Characteristics of welding laser beam and its influence on weld forming coefficient

Laser welding is a very complicated process. At present, there is no systematic research on the relationship between welding parameters and forming coefficient. By studying the internal relationship between laser parameters and the characteristics of welding laser, laser beam indexes, such as focusing index, are defined. They are simplified to a few laser parameters so as to facilitate the study of the influence of welding parameters on the forming coefficient. The results show that material, assembly gap, laser power and welding speed have great influence on weld penetration. When the laser power density reaches 106J/cm2, the characteristics of deep penetration welding appear. Under the condition of constant clearance, the weld depth increases with the increase of t/b. The weld penetration increases with the increase of laser power and tends to be stable when the welding speed is 12mm/s. Different materials also have a certain influence on the weld penetration depth. Under the same conditions, X6CrNiTi1810 obtains greater penetration depth, indicating that physical and chemical properties of materials are also one of the factors affecting the penetration depth. The mechanism and condition of laser welding were analysed. The relationship between laser absorptivity of sheet surface and physical properties of the materials is also studied and then obtained. The study will provide theoretical guidance for laser welding process design and welding parameter selection of steel plate.


Introduction
Laser welding is characterized by high-energy density, which can realize fast welding of sheet metal and reduce the influence of heat input on the performance of sheet metal.Laser welding is closely related to the characteristics of laser light, material, assembly gap, laser power and welding speed.Up to now, there are few researches on the relationship between them, and no systematic study methods and conclusions have been obtained.For laser light, there are many parameters, which will also cause great difficulties for choosing of laser welding parameters.At present, it is necessary to find the appropriate welding parameters through a large number of tests, which not only wastes a lot of manpower and time, but also hardly meet the requirements of various plate combination design.Therefore, it is very important to find out the influencing factors and rules of weld forming coefficient [1,5].
The purpose of this paper is to study the parameter characteristics of laser light, find the internal relationship between them in order to simplify the number of laser parameters.On this basis, the influence of major welding parameters such as laser parameters, laser power, assembly gap, materials and welding speed on the weld forming coefficient was studied, and find the laws between welding parameters and welding forming coefficient.It can provide high-effective and precise way for process design, reduce test time and cost.

Principle of laser welding and definition of laser parameter 2.1. Principle of laser welding
According to the characteristics of forming coefficient of laser welding joint, laser welding can be divided into thermal-conduction type and deep-penetration type.The thermal-conduction has a smaller ratio of weld depth/width, while the deep-penetration type possesses a large one.Generally, energy density of laser beam for laser welding with thermal-conduction is less than 10 5 J/cm 2 .The surface of workpiece is heated by laser radiation with slow welding speed [6,7].
In the case of deep-penetration, the energy density of laser beam is greater than 10 6 J/cm 2 .The metal surface is concave even with a "hole" under the action of heat, and eventually forms deep fusion welding with fast welding speed.
High laser energy of deep-penetration laser welding is converted to metal surface vaporizes metal, and forms eventually a "hole" in the surface where the energy conversion is realized.The pressure from the metal vapor blocks the molten metal around it, and keeps the "hole" open throughout the welding process.The laser energy is mainly absorbed by the boundary between vapor and melting liquid of the hole wall.The vapor-filled hole absorbs almost all the incoming laser energy.The temperature inside the hole is so high that the metal around it melts.The pore is filled with high-temperature steam, meanwhile high pressure is generated by liquid flow outside the hole wall.The surface stress of the wall and the steam pressure inside the hole maintain a dynamic balance.As light beam moves, the eyelet is always in a stable state of flow, and the eyelet and molten metal move along with the leading light beam [8,9].The molten metal fills the gap left by the eyelet and accumulates to form a weld beam, as illustrated in figure 1.
The laser beam and hole move continuously along the welding path.As a result the welding material is melted in front of the hole and re-solidified at the back to form a weld beam.
The type of laser weld beam is related to defocus value.Due to the high energy density in the center of the laser, hole structure is formed via evaporation with ease.The distribution of energy density is relatively uniform in each plane away from the laser focus point.According to the theory of geometrical optics, when the distance between the positive and negative defocus planes is equal to the welding plane, the energy density on the corresponding plane is approximately the same.However, shape of the molten pool is different.Therefore, when defocus value is negative, greater penetration can be obtained, which is related to the formation process of molten pool.When heated by laser, the material begins to melt, forms liquid phase and partial vaporization, generates high-pressure steam and sprays at extremely high speed.In the meanwhile, the high-density vapor makes the liquid metal move to the edge of molten pool, leading to a depression in the center of the molten pool.When the defocus value is negative, the energy density of internal material is higher than that of the surface, which enables a higher degree of melting and vaporization.Therefore, the laser beam can be transmitted deeper into the internal material.It is concluded that the forming coefficient of laser weld beam has a direct relationship with defocus value, which has an influence on the type of laser welding beam to a certain extent, as illustrated in figure 2.

