Basics You Should Know About Laser Welding Technology of Sheet Metal

Introduction

The advantages of laser welding can be obtained by comparing traditional arc welding with laser welding.

In order to get a better laser weld, this post discusses the laser welding process.

First, the depth width ratio and surface morphology of the weld are taken as the evaluation indexes, and then the process parameters that affect these indexes are studied.

The laser welding tests are carried out on stainless steel, aluminum plate and carbon steel plate, and the empirical conclusions that can be used in the welding production process are obtained.

Laser welding is a special production technology that uses laser with high energy density as the welding heat source to realize plate welding.

Because of its advantages of high energy density, fast welding speed, environmental protection, small plate deformation and so on, it has been well applied in the sheet metal manufacturing industry.

Comparison between laser welding and traditional arc welding

Introduction to arc welding

Traditional arc welding can be roughly divided into electrode arc welding, argon tungsten arc welding, gas metal arc welding, submerged arc welding, etc.

Electrode arc welding is to draw an arc between the weldment and the electrode, and the heat generated by the arc melts the contact part between the weldment and the electrode to form a metal molten pool.

With the direction of the electrode, new molten pools continue to be generated, and the old molten pool continues to cool and solidify, finally forming a weld.

The schematic diagram of welding process is shown in Figure 1.

Schematic diagram of electrode arc welding

Fig. 1 Schematic diagram of electrode arc welding

Tungsten electrode argon arc welding uses tungsten electrode as the discharge electrode, but the electrode will not be consumed.

Inert gas is used as the protective gas, and the heat generated by the arc is used to melt the base metal and welding materials.

The welding area is covered by protective gas, and there is basically no splash.

Gas metal arc welding is to generate an arc between the welding wire and the base metal by energizing the welding wire to melt the base metal and the welding wire.

At present, arc welding still occupies a dominant position in the welding field, but with the increasing requirements of the market for welding quality and efficiency, traditional arc welding has encountered bottlenecks in some high-end sheet metal manufacturing fields, and the shortcomings of arc welding are becoming more and more obvious.

Traditional arc welding requires argon as the shielding gas, and the cost of high-purity argon is high;

Arc welding operation requires professionals with special operation certificates, and the access threshold is high;

Argon arc welding will produce high-density current and high heat, which is not conducive to the welding of metal plates with low melting point;

Low welding efficiency and slow speed;

The welding surface is relatively rough. If the surface morphology is required to be high, grinding process needs to be added;

The arc welding intensity is limited.

Advantages of laser welding

Mainstream laser welding can be divided into self fusion welding, swing welding, wire filling welding, galvanometer welding and composite welding combined with various welding methods.

Compared with traditional arc welding, the advantages of laser welding technology are shown in Table 1.

For the high-end sheet metal manufacturing industry with high added value, good product consistency, small weld gap and efficiency, laser welding can be the first choice.

Table 1 Comparison of characteristics between laser welding and arc welding

Arc weldingLaser Welding
High density current is required, and the thermal effect is largeLow welding heat, small deformation and thermal effect
Shallow penetration and poor welding strengthDeep penetration and high welding strength
Contact type, limited by spaceNon contact type, less limited by space
Large arc starting current and large welding rangeSmall welding spot, capable of welding precision workpiece
Operators have high requirements and require special operation certificates.Low requirements for operators
Low welding efficiency and slow welding speedHigh welding efficiency and fast welding speed
Electrode pollution and lossNo electrode loss
The surface is rough and requires subsequent grinding.The surface morphology is stable, and there is basically no need for subsequent grinding.

Laser welding process parameters

Laser welding seam index

Due to different customer needs, the requirements for the welding effect of sheet metal parts are also different.

The requirements for welds are mainly reflected in the following indicators:

Surface morphology, width depth ratio of molten pool, whether there are pores, cracks, impurities, undercuts and other defects.

The surface morphology is the shape and appearance of the weld surface after the combination of welding material and base metal or the combination of base metal and base metal, or concave, or flat, or convex.

Different welding morphologies can be achieved by adjusting welding power, defocus, splicing mode, etc.

