Welding Fume: Formation Mechanism, Hazard and Protection

Summary

As a basic process, welding is widely used in various fields of industrial manufacturing.

Arc light, electromagnetic radiation, toxic gas and smoke particles generated during welding and operation not only pollute the environment, but also cause great harm to the health of employees.

Welding fume is the associated hazard that has the most complex impact on the health environment and is the most difficult to control in welding production.

Therefore, it is of great significance to carry out research on the control of welding fume to improve the welding production environment and protect the health of employees.

This article summarizes the formation mechanism, hazards and treatment measures of welding fume, analyzes the problems existing in the treatment of welding fume in engineering application, and points out the development direction of the treatment of welding fume.

Related reading: The Ultimate Guide to Welding

Welding Fume: Formation Mechanism, Hazard and Protection 1

Preface

As a basic process of the modern manufacturing industry, welding technology is gradually developing from the traditional single connection technology to a multi-discipline hot forming technology integrating electricity, machinery, materials and computers, playing an irreplaceable role in engineering machinery, water conservancy and hydropower, shipbuilding, transportation, military equipment and other fields.

Arc radiation, high temperature, noise, welding fume and toxic gas exist in the welding process, which not only pollute the environment but also endanger the health of employees.

For arc light, high temperature, noise, etc., good protection effect can be obtained by using masks and earplugs, but the treatment effect of welding fume as a primary carcinogen is not ideal.

Welding fume mainly contains toxic gases and soot particles, among which soot particles may cause asthma, bronchitis, pneumonia, pulmonary edema, acute poisoning, nervous system diseases, and even pneumoconiosis, metal fume heat, respiratory function changes, cancer and other diseases.

Toxic gases such as asphyxiating gas CO, irritating gases such as ozone, fluoride, chloride, sulfur dioxide, and nerve toxic gases such as nitrogen oxide, phosgene will cause employees to have headache, dizziness, cough, expectoration, chest pain, tinnitus, tension and anxiety.

Due to the welding smoke and other associated hazards, the welding environment is bad, which to some extent leads to the decline of the number of people willing to engage in welding work year by year, and becomes one of the prominent problems restricting the healthy development of the welding industry.

In recent years, with the substantial increase of welding manufacturing workload and the popularity of efficient welding methods, flux cored wire and other high dust-producing processes and materials, the occupational problems caused by welding fume have become increasingly prominent.

In the early days, the allowable concentration of smoke and dust in workshop air in China was 6mg/m3.

Welding Fume: Formation Mechanism, Hazard and Protection 2

At present, China Welding Association has reduced the allowable concentration of smoke and dust to 4mg/m3, and put forward clear requirements for the amount of dust generated by welding materials;

Considering the hazards of welding fume, the American Welding Society has also developed a corresponding ventilation instruction manual to reduce the welding fume concentration in the workshop.

At the beginning of this century, Japan formulated relevant standards to limit the concentration of smoke and dust in the welding workshop, and actively promoted the research and development of new welding materials.

However, in actual production, especially in places with high welding intensity and relatively closed space, for example, the soot concentration in the shipyard workshop is as high as 9~18 mg/m3, and even reaches 38~312 mg/m3 in the cabin sectional confined space, which is far beyond the minimum soot concentration required by the standard, and is very harmful to human health.

It can be seen that reducing the harmful substances in the smoke and dust and improving the working environment of welding practitioners have become an important problem that needs to be solved urgently in the welding industry.

At present, the treatment of welding fume at home and abroad is mainly carried out in the following three directions:

(1) Strengthen personal protection;

(2) Optimize welding process and welding materials;

(3) Ventilation and smoke exhaust.

By analyzing the generation mechanism of welding fume and comparing the advantages and disadvantages of current mainstream fume control measures, this article proposes to establish a new concept of welding fume control through intelligent manufacturing, which provides a certain reference for improving welding fume control.