Key laser parameters
The important parameters of laser fusion welding include energy density of beam on weld surface, diameter of focus point, depth of focus, symmetry of beam, location of focus point and protective gas, etc.The parameters and units of laser light are defined as shown in Table 1.The geometric dimensions are shown in figure 2. For the laser beam and welding parameters mentioned above, the stability of welding process can be judged by checking the fluctuation of these parameters.The stability of welding performance is also related to processing.
In this paper, key parameters are defined as follows [10,15]: Laser beam power (P): Beam guiding and beam shaping can result in power loss due to partial laser absorption.Stable power loss is obtained when laser module is in new state.With the increased use of laser equipment, however, pollution will lead to power loss.Therefore, it is necessary to measure the actual beam power of output on the surface of workpiece.
Focal diameter (df): About 86% of beam power is contained in the cross section of the beam focus diameter, which can be used as a reference for beam diameter at other location.If the beam geometry is not radially symmetric, the minimum and maximum beam diameters at the focal point must be specified.The focus point is the location of the beam with the smallest beam cross-sectional area.The focal diameter should be as small as possible to meet the energy density requirements.
Focal depth (Zf): The focus depth of is defined as the distance between two equal cross section of the beam.It is twice the cross sectional area of laser beam at focal point, and distributes in terms of radial symmetry.The focus depth plays an important role in obtaining the maximum welding penetration and maintaining the process stability caused by tolerance fluctuation of parts.The fluctuation of focal depth cannot be detected directly.It is determined by focal diameter and location.
Energy density distribution (I): The energy density distribution includes the beam intensity distribution in the focal plane and deep focal area.The change of maximum density and distribution in the focal plane and depth area will affect welding result, resulting in the fluctuation in key welding parameters, which can also be reflected by focus diameter and depth.
Beam symmetry: In the case of elliptic or non-rotationally symmetric beam distribution, the positions of the minimum and maximum beam diameters with respect to the welding direction characterize the welding results.Especially in welding with optical parts moving and in different directions, the location of laser beam must be appropriate to the welding direction.
Positioning focus: Positioning accuracy depends on the desired focus diameter and stability of focus position.For a thin plate butt joint, it is about 1/20 that of the mean transverse width of the weld beam or in 1/5 that of the focal depth of the beam axis.Positioning accuracy is influenced by the guiding IOP Publishing doi:10.1088/1742-6596/2671/1/0120034 accuracy of welding system, directional stability of laser beam, and tolerance of parts.A weld tracking system essential when part tolerances are too large during welding.

Definition of key laser parameter relationship
In order to study further the optical characteristics of laser welding, relevant physical parameters are defined, as shown in table 2. Beam index: Beam parameters cannot be selected independently of each other.It is of practical value to define beam index K.The larger the index K is, the better the beam quality is.Its maximum value can be 1, which can be realized through the Gaussian intensity distribution.The index is directly related to the type of intensity distribution in the beam.Experiments are conducted to determine accurate values, since it is difficult to calculate based on beam distribution types in high power laser systems.Therefore, two parameters are needed, since this is especially true for rotationally symmetric intensity distributions.The diameters of laser beam are defined by df and ds.
In addition, beam index K can be determined by measuring focal depth.
When defining the focus value F = f/ds, the value 1/F corresponds to the aperture ratio.Although beam index combined with beam power can describe the power of laser beam, welding results depend on focal parameters such as focus diameter, focus depth, and beam power.By specifying beam index K, focus value F and beam power P, the beam parameters of laser welding can be described clearly.
In order to realize the welding of metal plate above 3mm thickness, larger K value should be selected as high as possible.For laser welding of thin plate smaller than 3 mm, beam index plays a minor role, and the intensity distribution shape of laser beam cross section has a greater effect on the welding results.
In the case of lasers of solid-state, it is usually given in the form of SPP instead of index K.It is given by the product of focus radius of beam and semi-far field angle.SSP describes the function relationship between laser divergence behavior and fiber core diameter after leaving the cable.The relationship between the beam index K and SSP as follows: = / (3)

