It seems inappropriate to list steel as a lightweight material.
However, by selecting the appropriate alloy composition and heating, the mechanical properties of steel can be customized.
At this point, steel surpasses almost all other materials and has great application potential.
For example, smaller, lighter and thinner parts can be made of high-strength steel, which can absorb the same energy as those thicker and heavier parts made of conventional steel.
However, at this time, these materials pose a great challenge to the connection process because they are easy to crack.
In particular, the connection between the gear and shaft in the transmission device, which is an axisymmetric circular component, is a typical example.
In the process of connection, a specific problem often occurs: at the end of the weld, it is necessary to reduce the laser rate to prevent the formation of end weld scar.
These cracks will reduce the long-term strength of the weld under operation, so it can not meet the DIN EN iso13919 standard.
Hot and cold cracks
At present, there are two common ways to avoid cold cracks.
One is to add another material as filler wire in welding to change the local alloy composition.
The other is to preheat the parts to a certain high temperature.
The principle of both methods is to reduce the martensite (a high-hardness steel component) produced in the weld, because too much such crystal structure will lead to high hardening and large tension in the connection area due to volume expansion.
On the contrary, the cause of hot cracks is due to the complex interaction between alloy composition, structural design and weld location in the component.
In addition, the welding process parameters that determine the heat load of the workpiece will also affect it.
In addition, the welding process parameters that determine the heat load of the workpiece will also affect it.
The material will be affected by both pressure and tension, and fracture will occur along the grain boundary (internal crystallization), some of which show low melting point phase accumulation
Thermal cracks are small, usually no more than a few millimeters. This study focuses on the thermal cracks formed in the actual cladding.
These curing cracks are usually deeply buried below the surface and are difficult to detect.
Compared with the cold crack, the free-curing dendritic surface of the hot crack can be seen by scanning electron microscope (SEM) (see Fig. 1).
Fig. 1: the surface of free curing dendrites can be seen under scanning electron microscope, which is a typical characteristic of thermal cracks.
Keyhole is the key
In deep penetration welding, welding energy transfer occurs in the process of laser beam vaporization of materials.
The resulting vaporization pressure will form a deep hole filled with steam, that is, the keyhole.
Due to the dynamic characteristics of the keyhole itself, when advancing along the weld, the molten material generated by the laser beam moves in the molten pool.
Molten metal flows around the keyhole.
Eddy current is generated at the rear of the molten pool, which affects the three-dimensional geometry of the molten pool.
The relationship among keyhole, melt flow and thermal crack is that there is a periodicity among keyhole, melt flow and thermal crack, and this cycle is the oscillation frequency of the keyhole and the frequency of the thermal crack formed in the molten pool.
Vice versa, that is, the weld pool eddy current and weld pool geometry will be affected by the way of keyhole energy transfer.
A variety of measurement techniques can be used to analyze the motion of these molten pools.
A newly developed method is to obtain the footprints of fluid movement by creating a median image of fluid phenomena in the welding pool.
A newly developed method is to obtain the footprints of fluid movement by creating a median image of fluid phenomena in the welding pool.
Scientists have found that when the focus position moves a Rayleigh length, the rotation direction of the eddy current in the upper part of the molten pool will be reversed (see Fig. 2).
At the same time, they also found that the same focus position movement will reduce the thermal crack sensitivity.
Therefore, they used thermal simulation to detect the relationship between the changed flow phenomenon, the changed molten pool geometry and the reduction of crack formation.
The results show that the maximum strain position and focus position move with the change of molten pool geometry. These findings can be used to change the formation conditions of thermal cracks.
The results show that the maximum strain position and focus position move with the change of molten pool geometry.
These findings can be used to change the formation conditions of thermal cracks.
Researchers have proposed a variety of schemes to change the way energy is transferred to the keyhole.
One method is to use lasers with different brightness (beam parameter product 2 ~ 24 mm * mrad) under different focusing conditions.
In another method, scientists studied the influence of laser wavelength (1.03 μm and 10.6 μm) under the same optical and mechanical boundary conditions.
Unfortunately, these methods failed.
In both methods, the team successfully changed the form and characteristics of cracks, but could not completely eliminate cracks.
They also found that once the welding speed was increased, the crack sensitivity would be greatly improved.
Fig. 2: fluid dynamics changes with the movement of focus position during welding.
Double breakthrough
The first method to prevent cracks is double-beam welding, which distributes the output power of the main beam to the primary and secondary beams in a ratio of 72:28.
As long as the two beams are one front and one rear, that is, the secondary beam closely follows the primary beam, and the secondary beam is aimed at a specific point where the two beams share the molten pool, cracking can be completely avoided.
However, the spacing needs to be adjusted according to the specific weld pool length, which depends on the output power of the selected laser.
The second technique is to use a time-modulated laser beam for welding.
By carefully selecting the continuous modulation amplitude, no matter what welding depth and speed, cracking can be completely avoided in a wide range of modulation frequencies.
The analysis of the flow characteristics and geometry of the molten pool shows that the time modulation power can have a significant effect.
The thermal surface measurement of weld pool length fluctuation can also be carried out during welding, and such fluctuation can be greatly reduced.
The resulting measurable parameters can be used to monitor and stabilize the process.
A series of tests on real parts have proved that this method has great potential to eliminate cracks in the welding of high-strength steel in the future.
