1. Introduction
As we know, various stresses during the laser cladding process, including thermal stress, organizational stress, and constraint stress, along with the influence of temperature gradients and molten pool convection, can lead to undesirable effects like cracking in the cladding layer.
These adverse factors seriously hinder the reliable application of laser cladding technology to the quality and performance of experimental subjects, necessitating the elimination of these cracks.
In most cases, the occurrence and removal of cracks largely depend on the selection of laser cladding process parameters and other factors such as material selection.
Therefore, we have analyzed these parameters one by one and summarized the solutions for crack resolution.
This article integrates domestic and international research results and analyzes the influencing factors of laser cladding. It considers thermal stress, organizational stress, and constraint stress as the main factors causing laser cladding cracks.
It dissects the intrinsic reasons for the creation of cladding cracks from both the laser process and material selection perspectives, discussing methods to eliminate laser cladding cracks from aspects such as laser power, scanning speed, spot size, and material selection.
2. Mechanism of Crack Generation in Laser Cladding
We understand that the primary cause of crack formation is the impact of various stresses, including thermal stress, structural stress, and constraint stress, as well as factors like temperature gradient and melt pool convection. The following primarily introduces the impact of these stresses:
① Thermal stress
There are two types of thermal stress: one occurs within the same material. When the material is heated, it is caused by the temperature gradient of the material.
The other is due to the different coefficients of thermal expansion between the substrate and the cladding material, causing inconsistent rates of thermal expansion and contraction between layers, resulting in tensile or compressive stress.
② Structural stress
Structural stress arises from phase changes during the heating and melting of metallic materials, with elements bonding together to form various structures.
③ Constraint stress
Constraint stress is caused by two factors: first, due to the rapid process of cladding, the temperature of the melt pool is too high. The melted part of the material in the melt pool expands due to heat, but is constrained by the surrounding material, thus subject to compressive stress.
On the other hand, after the scanning beam moves away, the scanned area cools down quickly, and the cladding area cannot freely contract due to the integrity of the material, resulting in significant tensile stress.
In fact, during the laser cladding process, these factors can act independently or interact to generate cracks. They are all major contributors to crack formation.
By understanding the causes of crack formation, we can analyze each stress-related cause and correspondingly identify methods to eliminate cladding layer cracks.
3. Methods to Eliminate Laser Cladding Cracks
3.1 The Impact of Laser Power Parameters on Cladding Layer Cracks
3.1.1 The Effect of Laser Power
The selection of laser output power in the laser cladding process is usually very important, as it directly affects the thermal relationship changes between the cladding layer and the melt pool.
After reviewing a large number of documents, it was found that appropriately increasing the power, while meeting the processing requirements, has a good effect on eliminating the cladding layer cracks.
However, increasing the power will also directly increase the dilution rate, which may reduce the final effect of the experiment, thus creating a contradictory relationship.
Therefore, we have to analyze and select the power according to the specific situation of the experiment. But currently, due to the lack of quantitative data analysis, it’s difficult to find the optimal experimental result.
Therefore, I think we should study and clarify under permissible conditions to find out under what circumstances the best results can be achieved.
3.1.2 The Impact of Scanning Speed
The impact of scanning speed, i.e., the displacement of light moving at a constant speed per unit time, also directly affects crack control.
Research has found that appropriately reducing the scanning speed can eliminate cracks while meeting the experimental requirements.
This is because cracks are sensitive to changes in scanning speed, primarily referring to the increase in temperature gradient and thermal stress with increased scanning speed.
However, a review of other literature found that reducing the scanning speed will decrease the smoothness of the cladding surface, a contradiction that is worth considering for finding the optimal process parameters in the future.
3.1.3 The Impact of Spot Size
This refers to the spot size and shape, considering the uneven temperature distribution of the laser beam, the spot size should be appropriately increased.
This can be achieved by using broadband laser cladding or changing the beam mode, such as adopting a multimode laser beam, to create a “multi-peak distribution” of energy. This provides a more uniform heat distribution and reduces crack sensitivity.
Therefore, appropriately increasing the spot size under the experimental conditions is beneficial.
However, it’s worth noting that the spot size greatly affects the dimensional accuracy of the part contour. If the contour accuracy of the part needs to be improved, a smaller spot size is required.
3.2 Impact of Material and Other Parameters
Other parameters include: the composition of the molten layer, the powder used in the molten layer, the amount of powder supplied, the shape of the substrate, the thickness of the coating, etc.
3.2.1 Impact of Molten Layer Material
Different materials have different physical properties, such as thermal expansion coefficients, specific heat capacity, thermal conductivity, and solubility, etc. The most influential among these on laser cladding is the thermal expansion coefficient.
The difference in thermal expansion coefficients between the cladding material and the substrate is the main cause of thermal stress after laser cladding, and this thermal stress is a significant factor leading to cracking in the cladding layer.
Therefore, when selecting the laser cladding material, one should consider both the performance of the cladding layer to be achieved after cladding and the thermal expansion coefficients of the cladding and substrate materials.
It is generally required that the substrate’s thermal expansion coefficient should be slightly greater than that of the cladding material to minimize tensile stress.
3.2.2 Impact of Other Parameters
The composition of the cladding powder, primarily the size of the powder particles, is crucial. It has been found that larger particles require more energy to completely melt.
However, because laser cladding is a process of rapid melting and rapid solidification, the time of action is very short.
This could lead to some large particles not being completely dissolved, and residual stress in the melt pool can lead to cracking.
