In addition to process factors, other welding process factors, such as groove size and gap size, inclination angle of the electrode and workpiece, and spatial position of the joint, can also affect the weld formation and weld size.
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1. The influence of welding current on welding seam formation
Under certain other conditions, as the arc welding current increases, the penetration depth and residual height of the weld increase, and the penetration width increases slightly. The reasons are as follows:
As the arc welding current increases, the arc force acting on the weldment increases, the heat input of the arc to the weldment increases, and the heat source position moves downward, which is conducive to heat conduction toward the depth of the molten pool and increases the penetration depth. The penetration depth is approximately proportional to the welding current, that is, the weld penetration depth H is approximately equal to Km×I.
2) The melting speed of the arc welding core or welding wire is proportional to the welding current. As the welding current of arc welding increases, the melting speed of the welding wire increases, and the amount of welding wire melted increases approximately proportionally, while the melting width increases less, so the weld reinforcement increases.
3) After the welding current increases, the diameter of the arc column increases, but the depth of the arc penetrating into the workpiece increases, and the moving range of the arc spot is limited, so the increase in melting width is small.
During gas shielded arc welding, the welding current increases and the weld penetration depth increases. If the welding current is too large and the current density is too high, finger-like penetration is likely to occur, especially when welding aluminum.
2. The influence of arc voltage on welding seam formation
When other conditions are certain, increasing the arc voltage will increase the arc power accordingly, and the heat input to the weldment will increase. However, the increase in arc voltage is achieved by increasing the arc length. The increase in arc length increases the arc heat source radius, increases arc heat dissipation, and reduces the energy density of the input weldment. Therefore, the penetration depth slightly decreases while the penetration depth increases. At the same time, since the welding current remains unchanged, the melting amount of the welding wire remains basically unchanged, causing the weld reinforcement to decrease.
Various arc welding methods are used to obtain appropriate welding seam formation, that is, to maintain an appropriate welding seam forming coefficient φ, and to increase the arc voltage appropriately while increasing the welding current. It is required that the arc voltage and welding current have an appropriate matching relationship. . This is most common in metal arc welding.
3. Effect of welding speed on weld formation
Under certain other conditions, increasing the welding speed will lead to a reduction in welding heat input, thus reducing both the weld width and penetration depth. Since the amount of wire metal deposition per unit length of weld is inversely proportional to the welding speed, the weld reinforcement is also reduced.
Welding speed is an important indicator for evaluating welding productivity. In order to improve welding productivity, the welding speed should be increased. However, in order to ensure the required weld size in structural design, the welding current and arc voltage must be increased correspondingly while increasing the welding speed. These three quantities are interrelated. At the same time, it should also be considered that when increasing the welding current, arc voltage, and welding speed (that is, using high-power welding arc and high welding speed welding), welding defects may occur during the formation of the molten pool and the solidification process of the molten pool, such as bite. Edges, cracks, etc., so there is a limit to increasing the welding speed.
4. The influence of welding current type and polarity and electrode size on weld formation
1. Type and polarity of welding current
The types of welding current are divided into DC and AC. Among them, DC arc welding is divided into constant DC and pulsed DC according to the presence or absence of pulses of the current; according to the polarity, it is divided into DC forward connection (the weldment is connected to the positive) and DC reverse connection (the weldment is connected to the negative). AC arc welding is divided into sine wave AC and square wave AC according to different current waveforms. The type and polarity of the welding current affect the amount of heat input by the arc to the weldment, thus affecting the weld formation. It can also affect the droplet transfer process and the removal of the oxide film on the surface of the base metal.
When tungsten arc welding is used to weld steel, titanium and other metal materials, the penetration depth of the weld formed is the largest when direct current is connected, the penetration is the smallest when direct current is reverse connected, and the AC is between the two. Since the weld penetration is the largest during direct current connection and the tungsten electrode burning loss is the smallest, direct current connection should be used when welding steel, titanium and other metal materials with tungsten electrode argon arc welding. When tungsten argon arc welding uses pulsed DC welding, the pulse parameters can be adjusted, so the welding seam forming size can be controlled as needed. When welding aluminum, magnesium and their alloys with tungsten arc welding, it is necessary to use the cathodic cleaning effect of the arc to clean the oxide film on the surface of the base material. It is better to use AC. Since the waveform parameters of the square wave AC are adjustable, the welding effect is better. .
During metal arc welding, the weld penetration depth and width in DC reverse connection are larger than those in direct current connection, and the penetration depth and width in AC welding are between the two. Therefore, during submerged arc welding, DC reverse connection is used to obtain greater penetration; while during submerged arc surfacing welding, DC forward connection is used to reduce penetration. During gas shielded arc welding, the penetration depth is not only larger during DC reverse connection, but also the welding arc and droplet transfer processes are more stable than those during direct current connection and AC, and it also has a cathode cleaning effect, so it is widely used, while DC forward connection and Communication is generally not used.
2. Influence of tungsten tip tip shape, wire diameter and extension length
The angle and shape of the tungsten electrode front end have a great influence on the arc concentration and arc pressure, and should be selected according to the size of the welding current and the thickness of the weldment. Generally, the more concentrated the arc and the greater the arc pressure, the greater the penetration depth and the corresponding reduction in the penetration width.
During gas metal arc welding, when the welding current is constant, the thinner the welding wire, the more concentrated the arc heating will be, the penetration depth will increase, and the penetration width will decrease. However, when selecting the welding wire diameter in actual welding projects, the current size and molten pool shape must also be considered to avoid poor weld formation.
