature, it will be refined by recovery and recrystallization, and its mechanical properties will be improved.The fusion zone and overheating zone have the worst performance, and the smaller the heat-affected zone, the better. The influencing factors include welding method, welding specification, and joint form.

5.Factors affecting the performance of welded joints The mechanical properties of welded joints depend on their chemical composition and organization.Here the following ：

1）Welding materials：Both the wire and the flux affect the chemical composition of the weld.

2）Welding method：On the one hand, it affects the thickness of the tissue, and on the other hand, it affects the content of harmful impurities.

3）Welding process：During welding, the general term for each physical quantity (such as welding current, arc voltage, welding speed, heat input, etc.) selected to ensure welding quality is called welding parameters.；Heat input refers to the energy input from the welding energy to the unit length of the weld during fusion welding. Obviously, the welding parameters affect the input energy of the welded joint and the welding thermal cycle, thereby affecting the size of the heat-affected zone and the thickness of the joint structure.

4）Post weld heat treatment：Such as normalizing, can refine the joint organization and improve performance.

5）Connector form：The thickness of the workpiece, the temperature of the welding environment and the preheating will affect the cooling rate after welding, thereby affecting the structure and performance of the joint.

6.Welded joints Welded joints can usually be divided into butt joints, lap joints.

Welding groove form refers to the pre-processed structural form at the connection of the welded metal parts, which is generally determined by the welding process itself.The main factors considered in the selection of groove form are: to ensure penetration；Fill the weld with as little metal as possible；amusement ride Easy to weld, improve working conditions; reduce welding deformation.

1）Butt connector.Two metal parts are placed in the same plane (or in the curved surface) so that their edges are opposite, and the joints that are welded along the straight line (or curve) of the edge are called butt connector .The groove forms of the butt connector can be divided into no groove, V-shaped groove, X-shaped groove, single U-shaped groove and double U-shaped groove.

2）Lap Connector.A connector where two sheets are overlapped and fillet welded at the end or side is called a lap joint.Lap joints can be welded without beveling, and the assembly requirements are relatively loose.

3）Corner connector and T-connector.The two members form a right angle or a certain angle, and the joints welded at their connecting edges are called fillet connector.The connector where the two members are welded together in a T-shape is called a T-connector.Single-sided welded fillet connector and T-connector have extremely low ability to withstand reverse bending moments and should be avoided.Fillet connector and T-connector have V-shaped, unilateral V-shaped, U-shaped, K-shaped and other groove forms, which should be selected according to the thickness of the plate and the importance of the work.