In addition to process factors, other welding process factors, such as groove size and gap size, inclination angle of electrode and workpiece, and spatial position of joint, can also impact weld formation and weld size.
Influence of Welding Current on Weld Formation
Under certain conditions, as the arc welding current increases, the penetration depth and reinforcement of the weld seam increase, and the weld width increases slightly. The reasons are as follows:
1) As the welding current of arc welding 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 the conduction of heat in the depth direction of the molten pool and increases the penetration depth. The penetration depth is approximately proportional to the welding current. The weld penetration depth H is approximately equal to Km × I. In the formula, Km is the penetration coefficient (the number of millimeters by which the weld penetration depth increases when the welding current is increased by 100 A), which is related to the arc welding method, wire diameter, current type, etc. as shown in Table 1-1.
| arc welding methods |
electrode diameter/m m |
welding current/A |
voltage/V |
welding speed/mh-1 |
penetration coefficient/m m-100A-1 |
tungsten argon arc welding
|
3.2 |
100~350 |
10~16 |
6~18 |
0.8~1.8 |
|
|
1.6nozzle aperture |
50~100 |
20~26 |
10~60 |
1.2~2 |
| 3.4nozzle aperture |
220~300 |
28~36 |
18~30 |
1.5~2.4 |
|
|
2 |
200~700 |
32~40 |
15~100 |
1.0~1.7 |
| 5 |
450~1200 |
34~44 |
30~60 |
0.7~1.3 |
fusion electrode argon arc welding
|
1.2~2.4 |
210~550 |
24~42 |
40~120 |
1.5~1.8 |
| CO2 Welding |
0.8~1.6 |
70~300 |
16~23 |
30~150 |
0.8~1.2 |
| 2~4 |
500~900 |
35~45 |
40~80 |
|
Table 1-1 Melting depth coefficient Km for various arc welding methods and parameters (welding steel)
2) The melting speed of the welding core or welding wire in arc welding is proportional to the welding current. Since the increase in welding current in arc welding leads to an increase in the melting speed of the welding wire, the amount of welding wire melted increases approximately proportionally, while the weld width increases less, so the weld reinforcement increases.
3) After the welding current increases, the diameter of the arc column increases. However, the depth at which the arc penetrates into the workpiece increases, and the movement range of the arc spot is limited. Therefore, the increase in weld width is relatively small.
In gas-shielded metal inert gas welding (MIG), when the welding current increases, the weld penetration depth increases. If the welding current is too large and the current density is too high, finger-like penetration is prone to occur, especially when welding aluminum.
Influence of arc voltage on weld formation
Under certain conditions, when the arc voltage is increased, the arc power increases, and the heat input to the weldment also increases. However, the increase in arc voltage is achieved by increasing the arc length. The increase in arc length leads to an increase in the radius of the arc heat source and an increase in arc heat dissipation. As a result, the energy density input to the weldment decreases, so the penetration depth slightly decreases while the width of the weld bead increases. At the same time, since the welding current remains unchanged and the melting amount of the welding wire is unchanged, the reinforcement of the weld bead decreases.
For various arc welding methods, to obtain proper weld formation, that is, to maintain an appropriate weld formation coefficient φ. While increasing the welding current, the arc voltage should be appropriately increased. It is required that the arc voltage and the welding current have an appropriate matching relationship. This is most common in consumable electrode arc welding.
Influence of welding speed on weld formation
Under certain conditions, increasing the welding speed will lead to a reduction in welding heat input, thereby reducing both the weld bead width and penetration. Since the amount of deposited wire metal per unit length of weld is inversely proportional to the welding speed, it also leads to a reduction in weld bead reinforcement.
Welding speed is an important indicator for evaluating welding productivity. To improve welding productivity, the welding speed should be increased. However, to ensure the weld size required in structural design, while increasing the welding speed, the welding current and arc voltage should be increased accordingly. 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 such as undercut and cracks may occur during the formation of the molten pool and the solidification process of the molten pool. Therefore, the increase in welding speed is limited.