In view of the physical and chemical hazards of welding fume to employees, in order to better protect the health of employees and maintain the air quality of welding operation environment, it is necessary to start from the generation mechanism and influencing factors of welding fume, and through the exploration of the generation, generation, growth and polymerization process of welding fume, control and protect the fume from the source.

At the same time, it also lays a theoretical foundation for the admission standard of welding fume in industry.

Formation mechanism of welding fume

The earliest research believed that the generation mechanism of welding fume was the process of overheating evaporation oxidation condensation, as shown in Fig. 1.

Welding Fume: Formation Mechanism, Hazard and Protection 3
Fig.1   Schematic diagram of the fume formation process

In the welding process, the temperature of the arc center is high, and the evaporation of liquid metal and non-metallic substances generates high-temperature steam, and maintains a certain particle concentration.

When the high-temperature steam is located in the low temperature area at the edge of the arc, it is rapidly oxidized and condensed to generate “primary particles”.

The primary particles are basically spherical in shape, with a diameter of 0.01~0.4μm, mainly 0.1μm.

Under the action of static electricity and magnetism of the primary particles themselves, with the decrease of temperature, the primary particles will polymerize to form “secondary particles” and diffuse in a certain way.

Shi Yuxiang, Wuhan Jiaotong University of Science and Technology, made a more in-depth study on the formation mechanism of welding fume.

Aiming at the transformation process of steam and fume, he proposed the aerosol mechanism of welding fume.

He pointed out that the nucleation mechanism of welding aerosol particles near the arc was divided into homogeneous nucleation and heterogeneous nucleation.

The spectral distribution, morphology, composition and structural characteristics of primary particles were systematically investigated experimentally and theoretically by direct sampling electron microscopy and DMPS.

It was proved that Fe3O4 crystals were mainly composed of particles of 0.01μm scale of welding aerosols, and smoke particles of 0.1μm scale had two types of crystal structures-spinel type and fluoride type, which were formed by the heterogeneous condensation mechanism of vapor particle transition.

The soot particles above 1μm scale are mainly formed by the bubble particle transition mechanism.

At the same time, the model of welding arc particle nucleation zone is proposed, which is of great significance for analyzing the formation process of welding aerosol particles.

The soot particles after welding grow up in the diffusion process through aggregation and fusion.

The fusion process is a process in which several primary particles are fused into a single large particle.

The feature is that the total surface area of a single large particle after fusion is less than the sum of the surface areas of primary particles, and there is no boundary between particles;

Compared with the fusion process, the aggregation process is composed of dozens or even hundreds of primary particles, which adhere on the surface and have obvious boundaries between particles.

Regardless of particle aggregation or fusion, particle size, shape and quantity concentration in welding fume will change.

Hazards of welding fume

The welding process will produce a large number of smoke and toxic gases harmful to human health.

A large part of welding smoke is suspended in the air in the form of particles and diffuses in the air. The other part is dispersed in the air in the form of toxic gas.

The soot particles produced in the welding process mainly exist in the form of metal oxides, which have the characteristics of complex composition, high viscosity, high temperature and non-uniform particle size.

In general welding production occasions, the breathing capacity of the welder is about 20 L/min, so the breathing capacity for one year is 2300m3;

In the poor welding production environment, 100g particles will be inhaled a day, and 2.5kg harmful substances will be inhaled after 25 years of work.

Common metal oxide particles and their hazards during welding are shown in Table 1.

Table 1   Hazards of particles in welded fume

MaterialSourceHazard
Ferric oxideFrom filler material and base metalIron pneumoconiosis or iron deposition disease caused by long-term inhalation
Aluminum oxideWelding process from aluminum base materialsDust deposition in the lung causes pneumoconiosis
Manganese oxideWelding process from manganese containing welding materialsIrritating to the respiratory tract, causing pneumonia. Long term exposure will damage the nervous system
OxideBasic electrode or coated wireIrritating to gastric mucosa, causing bone damage
Barium compoundBarium containing welding fillerToxicity, causing potassium deficiency in human tissues
Nickel oxide  Welding materials of pure nickel or nickel base alloyNasal mucosa damage and lung cancer, Class I carcinogen

According to different particle size, welding fume will cause different harm to human body.