Test results and analysis 3.1 Relationship between laser and properties of welded materials
In the process of laser welding, only part of laser energy is transferred on the workpiece and is absorbed by material.Laser absorption rate is higher when welding steel materials, but is lower for aluminum materials.This is determined by the properties of material.
The absorption rate of metal for laser can be approximated by the following formula [13]: whereεh (t) is the absorption rate of metal at temperature t, γ is the resistivity of metal material at temperature 20℃, β is resistance temperature coefficient, t is temperature, λ is laser wavelength.
It can be seen from the above formula that laser absorption rate increases with the increase of temperature and resistivity.By pre-heating, the laser performance on metal surface can be increased to improve the absorption coefficient of laser and reduce appropriately laser power.
Figure 3 shows the relationship between different materials, laser wavelengths, and absorptivity.At room temperature, the absorption rate of laser is 35% for steel and only 4% for aluminium.With the increase of temperature, the absorption rate of laser increases sharply.Based on the characteristics, it is necessary to preheat the welding parts.So generally after welding process is started, under the action of welding heat, laser absorption rate is improved significantly.However, at the beginning of welding, welding time should be increased appropriately to ensure the full melting of the welded parts.
In general, the smaller the resistivity is, the lower the absorption rate of laser is.Therefore, absorption rate of laser increases in the order of silver, copper, aluminium, nickel and steel, depending on the physical properties of these materials.The state of surface has a great influence on absorption rate of laser.The higher the surface roughness is, the higher the absorption rate of laser is.The surface with coating materials also have a greater influence on absorption rate, such as zirconia or phosphate, improving significantly absorption rate of laser.
Because the laser brazing of steel plate mostly uses copper alloy as filling wire, in order to melt fully the wire, when keeping laser power unchanged, preheating the wire by current is essential to improve absorption rate.Laser brazing is extremely difficult when preheating is absent.
When the preheating current flows through the connection point between wire and welding part, the wire is heated due to resistance heat.The wire makes contact with the part in front of the focus point, at which point the laser energy is insufficient to melt the wire.In order to bend the wire in the desired manner, a sufficient positioning angle is maintained in the various joint types.In general an angle greater than 40 degree is necessary.
In addition, serious splash in the welding process indicates that the preheating current has reached the upper limit and the welding wire has melted in advance.Similarly, poor quality is obtained when lowering the limit of preheating current, meanwhile the surface of weld beam becomes rougher.Generally, wire preheating current should be as close to the upper limit as possible.
This phenomenon also occurs in laser fusion welding of Al alloys.Due to the low laser absorption rate of Al alloy, Al alloy must be passivated before welding.Surface oil and dense, hard, refractory Al2O3 should be removed, followed by coating a layer of titanium phosphate on its surface.The coating layer has a high absorption rate and can significantly reduce the power of laser source.In addition, due to the removal of Al2O3, the content of oxide inclusion in weld beam can be reduced and the strength of weld beam can be improved.

Welded materials
In this paper, several typical materials, X6CrNiTi1810, St 52-3 and C22 were employed for laser welding.The relevant physical and chemical properties are shown in table 3, including parameters that have an impact on laser welding, such as thermal conductivity, specific heat capacity, resistivity, etc.

Joint gap design
The maximum allowable gap width depends on the focus diameter of laser beam, the weld beam geometry, weld type, part geometry and metallurgical properties of material.For steel plate joints, the following experiences can be referred: The maximum gap width bsmax should be less than the lateral contraction in order to avoid joint collapse, i.e.
bsmax < SQ The transverse contraction SQ of steel plate is about 6% of the average weld width bs, namely: SQ = 0.06bs Via test and product verification, for automobile body steel plate, the gap should be between 0.05 and 0.2mm.
The definition of relevant parameters and their relationship are shown in figure 4.
During laser welding, the ratio of weld/depth (t/b) can vary over a wide range, generally from 1 to 10, depending on the design requirements of product.Figure 5 shows the morphology of a typical deep-penetration welded joint.The joint is a C22 plate with a butt joint type and a plate thickness of 4mm.It is obtained under high energy density and welding speed.It can be seen that the weld depth /width are relatively high.
In order to avoid collapse of weld beam, the gap width is closely related to the depth of weld beam.The related results are shown in figure 6.It shows the relationship between the maximum gap width  bsmax and the depth of weld beam under the condition of two constant depth-width ratios.Larger gap widths can be bridged using metal-filled laser welding.
It is important to study the relationship between maximum gap width and weld depth under the condition of constant depth-width ratio, which provides theoretical reference for reasonable selection of plate thickness, tolerance, assembly requirements and satisfies weld-forming coefficient.The two t/b curves in figure 6 are two common usage scenarios in low-alloy high-strength steel.In this paper, C22 of 4mm plate is employed for related analysis.
As shown in figure 6, when t/b = 2~6, both weld depth and welding gap show a parabolic relationship, in the same curvature direction.However, the curvature radius is obviously different, and generally decreases with the increase of the depth-width ratio.
When t/b = 2 or 6, the relationship between weld depth and gap should be between the two curves demonstrated in Figure 6.

