Experimental Study on Residual Stress in Laser Cladding Layers Based on Nanoindentation

Experimental Study on Residual Stress in Laser Cladding Layers Based on Nanoindentation

Wang Meixia

School of Mechanical Engineering, Tianjin University of Technology and Education, Tianjin, China

Abstract : This study investigates the influence of process parameters on the residual stress in laser cladding layers. Using Cr12MoV die steel as the substrate, Ni60 alloy powder cladding layers were fabricated. Single-track cladding experiments were conducted under varying laser powers (10001800 W), scanning speeds (513 mm/s), and powder feed rates (0.41.2 r/min). The residual stresses on the cladding layer surface were measured using nanoindentation technology, and the resulting data were analyzed with a modified energy-based model. The results indicate that the residual stress increases with higher laser power but decreases with higher scanning speeds and increased powder feed rates. These findings establish the relationship between cladding parameters and residual stress, offering a theoretical foundation and experimental support for process optimization.

Keywords : laser cladding, process parameters, nanoindentation, residual stress, influence mechanisms

1 INTRODUCTION

Laser cladding is an additive manufacturing technology widely employed in engineering due to its advantageous characteristics, which include a high cooling rate, stable performance, low dilution rate, broad applicability for various powder materials, and high flexibility [1-4]. This technique is capable not only of repairing damaged regions of components but also of modifying their surfaces to meet specific requirements. As a critical method for metal additive manufacturing and surface modification, the stress-induced deformation generated during the laser cladding process significantly affects the performance and dimensional accuracy of the parts, thereby limiting the further promotion and application of this technology [5-8].

Several studies have investigated the optimization of laser cladding process parameters. Kendall et al. [9] examined the effects of cladding parameters on molten pool temperature and cladding layer quality. Their research explored the influence of varying laser power and scanning speed on the temperature at the pool center. Using cladding parameters as independent variables and aiming for optimal dilution rate and forming coefficient, a multi-objective optimization was performed with a genetic algorithm. The optimal parameters obtained were a laser power of 1756 W, a scanning speed of 19.43 mm/s, and a powder feed rate of 19.878 g/min.Cui Zihan et al. [10] investigated the optimization of process parameters for preparing Mo2FeB2 coatings on 45 steel surfaces via laser cladding. Laser power, scanning speed, and powder feed rate were selected as the process parameters, with dilution rate and microhardness as the response targets for evaluating forming quality. The optimal combination was found to be a laser power of 5000 W, a scanning speed of 13.6 mm/s, and a powder feed rate of 15 g/s.Further parametric studies have been conducted on different material systems. Liu Lilan et al. [11] optimized the laser cladding process for Ni60 alloy powder on 56NiCrMoV7 steel surfaces. Their results indicated that the interaction between powder feed rate and scanning speed had the most significant impact on surface flatness. Laser power, powder feed rate, and overlap rate all significantly influenced the dilution rate.

Corresponding author: Meixia Wang. Email address: 2891152433@qq.com

1 College of Mechanical Engineering, Tianjin University of Technology and Education, Tianjin, 300222, China.

Powder feed rate had the greatest effect on microhardness, followed by laser power. The optimized parameters were a laser power of 1647 W, a powder feed rate of 0.5 rad/min, and a scanning speed of 5 mm/s.In another study, Liu Lilan et al. [12]

optimized cladding parameters for Ni60 alloy cladding layers, obtaining an optimal combination of laser power 1405 W, scanning speed 5.7 mm/s, and powder feed rate 0.4 r/min. Li Sicong et al. [13] investigated laser cladding of Ni60 alloy on 316L stainless steel and determined that the optimal parameters for a defect-free single-layer, single-track cladding layer were a laser power of 1405 W, a scanning speed of 5.3 mm/s, and a powder feed rate of 0.4 r/min.Additional research has explored composite coatings and the influence of preheating. Zhang Hongyan et al. [14] studied the effects of process parameters on the geometric characteristics and hardness of Stellite157/WC/TiC composite coatings produced by laser cladding on high-speed steel. The optimal parameter combination was reported as laser power 820 W, scanning speed 237 mm/min, and powder feed rate 9 g/min.Zheng Xiaohao et al. [15] investigated the influence of laser power, scanning speed, and preheating temperature on the temperature and stress fields during cladding. Using response surface methodology for parameter optimization with dilution rate as the target, the optimal parameter ranges were identified as a scanning speed of 0.0860.111 rad/s, a laser power of 2.7064.25 kW, and a preheating temperature of 103.836190.56 °C.

Current research on laser cladding processes primarily focuses on two optimization objectives: achieving dense cladding layers and enhancing material hardness. However, studies specifically addressing residual stress control during the laser cladding process remain relatively limited. As a critical factor influencing cladding layer performance, the magnitude and distribution of residual stress directly affect the fatigue resistance, corrosion resistance, and service life of the cladding layer.Therefore, this study conducts experiments to fabricate laser cladding layers under various process parameters. Residual stress measurements are performed on the cladding layers produced with different parameters using the nanoindentation method. Through single-factor experiments, the influence patterns of laser power, scanning speed, and powder feed rate on residual stress are analyzed.