The Yang Lijun team of Tianjin University made statistics on the particle size distribution law of MIG welding fume, analyzed the influence of welding parameters and droplet transfer form on the particle size of fume, and the research results showed that: the soot particles were quasi quantized distribution characteristics, the particle size was mostly within the range of 0.1~1μm, accounting for more than 85%, and the particle size less than 0.1μm accounted for about 10%.

At the same time, welding process, droplet transfer form and welding parameters have certain effects on the particle size of soot.

With the decrease of welding voltage, the particle size of soot tends to decrease.

Gomes J F et al. calculated the particle size of welding fume generated in the welding process and found that its size was basically about 0.5μm.

Relevant research shows that most smoke particles with a diameter of more than 10μm in the air are deposited in the nasopharynx, smoke particles with a diameter of less than 10μm will be inhaled by the human body, smoke particles with a diameter of 2~10μm can be discharged by the human body, but smoke particles with a diameter of less than 0.5μm will be deposited in the lungs, which is difficult to be discharged.

Table 2 shows the residual amount of TiO2 with different particle sizes in rat lung tissue for several days (unit: μg) .

The smaller the size, the stronger the penetrability of the smoke particles, and the more difficult it is to be discharged from the body.

At the same time, the smoke particles will disperse into smaller primary particles in the human alveoli, which will aggravate the harm to the human body.

Table 2   Content of different sizes of TiO2 in rat lung tissue (μg)

Time/dayTiO2-D(0.03μm)TiO2-F(0.25μm)
1347.7±13.1324.3±6.1
29202.8±23.0172.8±12.1
59140.9±22.6128.5±16.6

Lauryn M F et al. found that Fe2O3 is the only metal oxide that promotes lung cancer, and the trend of metal oxide that causes lung inflammation is Fe2O3>Cr2O3+CaCrO4>NiO.

Among them, the toxic effect of Fe2O3 on the lung is continuous, and the toxic effect of Cr2O3+CaCrO4 on the lung is acute.

Roth J A et al. found that long-term exposure to welding fume and inhalation of excessive manganese would cause adverse effects on human health, including damage to lung, liver, kidney and central nervous system, and male workers had a higher risk of infertility.

Long term exposure to the environment with manganese concentration exceeding 1 mg/m3 will lead to an increased risk of manganese poisoning similar to Parkinson’s disease.

In addition to many harmful smoke particles, welding will also produce many harmful gases, which contain carbon monoxide, nitrogen oxides, ozone, phosgene, hydrogen fluoride and other harmful components.

Table 3 lists the hazards of harmful gases in some welding fume to human body.

Table 3   Harmful gases and hazards in welding fumes

Harmful gasProduceHazard
Carbon monoxideThe welding flux or shielding gas is produced by carbon dioxide combustion and decomposition.Headache, dizziness, confusion, suffocation
Nitric oxideIt is produced by the action of ultraviolet ray generated by electric arc on nitrogen in the airIrritating eyes and respiratory tract, leading to pulmonary congestion
OzoneIt is produced by the interaction of ultraviolet ray generated by arc and nitrogen in airThe respiratory tract feels dry, causing headache, fatigue, pulmonary congestion and pulmonary disease
PhosgeneIt is produced by decomposition of fluoride containing solvent, polytetrafluoroethylene, surface coating, etc.Irritating respiratory tract, nose and eyes, toxic, leading to pulmonary edema.
Hydrogen fluorideElectrode coating and fluxIrritating eyes, nose, throat, lung congestion, bone changes

Protection of welding fume

In order to purify the welding working environment and protect the health of employees, comprehensive treatment should be carried out from the source to reduce emissions, strengthen protection and technological innovation, so as to ensure that the concentration of harmful substances generated by welding is within the allowable concentration range.

At present, common treatment measures include personal protection, optimization of welding process and welding materials, and ventilation and smoke exhaust.