Influence of welding current type and polarity and electrode size on weld formation
1. Types and polarities of welding current
The types of welding current are divided into direct current and alternating current. Among them, direct current arc welding is further divided into constant direct current and pulsed direct current according to whether there is a pulse in the current; it is divided into direct current positive connection (the weldment is connected to positive) and direct current reverse connection (the weldment is connected to negative) according to polarity. Alternating current arc welding is further divided into sine wave alternating current and square wave alternating current according to different current waveforms. The type and polarity of welding current can affect the amount of heat input from the arc to the weldment, so it can affect the weld formation. At the same time, it can also affect the droplet transfer process and the removal of the oxide film on the surface of the base metal.
When tungsten inert gas arc welding is used to weld metal materials such as steel and titanium, the weld penetration is the deepest when direct current is connected in the positive direction, the penetration is the shallowest when direct current is connected in the reverse direction, and alternating current is between the two. Since the weld penetration is the deepest when direct current is connected in the positive direction and the tungsten electrode has the least burn loss, the direct current positive connection should be used when tungsten inert gas arc welding is used to weld metal materials such as steel and titanium. When pulsed direct current welding is used in tungsten inert gas arc welding, since the pulse parameters can be adjusted, the weld formation size can be controlled as needed. When tungsten inert gas arc welding is used to weld aluminum, magnesium, and their alloys, it is necessary to use the cathode cleaning effect of the arc to clean the oxide film on the surface of the base metal. Alternating current is better. Since the waveform parameters of square wave alternating current can be adjusted, the welding effect is better.
In gas metal arc welding, when the direct current is reverse connected, the weld penetration and weld width are both greater than those in the case of direct current positive connection. The penetration and width of alternating current welding are between the two. Therefore, in submerged arc welding, direct current reverse connection is generally used to obtain greater penetration; while in submerged arc surfacing welding, direct current positive connection is used to reduce penetration. In gas metal arc welding with shielding gas, since reverse direct current connection not only has a large penetration depth, but also the welding arc and droplet transfer process are more stable than those in direct current positive connection and alternating current, and it has a cathode cleaning effect, it is widely used. Direct current positive connection and alternating current are generally not used.
2. Influence of tungsten electrode tip shape, welding wire diameter & extension length
The angle and shape of the front end of the tun, gsten electrode have a greater influence on the concentration of the arc and arc pressure. They should be selected according to the welding current and the thickness of the workpiece. Generally, the more concentrated the arc and the greater the arc pressure, the greater the formed penetration depth, while the weld width correspondingly decreases.
In gas metal arc welding, when the welding current is constant, the thinner the welding wire, the more concentrated the arc heating is, the penetration depth increases, and the weld width decreases. However, when choosing the welding wire diameter in actual welding projects, the current magnitude and weld pool morphology should also be considered to avoid poor weld formation.
When the wire extension length in gas metal arc welding increases, the resistance heat generated by the welding current passing through the extended part of the wire increases, which makes the wire melting speed increase. Therefore, the weld reinforcement increases, while the penetration depth decreases somewhat. Due to the relatively large resistivity of steel welding wires, the influence of wire extension length on weld formation is relatively obvious in welding with steel and fine wires. The resistivity of aluminum welding wires is relatively small, so its influence is not significant. Although increasing the wire extension length can improve the wire melting coefficient, considering comprehensively the aspects of wire melting stability and weld formation, there is an allowable variation range for the wire extension length.
Influence of other process factors on weld formation factors
In addition to the above process factors, other welding process factors, such as groove size and gap size, inclination angle of electrode and workpiece, and spatial position of joint, can also affect weld formation and weld size.
1. Groove and gap
When welding butt joints by electric arc welding, usually determine whether to reserve a gap, the gap size and the form of the groove opened according to the thickness of the welding plate. Under certain other conditions, the larger the size of the groove or gap, the smaller the reinforcement of the welded weld, which is equivalent to the weld position dropping. At this time, the fusion ratio decreases. Therefore, leaving a gap or opening a groove can be used to control the size of the reinforcement and adjust the fusion ratio. Compared with leaving a gap and not leaving a gap and opening a groove, the heat dissipation conditions of the two are somewhat different. Generally speaking, the crystallization conditions of opening a groove are more favorable.