Characteristics of laser molten pool
In general, CO2 laser beam has the following physical characteristics: At vertical incidence, more than 90% of laser radiated on the metal surface is reflected for CO2 laser (λ= 10.6μm).The higher the conductivity of metal is, the greater the reflection is.Laser absorption rate is proportional to square root of the resistance coefficient.The resistivity, in turn, varies with temperature.If the characteristic power density is exceeded, a steam channel is formed and the deeppenetration effect occurs.
With the appearance of deep welding effect, radiation absorption efficiency increases suddenly, as shown in figure 7. The effect of deep-penetration welding is directly related to energy density, which in turn depends on beam power and focus area.Due to the restriction of plate thickness, the minimum focus diameter will also have certain requirements, coupled with limitation of laser power, so laser deeppenetration welding is conducted under certain condition.In general, it is difficult to start deeppenetration welding under the condition of energy density < 10 6 (W/cm 2 ).When the value is higher than it, the energy density will increase due to the constant focus radius under the condition that other conditions remain unchanged, thus leading to deep-penetration welding.
As energy density increases, metal surface is vaporized, with "hole" formed nearby.More energy goes into metal through the hole.Due to presence of the "hole", laser absorption rate is increased by multiple times.The hole can absorb all laser energy, so the temperature rises rapidly, which results in fast melting of the metal around.Under the pressure of the liquid flow, more heat is transferred to the bottom of pool, melting more metal and increasing the pool depth.In addition, deep-penetration welding is also associated with the thermal physical and chemical characteristics of material, such as thermal conductivity, specific hot melt, heat of melting, heat of evaporation, thermal diffusion coefficient and material density, which determine the energy absorption and heat transfer capacity of welded material.
Deep-penetration welding is related to a rapid increase in the absorption of laser beam.When improper process parameters are used, plasma cloud above the workpiece will shield laser beam and thus reduce energy coupling.The plasma shield can be weakened or eliminated by providing appropriate gas protection or by increasing the amount of airflow.With higher energy density and faster welding speed, the heat affected zone (HAZ) is smaller, together with larger weld depth-width ratio.Moreover, deformation and shrinkage related to parts are also hampered.

Relationship between welding and weld beam depth
Different materials have their intrinsic physical and chemical properties, which lead to the different absorption effect even with the same wavelength, perforation and the molten pool disturbance.During laser welding, metal temperature changes significantly, indicating that the physical and chemical properties of metal, especially resistivity, which has an important influence on laser absorption rate, also changes obviously, and increases the influence on laser absorption rate.
Figure 8 summarizes the relationship between welding speed and weld beam depth of St 52-3 steel with varying focus value and power.It should be noted that power has greater influence on weld beam depth as compared with focus value.Even with larger F value, high power joints can still generate greater depth at the same welding speed.In the low-speed stage, the slope of curve is larger, and the growth rate of weld beam depth increases obviously.The power of 2KW can also generate a depth of 3.5mm at the speed of 20mm/s.However, low power welding is not recommended, due to quality requirements of HAZ.
Under the condition of high welding speed, due to weakening of the perforation effect, laser absorption rate is difficult to improve rapidly.Even with varying power, the depth remains low in stable range.For different materials, however, even with the same power and other laser welding conditions, different results can be obtained due to varying physical and chemical state on material surface.
Figure 9 summarizes the relationship between welding speed and weld beam depth of X6CrNiTi1810, St 52-3 and C22 under the same focus value of 7, power of 8kW.They all decrease with the increase of welding speed.For different steels, the magnitude and direction of curvature radius changes in a similar

Figure 2 .
Figure 2. Physical parameters of the laser beam.

Figure 3 .
Figure 3. Laser absorptivity of different materials. bs

Figure 6 .
Figure 6.Maximum allowable gap of steel butt joint.where t stands for weld depth and b for joint gap width.Weld beam depth(mm) Maximum gap(mm) t/b=2

Figure 7 .
Figure 7. Relation between energy density and weld depth.Energy density of laser beam(J/cm 2 )

Figure 8 .Figure 9 .
Figure 8. Relationship of Welding speed and weld beam depth.Welding speed(mm/s)

Table 1 .
Parameter definition of laser welding

Table 2 .
Definition of relevant laser parameter relationships