The amount of powder supplied, mainly referring to the volume of powder, is also impactful. It’s been found that a larger volume of powder in a unit time can result in the powder not being entirely melted, that is, some of the powder enters the molten layer and mixes without being melted, which can also lead to cracking.
The shape of the substrate, specifically the regularity of the substrate’s shape, is another factor. Under permissible conditions, the substrate must be regular, and its surface must be polished to be very smooth and flat.
In special cases, i.e., when the substrate shape cannot be changed and is not smooth, real-time beam transformation is used for cladding, otherwise, it can lead to cracking.
The thickness of the coating, specifically the thickness of the pre-coated powder, is also influential.
On the premise of meeting experimental requirements, the coating thickness theoretically should be as thin as possible. This is related to heat transfer during laser cladding; otherwise, cracks can occur during thick layer cladding.
4. Comprehensive Analysis of Factors in Crack Elimination in Laser Cladding
Through the above analysis, the relationship between process parameters in laser processing and the impact on cladding cracks can be approximately represented by the following equation:
E = P/DV
Here, E represents laser energy density, P stands for laser power, D is the spot size, and V is the scanning speed.
The above equation allows us to understand the relationship between various laser process parameters.
By adjusting these parameters, under other satisfactory conditions, the optimal parameters can be found to ensure a cladding layer free from cracks, guaranteeing the best experimental results.
Combining domestic and international research results, we have analyzed methods for eliminating cladding cracks from the perspective of material parameter selection, including heat treatment, ductile alloying elements, and ultrasonic vibration.
Heat treatment mainly involves preheating and slow heating of the base material, which greatly benefits the reduction of thermal stress and temperature gradient in laser cladding.
The methods include using a resistance furnace specially designed for laser processing, heating the workpiece by remote infrared radiation, and heating the workpiece using a flame gun and wrapping it with insulation cotton to increase heat capacity.
Ductile alloying elements refer to the addition of toughening and plasticizing elements to control crack formation. The addition of such elements depends on the specific relationship between the base and cladding materials.
Ultrasonic vibration refers to the cavitation and stirring action of ultrasonic waves in the molten pool, which breaks the growing dendrite network into evenly distributed small crystal nuclei, rendering the hard phase and temperature gradient evenly distributed, reducing the gas content in the molten pool, and thus reducing crack formation.
Additionally, methods such as secondary scanning and the introduction of transition layers can also eliminate crack formation.
From the perspective of laser equipment, the energy distribution uniformity and stability of domestic laser processing equipment and supporting devices have not yet fully met the level of large-scale industrial production applications.
The stability of high-power beams of domestic laser equipment is generally within 5%, and such power deviation affects the consistency of the performance of laser cladding samples.
Therefore, I believe that after fully understanding and comprehending the basic knowledge of laser manufacturing principles, if a method can be found to increase the laser output power, even if it is only increased by 1%, it can greatly accelerate the industrial production of lasers, so this is also worthy of our research.
Furthermore, we can see that the selection of powder materials also has different cladding effects on different base materials, so it is crucial to choose the best cladding powder for different industries.
Below are some commonly used powder materials, such as: elemental substances and compounds containing such elements in cladding materials, B, C, Ni, Si, B4C, Cr3C2, etc., self-melting alloy powder Fe-based powder, Co-based powder, Ni-based powder, ceramics TiC, BN, SiC, TiB, TiB2, etc., intermetallic compounds Ti3Al, TiAl, Ti5Si3, etc., oxides Al2O3, Zr2O3, R2O3(R are rare earth elements), biocoating hydroxyapatite (HA), CP coating, etc., composite powder NiCrBSi, CoCrWB, NiCrFeBSi mixed powder, etc.
Therefore, it is evident that the research on cladding materials in the laser cladding process is both a focus and a challenge. If a material with excellent performance and widespread applicability can be found in laser cladding, it would be of great significance, making it a worthwhile field of study.
Additionally, in conjunction with the review of other literature , we find that in the laser process parameters, appropriately reducing the scanning speed greatly helps in eliminating cracks under the premise of satisfying experimental requirements.
However, it does not effectively control the surface smoothness of the melt layer. This is a problem that deserves to be solved.
For powder feeding cladding, we conclude that under the premise of meeting experimental requirements, reducing the amount of powder supply significantly influences the generation of cracks and the smoothness of the surface.
Therefore, we can identify the optimal powder supply amount through multiple comparative experiments.
Moreover, increasing the laser power can effectively eliminate cracks, but it has a counterproductive effect on the surface smoothness of the melt layer and dilution rate.
On this basis, if we can find the optimal experimental data, it would be highly beneficial for the entire laser cladding process, and further promote the application of lasers in industry.
5. Conclusion
Laser cladding technology is a broad and complex field. Each detail can impact the experimental results.
At the current stage, both domestic and foreign researches on laser cladding processing technology are not comprehensive and in-depth, including the research on the melt pool, stress, cracks, and convection mass transfer.
In the future, we will intensify efforts to explore new methods in the direction of laser cladding.
Regarding the impact of stress on cracks in laser cladding and the condition of melt pool convection mass transfer during the cladding process, we may use the isotope element labeling method learned in chemistry to track the thermal changes and combination conditions of the elements, to thoroughly grasp their distribution and combination with other elements, thus gaining a deeper understanding of stress and helping to eliminate cracks.
We may also use a CCD high-speed camera for synchronous observation to grasp the relationship of convection mass transfer in the melt pool, which can lead to a more in-depth and detailed understanding of the impact of convection mass transfer in laser cladding on the combination of the cladding layer and the distribution of elements.