The ratio of weld pool width to depth refers to the ratio of weld pool depth to width.

If customers have strength requirements for welding products, they need to pass a series of steps of wire cutting → inlay → grinding and polishing → corrosion → microscopic metallographic analysis to reflect the welding strength according to the depth width ratio of the weld, which is more the embodiment of hardness.

They can also feed back the tensile strength index of welding through tensile strength test. Metallographic analysis of penetration ratio is shown in Fig. 2.

metallographic analysis of penetration ratio

Fig. 2 metallographic analysis of penetration ratio

In a specific working environment, the weldments have defects such as pores, cracks, impurities, undercuts, etc., which are likely to cause serious safety accidents.

For example, some products have strict requirements on air tightness, water leakage, etc.

The schematic diagram of normal welds and defective welds is shown in Fig. 3.

weld seam diagram

Fig. 3 weld seam diagram

Influencing factors of laser welding

The factors that directly affect laser welding include welding temperature, melting point of welding materials, laser absorption rate of welding materials, thermal influence, etc.

Corresponding to the welding process, it can be considered from the aspects of material performance, laser power, welding speed, focus position, shielding gas, weld gap, etc.

The laser absorptivity of welding materials affects the welding effect. For general materials, aluminum and copper have higher laser absorptivity, while carbon steel and stainless steel have lower laser absorptivity.

Welding materials with high absorptivity often need to spend more energy to melt and form a stable weld pool and weld.

Laser power is the energy source of laser welding, which has a decisive impact on the welding effect, and the size of laser power also affects the welding speed.

The higher the laser power is, the better.

After reaching a certain laser power, the corresponding welding penetration will also reach a threshold.

Beyond this threshold, the weld pool will be more unstable, but the weld pool depth will be reduced.

Therefore, choosing the appropriate laser power value becomes particularly critical.

There is a negative correlation between welding speed and penetration.

The faster the speed is, the lower the energy input on the welding material will be, and vice versa.

Too fast speed can not make the welding material obtain enough energy to form an ideal weld. Too low speed will lead to over melting, especially for heat sensitive aluminum.

The position of the focus directly affects the penetration and width of the weld.

The laser focus just located on the surface of the welding material is called zero focus, and it is eccentric focus when it is above or below the welding material.

The zero focus spot is the smallest, and the spot energy density is the highest;

The off focus welding power density decreases but the light spot becomes larger, which is suitable for welding workpieces with a large range.

The influence of shielding gas on welding is not only reflected in the type of blowing, but also in different ways of blowing.

The function of air blowing is not only to prevent the workpiece surface from being oxidized during welding, but also to suppress the plasma cloud generated during laser welding.

Shielding gas will directly affect the appearance and color of the surface. Welding products with requirements for these two points must pay attention to the importance of shielding gas.

The weld gap of the workpiece to be welded is directly related to the weld penetration, weld width and weld morphology.

If the weld gap is too large and the light spot is small, the weld cannot be fused and combined, and the fusion is difficult;

At the same time, the laser is exposed, which is likely to damage the tooling or workpiece.

If the gap is too large and within a certain range, it can be improved by increasing the light spot and increasing the swing, but the improvement effect is limited.

Welding test analysis

In the welding test, Yaskawa robot GP25, Prima laser, ospri welding joint (core diameter 100μm, focal length 300mm) and WSX wire feeder were used to test the welding effect of 1.5mm Q235 carbon steel plate, SS304 stainless steel and 3-Series aluminum alloy plate respectively.

According to experience, a reference can be provided in the process of testing:

The test welding of 1mm thin plate can be started from 1kW power and 30mm/s welding speed.

The reference power can be P=A · X, where a is the constant coefficient (A ≥ 0), and X is the plate thickness.

When the welding speed, material, gas and other welding conditions remain unchanged, the size of coefficient a gradually decreases with the increase of plate thickness, and the coefficient A is also affected by the welding method.

Analysis of welding process of carbon steel plate

See Table 2 for the swing welding process parameters of Q235 carbon steel plate with thickness of 1.5mm.