When the extension length of the welding wire in gas metal arc welding increases, the resistance heat generated by the welding current through the extended part of the welding wire increases, which increases the melting speed of the welding wire, so the weld reinforcement increases and the penetration depth decreases. Since the resistivity of steel welding wire is relatively large, the influence of the extension length of the welding wire on the welding seam formation is more obvious in steel and fine wire welding. The resistivity of aluminum welding wire is relatively small and its influence is not significant. Although increasing the extension length of the welding wire can improve the melting coefficient of the welding wire, considering the stability of the melting of the welding wire and the formation of the weld seam, there is an allowable range of variation in the extension length of the welding wire.
5. The influence of other process factors on welding seam forming factors
In addition to the above-mentioned process factors, other welding process factors, such as groove size and gap size, inclination angle of the electrode and workpiece, and spatial position of the joint, can also affect the weld formation and weld size.
1. Grooves and gaps
When arc welding is used to weld butt joints, whether to reserve a gap, the size of the gap, and the form of the groove are usually determined based on the thickness of the welded plate. When other conditions are constant, the larger the size of the groove or gap, the smaller the reinforcement of the welded seam, which is equivalent to a decrease in the position of the weld seam, and at this time the fusion ratio decreases. Therefore, leaving gaps or opening grooves can be used to control the size of the reinforcement and adjust the fusion ratio. Compared with beveling without leaving a gap, the heat dissipation conditions of the two are somewhat different. Generally speaking, the crystallization conditions of beveling are more favorable.
2. Electrode (welding wire) inclination angle
During arc welding, according to the relationship between the electrode tilt direction and the welding direction, it is divided into two types: electrode forward tilt and electrode backward tilt. When the welding wire tilts, the arc axis also tilts accordingly. When the welding wire tilts forward, the effect of the arc force on the backward discharge of the molten pool metal is weakened, the liquid metal layer at the bottom of the molten pool becomes thicker, the penetration depth decreases, the depth of the arc penetrating into the weldment decreases, the arc spot movement range expands, and the melt width increases, and the coheight decreases. The smaller the forward angle α of the welding wire, the more obvious this effect is. When the welding wire is tilted backward, the situation is opposite. When using electrode arc welding, the electrode back-tilt method is often used, and the inclination angle α is between 65° and 80°.
3. Inclination angle of weldment
The tilt of the weldment is often encountered in actual production and can be divided into upslope welding and downslope welding. At this time, the molten pool metal tends to flow downward along the slope under the action of gravity. During uphill welding, gravity helps the molten pool metal move toward the rear of the molten pool, so the penetration depth is large, the molten width is narrow, and the remaining height is large. When the upslope angle α is 6° to 12°, the reinforcement is too large and undercuts are prone to occur on both sides. During downslope welding, this effect prevents the metal in the molten pool from being discharged to the rear of the molten pool. The arc cannot deeply heat the metal at the bottom of the molten pool. The penetration depth decreases, the arc spot movement range expands, the molten width increases, and the residual height decreases. If the inclination angle of the weldment is too large, it will lead to insufficient penetration and overflow of liquid metal in the molten pool.
4. Weldment material and thickness
The weld penetration is related to the welding current, as well as the thermal conductivity and volumetric heat capacity of the material. The better the thermal conductivity of the material and the greater the volumetric heat capacity, the more heat is required to melt unit volume of metal and raise the same temperature. Therefore, under certain conditions such as welding current and other conditions, the penetration depth and width will be Just decrease. The greater the density of the material or the viscosity of the liquid, the more difficult it is for the arc to displace the liquid molten pool metal, and the shallower the penetration depth. The thickness of the weldment affects the conduction of heat inside the weldment. When other conditions are the same, the thickness of the weldment increases, the heat dissipation increases, and the penetration width and penetration depth decrease.
5. Flux, electrode coating and shielding gas
Different compositions of flux or electrode coating lead to different polar voltage drops and arc column potential gradients of the arc, which will inevitably affect the formation of the weld. When the flux density is small, the particle size is large, or the stacking height is small, the pressure around the arc is low, the arc column expands, and the arc spot moves in a large range, so the penetration depth is small, the melting width is large, and the residual height is small. When welding thick parts with high-power arc welding, using pumice-like flux can reduce the arc pressure, reduce the penetration depth, and increase the penetration width. In addition, the welding slag should have appropriate viscosity and melting temperature. If the viscosity is too high or the melting temperature is high, the slag will have poor air permeability, and it is easy to form many pressure pits on the surface of the weld, and the surface deformation of the weld will be poor.
The composition of the shielding gas (such as Ar, He, N2, CO2) used in arc welding is different, and its physical properties such as thermal conductivity are different, which affects the polar pressure drop of the arc, the potential gradient of the arc column, the conductive cross section of the arc column, and the plasma flow force. , specific heat flow distribution, etc., all of which affect the formation of the weld.
In short, there are many factors that affect weld formation. To obtain good weld formation, you need to select based on the material and thickness of the weldment, the spatial position of the weld, the joint form, the working conditions, the requirements for joint performance and weld size, etc. Appropriate welding methods and welding conditions are used for welding, and the most important thing is the welder's attitude towards welding! Otherwise, the welding seam formation and performance may not meet the requirements, and various welding defects may even occur.
Post time: Feb-27-2024