1. Personal protection

The personal protection measures for welding fume are mainly to wear ventilation and dust removal masks and other respiratory protection equipment to reduce the damage of welding fume to the welding workers.

Fig. 2 shows four filtering mechanisms of respirators for smoke and dust particles of different sizes.

Welding Fume: Formation Mechanism, Hazard and Protection 4
Fig.2   Schematic diagram of the filtering mechanism of masks

(1) Gravity effect:

When the air containing dust particles passes through the filter material fiber layer, the particles will shift away from the airflow direction under their own gravity, thus depositing on the filter material.

Generally, for dust particles larger than 1μm in size, particles with smaller sizes can be ignored due to gravity compared with gas flow rate and other factors, and basically have no filtering effect.

(2) Interception effect:

The fibers in the filter material are stacked irregularly and intertwined with each other.

When the high-speed smoke particles in the air enter the fiber material, they contact with the fiber surface and bond to the fiber surface of the filter material, so as to achieve effective interception of particles.

(3) Inertia effect:

Because the air flow will frequently change its direction when passing through the entire filter material, smoke particles will be deposited by breaking away from the streamline and hitting the fiber surface under the effect of inertial force.

In particular, smoke particles with a particle size of 0.5~1.0μm are mainly intercepted by the inertial effect.

(4) Diffusion effect:

Particles with a diameter less than 0.1 μm at room temperature mainly move in Brownian motion, and the smaller the particles, the easier they are to be removed;

Particles larger than 0.5μm are mainly in inertial motion, and the larger the particles are, the easier they are to be removed;

The particles between 0.1μm and 0.5μm have no obvious diffusion and inertia effects and are difficult to remove.

During welding, the size of smoke particles is about 10-3~102μm in five orders of magnitude, of which 0.1~0.5μm is the most penetrating.

At present, no respirator can achieve ideal filtering effect on all smoke particles.

At present, personal protective equipment has poor protection effect on toxic gas, and the prevention of toxic gas cannot be achieved only by personal protection.

Welding Fume: Formation Mechanism, Hazard and Protection 5

2. Optimization of welding process and welding materials

The optimization of welding process and welding materials mainly refers to the control of welding fume by reducing the generation rate of welding fume and the content of toxic substances in the fume.

There are many factors affecting the amount of welding dust.

At present, the research on the amount of welding dust at home and abroad mainly focuses on two aspects:

One is to study the influence of different welding methods and process parameters on the amount of dust, and the other is to study the influence of the composition of welding wire, coating and shielding gas on the amount of dust.

2.1 Impact of welding process on dust emission

The amount of dust generated by different welding methods is different.

Under the same process parameters, the dust generation rate of MIG welding is much higher than that of non MIG welding, while the smoke generated by submerged arc welding is very little.

Related reading: MIG vs TIG Welding

Under the same specification, the amount of dust generated by different welding methods is shown in Table 4.

Generally speaking, under the same welding method, the amount of dust generated increases with the increase of welding current and voltage.

AC welding produces more dust than DC welding, and the amount of dust decreases with the increase of welding speed.

Table 4   Dust generation rate of different welding methods

Welding processGeneration rate/(mg·min-1)
FCAW900~1300
SMAW300~800
MIG/MAG200~700
GTAW3~7
SAW3~6

Due to the large amount of dust generated by flux cored wire welding, shielded metal arc welding, and MIG welding, which has a serious impact on welders and the environment, it has become the focus of researchers at home and abroad.

Shi Qian of Lanzhou University of Science and Technology and others conducted relevant research on the amount of dust generated by self-shielded flux cored wire welding under different process parameters.

In small specification welding, the amount of dust generated by short circuit transition and slag column transition increased significantly due to the increase of spatter;

In large specification welding, due to the increase of heat input, the evaporation rate of droplet and base metal heated is accelerated, and the amount of dust generated is increased.

At this time, the droplet transfer mode has little effect on the amount of dust generated.