Table 2 swing welding process parameters of Q235 carbon steel plate

NO.Laser power
(kW)
Welding speed
(mm/s)
Swing range
(mm)
Swing speed
(mm/s)
Welding effect
11.6321143good
21.6311148good
31.6301148good
41.7331151good
51.7321152good
61.7341150good
71.8351153good
81.8351154good
91.8361154good
101.9361156good
111.9371155good
121.9371160good

It can be seen from the above test data that during the swing welding of carbon steel plates, the laser power should be appropriately increased with the increase of welding speed.

Under the condition of ensuring that the swing range remains unchanged, the swing speed needs to be increased to ensure the welding effect.

If the swing speed is too small, the weld will be uneven.

Generally speaking, the energy required for carbon steel self fusion welding is less than that required for carbon steel self fusion swing welding.

The energy required for carbon steel self fusion swing welding is less than that required for carbon steel swing wire filler welding.

When other factors remain unchanged, the energy is mainly controlled by power and speed.

The same welding effect, higher power, faster speed;

The power is low and the speed is slow.

Ideally, in order to give consideration to welding quality and efficiency, the welding speed should be improved as much as possible, but too fast welding joints will shake, and will be limited by laser power and material properties.

Therefore, the balance between power and speed is generally sought.

Analysis of aluminum plate welding process

The core diameter of optical fiber selected in the test is 100μm.

If aluminum, copper and other high reflective and heat absorbing materials are to be welded, higher power density is required to melt aluminum.

At this time, it is necessary to choose zero focus welding metal.

When the weld is small, it can make the laser form the maximum power density with the minimum power, melt the metal and form a molten pool, which is one of the reasons why zero focus needs to be found.

See Table 3 for welding process parameters of different materials.

Table 3 Comparison of welding process parameters of different materials

NO.Laser power
(kW)
Welding speed
(mm/s)
Plate thicknessSwing range
(mm)
Swing speed
(mm/s)
Whitening effectMaterial
11.52.11.51300GoodQ235 carbon steel
21.51.81.51300Good3 series aluminum alloy
322.021300GoodQ235 carbon steel
421.721300Good3 series aluminum alloy

From the above test data, it can be seen that under the condition that other parameters are almost unchanged, the welding speed of 3-Series aluminum alloy is slower than that of Q235 carbon steel to achieve the ideal welding effect, which essentially requires more heat.

Analysis of stainless steel white blowing process

See Table 4 for the comparison of whitening process parameters of stainless steel welding seams with a thickness of 1.5mm. The welding effect is shown in Fig. 4.

The welding parameters of the three welds (from left to right) correspond to the serial numbers 1, 2 and 3 in table 4 respectively.

Comparison of welding effect

Fig. 4 Comparison of welding effect

Table 4 Comparison of process parameters of stainless steel welding seam whitening

NO.Laser power
(kW)
Welding speed
(mm/s)
Plate thicknessSwing range
(mm)
Swing speed
(mm/s)
Whitening effect
11.21.71.51300poor
21.51.81.51300good
31.61.81.51300poor

In order to whiten stainless steel, it is necessary to quickly cool and crystallize in the atmosphere of shielding gas after laser melting the metal.

If the power is too high, a lot of heat will be accumulated on the metal plate, the cooling speed will be slow, and it is easy to oxidize and discolor;

The power is too low to melt through;

If the speed is too fast, and the blowing tooling may not be large enough, the blowing effect will be bad;

If the speed is too slow, there is too much heat accumulation.

In general, to blow white, it is necessary to balance the power, speed and blowing.

If it is really impossible to blow white at one time, you can weld one layer under the condition of slightly higher power, and then reduce the power and then weld another layer.

Conclusion

In the process of laser welding, in order to ensure the welding quality, comprehensive consideration can be given to material properties, laser power, welding speed, focus position, shielding gas, weld gap, etc.

For mainstream carbon steel, stainless steel and aluminum plates, the initial test parameters mentioned above can be selected, and then various parameters can be adjusted according to material characteristics, customer requirements, etc., so as to achieve the desired effect of the weld.

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