1. PREPARATION OF LASER CLADDING LAYER SPECIMENS

   In this study, Cr12MoV cold work die steel was used as the substrate material, with specimen dimensions of 170 mm × 100 mm × 10 mm. After quenching, this material achieves a hardness of 50-60 HRC, exhibiting both high strength and wear resistance, making it a commonly used material for automotive molds. The chemical composition is presented in Table 1.Prior to the experiment, the substrate surface was ground and polished as a pretreatment. The substrate was then rigidly connected to a force measurement platform using bolts to ensure process stability during cladding.

   Table 1 Chemical Composition of Cr12MoV Die Steel (%)

   Element	Cr	C	Mo	Mn	V	Si
   Mass fraction(%)	11.25	1.55	0.45	0.35	0.2	0.35

   In this study, Ni60 self-fluxing alloy powder with a particle size range of 140-325 mesh was selected as the cladding material; its chemical composition is provided in Table 2. The powder exhibits high sphericity and a smooth surface, ensuring continuous and stable powder delivery during the process, thereby establishing a foundation for achieving quality control of the cladding layer.During the cladding process, the Si and B elements within the powder play a critical role. On one hand, Si and B react with oxygen to form protective films of SiO2 and 2O3, effectively inhibiting the oxidation of other alloying elements. On the other hand, the unique wetting characteristics of Si and B regulate the surface tension of the molten pool, optimizing the melt flow behavior.

   Table 2 Chemical Composition of Ni60 Powder (%)

   Element	C	Cr	B	Fe	Si	Ni
   Massfraction( %)	0.8	15.7	3.1	13.0	3.9	Bal.

   The laser cladding system employed in this study is illustrated in Fig 1. The system primarily consists of a YLS-4000-KS fiber laser (4000 W), a Yaskawa industrial robot with a positioning accuracy of ±0.005 mm, an RC-PGF-D-2 synchronous powder feeder (0-50 g/min), and a high-purity argon shielding system. The coordinated operation of these modules ensures the realization of high-quality cladding layers.

   Fig 1 Laser Cladding Equipment

   For the preparation of Ni60 laser cladding layers, single-track cladding experiments were conducted on Cr12MoV die steel substrates. Cross-sectional specimens of the resulting cladding layers were prepared. After cladding, the specimens underwent pretreatment procedures including grinding, polishing, cleaning, and etching in preparation for subsequent nanoindentation testing.The investigation focused on three key process parameters: laser power, scanning speed, and powder feed rate. Based on relevant literature, the parameter ranges were established as follows: laser power from 1000 to 1800 W, scanning speed from 5 to 13 mm/s, and powder feed rate from 0.4 to 1.2 r/min. The detailed parameter configurations and the single-factor experimental design are presented in Tables 3 and 4, respectively.Specimens exhibiting good morphological quality were selected and sectioned using wire electrical discharge machining, followed by grinding and polishing. Notably, no cracking was observed in the laser cladding layers after processing, indicating that the employed cladding parameters were within reasonable ranges.

   Table 3 Laser Cladding Process Parameters

   Factor Level	Laser powerP(W)	Scanning speedV(mm/s)	Powder discharge rateG(r/min)
   1	1000	5	0.4
   2	1200	7	0.6
   3	1400	9	0.8
   4	1600	11	1.0
   5	1800	13	1.2

2. NANOINDENTATION TESTING

   Fig 2 Nanoindentation Experiment (a) Sample image (b) Schematic diagram of indentation dot positions

   To ensure measurement accuracy, the ground and polished cladding layer specimens were ultrasonically cleaned prior to testing to obtain a contamination-free surface. Nanoindentation tests were conducted using an Anton Paar NHT2 nanoindenter equipped with a Berkovich diamond indenter. A maximum load control mode was employed, with the peak indentation load set to 50 mN. Six valid indentation locations were predetermined through pre-indentation targeting. The Ni60-Cr12MoV cladding layer was then subjected to nanoindentation testing, from which load-displacement curves were obtained.The corresponding load-indentation depth curves are presented in Fig 3. These data provide reliable raw data for subsequent residual stress analysis.

   Fig. 3: Load-Indentation Depth Curve for Nanoindentation Testing of Specimen

3. Energy-Based Method and Determination of Mechanical Parameters

   The classic Oliver-Pharr method does not account for the influence of residual stress on indentation contact behavior; therefore, its direct application for residual stress calculation can introduce significant errors. To address this limitation, an energy-based correction method was adopted in this study. The energy-based approach treats the nanoindentation process as an energy work process. By analyzing the load-displacement curve, the relationship between the total work and the elastic work is calculated, from which the hardness and elastic modulus of the material are subsequently determined. Because this method does not rely on the measurement of the contact area, it effectively avoids the influence of material pile-up on measurement accuracy.

   As illustrated in Fig 4, the area under the loading curve represents the total work (Wt), while the area under the unloading curve represents the elastic work (We). The difference between these two values corresponds to the plastic work (Wp).