2. Electrode (welding wire) inclination
During arc welding, according to the relationship between the electrode inclination direction and the welding direction, it is divided into two types: electrode forward inclination and electrode backward inclination. When the welding wire is inclined, the arc axis is also inclined accordingly. When the welding wire is inclined forward, the effect of the arc force on discharging the molten pool metal backward is weakened. The liquid metal layer at the bottom of the molten pool becomes thicker, the penetration depth is reduced, the depth at which the arc penetrates the weldment is reduced, the movement range of the arc spot is expanded, the weld width is increased, and the reinforcement is reduced. The smaller the forward inclination angle α of the welding wire, the more obvious this influence is. When the welding wire is inclined backward, the situation is the opposite. In shielded metal arc welding, the electrode backward inclination method is mostly adopted, and an inclination angle α between 65° and 80° is relatively appropriate.
3. Welding piece inclination
Weldment inclination is often encountered in actual production and can be divided into uphill welding and downhill welding. At this time, under the action of gravity, the molten pool metal tends to flow downward along the slope. In uphill welding, gravity helps to discharge the molten pool metal to the tail of the molten pool, so the penetration is deep, the weld width is narrow, and the reinforcement is high. When the uphill angle α is 6° to 12°, the reinforcement is too large, and undercuts are easily generated on both sides. In downhill welding, this effect prevents the molten pool metal from being discharged to the tail of the molten pool. The arc cannot deeply heat the metal at the bottom of the molten pool, the penetration is reduced, the moving range of the arc spot is expanded, the weld width is increased, and the reinforcement is reduced. If the inclination angle of the weldment is too large, it will lead to insufficient penetration and overflow of molten pool liquid metal.
4. Welding material and thickness
Weld penetration is related to welding current and also to 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 a unit volume of metal and raise the temperature by the same amount. Therefore, under certain other conditions such as welding current, the penetration depth and weld width will decrease. The greater the density or liquid viscosity of the material, the more difficult it is for the arc to displace the liquid molten pool metal, and the shallower the weld penetration. The thickness of the welded part affects the heat conduction inside the welded part. When other conditions are the same, as the thickness of the welded part increases, the heat dissipation increases, and both the weld width and penetration depth decrease.
5. Flux, electrode coating and shielding gas
The different compositions of fluxes or electrode coatings lead to different voltage drops at the electrode regions of the arc and different potential gradients of the arc column, which will inevitably affect the weld formation. When the flux has a low density, large particle size, or small stacking height, the pressure around the arc is low, the arc column expands, and the arc spot has a large movement range. Therefore, the penetration is small, the weld width is large, and the reinforcement is small. When high-power arc welding is used to weld thick workpieces, using pumice-like flux can reduce arc pressure, decrease penetration, and increase weld width. In addition, the welding slag should have appropriate viscosity and melting temperature. If the viscosity is too high or the melting temperature is relatively high, the slag will have poor ventilation, and it is easy to form many depressions on the weld surface, resulting in poor weld surface formation.
The composition of shielding gases for arc welding (such as Ar, He, N2, CO2) is different, and their physical properties such as thermal conductivity are also different. This makes the polar region voltage drop of the arc and the potential gradient of the arc column, the conductive cross-section of the arc column, the plasma flow force, and the distribution of specific heat flux different. All these factors affect the formation of weld seams.
In short, there are many factors affecting weld formation. To obtain good weld formation, it is necessary to select appropriate welding methods and welding conditions for welding according to the material and thickness of the welded part, the spatial position of the weld, the joint form, working conditions, requirements for joint performance and weld size. At the same time, the most important thing is the welder's attitude towards welding! Otherwise, the weld formation and its performance may not meet the requirements, and even various welding defects may appear.