This result has also been verified in Zhang Junqiang’s research on the generation mechanism of smoke and dust of self shielded flux cored wire, that is, the aggregate smoke and dust generated in the splash smoke and dust area and the droplet smoke and dust area will greatly increase the total amount of smoke and dust.

Yamamoto et al. used CO2 as shielding gas when using 26% flux cored wire for welding.

With the increase of welding current, the amount of welding dust decreases gradually.

At the same time, the author has developed an advanced pure carbon dioxide gas shielded arc welding process by using the method of pulse current to control the droplet.

By using the pulse current and using high current to melt the welding wire, the current is reduced during the droplet transfer, so as to ensure that the droplet can be smoothly transferred to the molten pool with a constant length, realizing the regular formation and separation of metal droplets, and reducing the amount of dust generated by 50% through this method.

Scotti studied the influence of arc length, droplet diameter and short circuit current on the amount of dust generated by GMAW through the control variable method.

The results show that during short circuit transition, the increase of droplet diameter, short circuit current and arc length will lead to the increase of the amount of dust generated.

Higher short circuit current will make the metal evaporation on the surface of the liquid bridge more intense when the droplet enters the molten pool, thus increasing the amount of dust generated.

When they work together, the increase in dust emission is more obvious.

Bu Zhixiang of Hubei University of Technology and others took CO2 gas shielded welding of solid welding wire as the research object, adopted welding current, welding voltage and welding speed as the three factors of the experiment, and took the welding dust rate and amount as the experimental indicators, carried out an orthogonal experiment.

Through variance analysis and range analysis of orthogonal test data, the results show that the main factors affecting the formation rate of welding fume are welding current and welding voltage, and the welding speed has no significant effect on the formation rate of welding fume;

When the welding voltage is 22~24 V, the welding current is 290~320 A, and the welding speed is 26 cm/min, the amount of welding dust is the lowest.

The amount of welding fume is not only related to the filler material, but also closely related to the composition of shielding gas.

K. R. Carpenter et al. added O2 and CO2 into the protective gas of GMAW, and found that adding 2% O2 into the binary Ar-CO2 mixture had no effect on the dust generation rate.

When O2 in the ternary mixture increases, the dust generation rate increases at the level of 5% CO2, but does not increase significantly at the level of 12% CO2.

Therefore, the amount of dust generated can be controlled by changing the amount of CO2 added in the mixed gas.

Li Zhuoxin’s team from Beijing University of Technology studied the Cr (Ⅵ) content in the stainless steel welding fume.

The results showed that the stronger the oxidation of the shielding gas during gas shielded welding, the greater the mass fraction of Cr (Ⅵ) in the fume.

When the MAG current was 150~250 A, Cr (Ⅵ) increased with the increase of electric current.

When GMAW was welding stainless steel, the mass ratio of Cr (Ⅵ) in the short circuit transfer fume to total Cr was higher than that of the jet transfer fume.

Vishal Vats pointed out in the report of the interim meeting of the Eighth Committee of the 2022 IIW that adding oxygen to the GMAW protective gas will promote the formation of Cr3+and Cr6+, and will also lead to the increase of Mn, Fe and Ni harmful elements in the smoke.

The above research shows that the amount of welding dust is related to the welding process parameters, and the amount of dust can be controlled by selecting the process parameters conducive to health and environment.

However, there is a coupling effect between welding process and welding quality, which can reduce welding smoke at the cost of sacrificing welding quality and efficiency.

There are great limitations in the actual application process. With the extensive application of efficient welding methods (double wire/multi wire welding, laser arc hybrid welding) in the engineering field, the requirements for welding specifications are higher, which makes the treatment of welding fume more difficult.

Welding Fume: Formation Mechanism, Hazard and Protection 6

2.2 Effect of welding materials on dust emission

In the welding process, metal oxides produced by welding materials under high temperature are mixed with various carcinogens, and excessive inhalation by operators can cause a variety of diseases.

Therefore, through the development of green welding materials, the harmful components of smoke and dust can be effectively controlled from the source.

The research on green welding materials at home and abroad mainly focuses on three aspects:

(1) By changing the composition of the drug skin, the amount of dust generated by the material is reduced;

(2) Reduce the content of heavy metal elements in smoke and dust during welding;

(3) Welding fume shall be treated by dealloying welding materials.

For the electrode, its coating composition, chemical composition of powder and welding wire steel strip have influence on the amount of dust, and the influencing factors are complex.

Fluorite and sodium silicate play a major role in the dust generation of electrode coating, and their reaction products account for more than 50% of the total amount of smoke and dust.

Materials containing K and Na will increase the amount of dust generation. Silicon calcium alloy and magnesium powder can inhibit the amount of dust generation.

For flux cored wire, Jiang Jianmin of Beijing University of Technology and others found that reducing the content of iron powder in the flux core can reduce the amount of dust generated during welding by 33%~47%.

Mruczek MF report points out that a foreign welding material manufacturer has developed a flux cored wire with low manganese content, which can effectively reduce the content of Mn in smoke, but will lead to poor mechanical properties of the weld.

North T H find that it can significantly reduce the content of Mn element in smoke by adding Mn containing composite particles into the core to prevent Mn oxidation and leaving more Mn in the weld.

Dennis J H et al. added active elements (Zn, Al, Mg) to the flux cored wire, which can significantly reduce the content of Cr6+in welding fume by allowing the active elements to preferentially oxidize.

However, adding Zn to stainless steel welding wire can reduce the Cr content in the smoke, but accelerate the formation rate of smoke.

Mortazavi S B et al. find by reducing the content of K element in welding materials, increasing the content of Li element, reducing potassium chromate through Li to reduce the content of Cr6+in smoke.

In addition, Topham N et al. showed that reducing the content of Na and K in austenitic stainless steel welding materials and adding 30% tetraethyl silane (TEOS) in shielding gas can reduce the content of Cr (VI) in stainless steel welding fume.

In order to reduce the influence of harmful components in welding fume, the method of de alloying of welding materials is often inconsistent with the requirements of mechanical properties, corrosion resistance and wear resistance required by welding structures.

At present, the degree of alloying of the base metal used is very high.

From low-carbon steel to low-alloy steel, and then to high entropy alloy, the degree of alloying is getting higher and higher.

At the same time, adding Mn, Cr, Ni, Mo, Co and other alloy elements to the welding materials (base material+welding wire) can effectively improve the mechanical properties and corrosion resistance of the welding components, increase the service life and broaden the application field of metal materials.

Therefore, it is often unacceptable to treat smoke and dust by dealloying welding materials in actual production.

3. Ventilation and smoke exhaust

Welding Fume: Formation Mechanism, Hazard and Protection 7

Ventilation and smoke exhaust is the most effective treatment method in production at present, mainly including two types of treatment methods:

One is to control the further diffusion of welding smoke and harmful gases by installing local smoke extraction devices or using smoking welding guns on the welding station, and to control them from the source;

The other is to improve the working environment of the welding workshop through comprehensive ventilation and displacement ventilation of the plant.

3.1 Local smoke extraction

At present, the mainstream local smoke extraction methods mainly include smoking welding gun and local ventilation and dust removal.

The principle of smoking welding gun is shown in Fig.3.

The smoke and dust will be captured by the suction generated by the smoking mouth, so as to avoid environmental pollution by diffusion.

Compared with other local processing equipment, smoking welding gun can change the position and angle of the welding gun at any time, and the welder’s operation is less limited.

Welding Fume: Formation Mechanism, Hazard and Protection 8
Fig.3   Schematic diagram of a smoking torch

Local ventilation refers to the use of professional dust hoods to directly suck away welding smoke from the welding operation area, and then discharge the collected smoke to the outside after dust reduction treatment. Its principle is shown in Fig. 4.

Welding Fume: Formation Mechanism, Hazard and Protection 9
Fig.4   Schematic diagram of local ventilator

Relevant research shows that the efficiency of local ventilation is better than that of general ventilation.

Flynn MR compared and analyzed the dedusting effect of the local ventilation dedusting system under three conditions of indoor no ventilation, natural wind and mechanical ventilation.

The results showed that the dedusting efficiency of the fan combined with the local ventilation dedusting system was the highest.

Meeker J D conducted an experiment with a commercial local ventilation and dust removal equipment.

It was found that after using the local ventilation and dust removal equipment, the concentration of Mn in the air smoke decreased by 25%, the concentration of particulate matter decreased by 40%, and the concentration of Cr6+decreased by 68%.

Thus, local ventilation and dust removal is a very effective ventilation method.

However, local smoke extraction equipment is only suitable for the welding of small-size workpieces, which is quite limited in the application of heavy structure welding workshops.

Because the station of heavy structure is moving during welding, and the smoke and dust points are constantly changing, it is difficult to give consideration to the overall space by using local dedusting.

3.2 General ventilation and displacement ventilation

General ventilation, also known as dilution ventilation, refers to dilution of indoor polluted air with clean air through doors, windows and roofs, so as to reduce the concentration of harmful substances in indoor air, so as to ensure that indoor air environment complies with air quality standards.

Its principle is shown in Fig. 5.

Welding Fume: Formation Mechanism, Hazard and Protection 10
Fig.5   Schematic diagram of general ventilation

General ventilation is only applicable to the environment with low concentration of harmful substances, and is generally used as the auxiliary mode of local ventilation and dust removal.

C. E. Feigley et al. studied and discussed the safety factor K in the formula for calculating the dilution ventilation air volume, and proposed a more objective mixing factor Km based on the experimental measurement value.

Liu Siyan et al. tested and evaluated the concentration of chemical hazards in the welding workshop before and after mechanical ventilation treatment.

After ventilation treatment, the content of manganese and its compounds, welding fume, ozone, carbon monoxide and nitrogen oxides in the air of the workshop decreased, among which manganese and its compounds had the best effect, with the concentration reduced by 82%.

Displacement ventilation is developed on the basis of general ventilation, and its principle is shown in Fig. 6.

Welding Fume: Formation Mechanism, Hazard and Protection 11
Fig.6   Schematic diagram of displacement ventilation

Due to the inherent heat source in the welding process, there will be a stable temperature gradient in the welding workshop, which will reduce the wind speed and temperature difference( Δ T=2~4 ℃) of fresh air is directly sent into the indoor working area.

At this time, the air with lower temperature will first go down under the action of gravity, and then slowly spread on the ground to form a layer of fresh air.

With the rise of temperature, it will float up, and continuously take away the polluted air.

In addition, fresh air is continuously sent into the room through the air duct.

The air return opening above the workshop continuously draws indoor air under multiple effects.

The fresh air above the ground in the work area moves slowly upward, and finally forms an upward uniform airflow.

The dirty air in the work area is gradually replaced by the follow-up fresh air, so as to purify the air in the workshop.

The displacement ventilation dedusting method not only saves energy consumption, but also has higher purification efficiency.

R. Nienel et al. studied the displacement ventilation system of large welding plants.

Through studying the distribution of particles formed in the welding process in space, they pointed out that the particle concentration in the personnel activity area at the lower part of the plant was far less than the particle concentration at the upper part of the plant, and demonstrated the efficiency of displacement ventilation in discharging particles from the welding plant.

At present, the research on displacement ventilation is mainly to optimize the air distribution, air supply parameters and outlet position of displacement ventilation through CFD numerical simulation to improve ventilation efficiency and provide theoretical guidance for the optimization design of displacement ventilation.

Conclusion

(1) The generation and hazard of welding fume are determined by complex physical and chemical processes, and comprehensive measures are required for its treatment.

(2) The basic control of welding fume and other dangerous factors cannot be realized completely through passive protection.

(3) The innovation of intelligent and automatic welding process and welding process system has opened up a new way to realize green, efficient welding and clean production.

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