   Wp =Wtot -We

   (1)

   Fig 4: Schematic Diagram of Load-Unload Curve

   Through dimensional analysis combined with a power-law constitutive model , the indentation load can be expressed as a function of material parameters and geometric parameters. Based on the theorem, the following dimensionless relationship can be derived:

   F Y

   E p E

   , n, ,

   (2)

   r r

   Furthermore, the total work and the elastic work can be expressed as:

   W

   hmax Fdh W

   hmax Fdh

   (3)

   t 0 e hr

   By incorporating the results from finite element simulations, a linear relationship can be fitted between the hardness ( H ), the reduced modulus ( Er ), and the indentation work

   We

   Wt

   hr hmax

   1 We

   H W E

   r

   t

   (4)

   Where ( k ) and ( ) are constants, ( hf) is the residual indentation depth, and ( hmax) is the maximum indentation depth.

   According to Hertzian contact theory and the Sneddon model, the relationship between the elastic modulus ( E ) and the unloading stiffness (S) is given by:

   1

   2 S

   H Fmax

   Er 2

   1 , A

   A2

   (5)

   By combining the aforementioned equations, the expression for residual stress can be derived (Suresh model, under fixed load conditions):

   p

   R H 0 1

   (6)

   p

   In the formula, (h0) represents the indentation depth in a stress-free state, and (h) is the measured indentation depth. By obtaining the unloading stiffness, elastic work, and total work of the tested specimen, the elastic modulus and hardness can be calculated, from which the residual stress is subsequently derived. This entire process eliminates the need for a contact area function, effectively avoiding the influence of the pile-up effect.

   During nanoindentation testing, the system defaults to 90% of the maximum load as the endpoint for unloading. Therefore, to determine the maximum residual indentation depth, a tangent line extrapolation of the unloading curve is required. In this study, the tangent line was drawn at one-third of the maximum indentation load, and its intersection with the X-axis was defined as the maximum residual depth ( hf ). The unloading stiffness (S) was extracted using the upper 50% of the unloading curve data.Following these calculation principles, the load-displacement curve of Specimen No. 1 was processed. The unloading stiffness (S), total work ( Wtot), elastic work ( We ), and maximum residual indentation depth ( hf ) were obtained, as shown in Fig 5.

   Fig. 5: Mechanical Parameters of Test Specimens (a) Total Work (b) Elastic Work (c) Unloading Stiffness (d) Maximum Residual Indentation Depth

   Nanoindentation tests were conducted on cladding layer specimens prepared under different laser power conditions

   while maintaining constant scanning speed and powder feed rate. The load-displacement curves were analyzed, and the surface residual stresses were calculated to quantitatively reveal the influence of laser power on residual stress.Following the calculation principles described above, the mechanical parameters required for the energy-based method were extracted. These parameters are summarized in Table 5.

   Table 5: Mechanical Parameters Required for Energy Method Calculations

   Laser powerP(W)	Pmax (N)	S (N/m)	Wtot (J)	We (J)	hf / hm
   1000	0.05	5.63e5	1.10e-8	2.02e-9	0.81
   1200	0.05	5.41e5	1.11e-8	2.09e-9	0.81
   1400	0.05	5.17e5	1.06e-8	2.22e-9	0.80
   1600	0.05	5.10e5	1.08e-8	2.23e-9	0.81
   1800	0.05	4.89e5	1.16e-8	2.33e-9	0.81

   Nanoindentation tests were performed on cladding layer specimens fabricated under varying scanning speeds while maintaining constant laser power and powder feed rate. The load-displacement curves were analyzed, and the surface residual stresses were calculated to quantitatively investigate the effect of scanning speed on residual stress. The mechanical parameters required for the energy-based method were extracted and are summarized in Table 6.

   Table 6: Mechanical Parameters Required for Energy Method Calculations

   Scanning speedV(mm/s)	Pmax (N)	S (N/m)	Wtot (J)	We (J)	hf / hm
   5	0.05	5.16e5	1.10e-8	2.20e-9	0.80
   7	0.05	5.13e5	1.16e-8	2.26e-9	0.80
   9	0.05	5.20e5	1.12e-8	2.22e-9	0.80
   11	0.05	5.26e5	1.15e-8	2.27e-9	0.81
   13	0.05	5.22e5	1.17e-8	2.34e-9	0.81

   Nanoindentation tests were conducted on cladding layer specimens prepared under different powder feed rates while maintaining constant laser power and scanning speed. The load-displacement curves were analyzed, and the surface residual stresses were calculated to quantitatively determine the influence of powder feed rate on residual stress.The mechanical parameters required for the energy-based method were extracted and are summarized in Table 7.

   Table 7: Mechanical Parameters Required for Energy Method Calculations

   Powder discharge rateG(r/min)	Pmax (N)	S N/m	Wtot (J)	We (J)	hf / hm
   0.4	0.05	5.24e5	1.11e-8	2.22e-9	0.81
   0.6	0.05	5.51e5	1.05e-8	2.13e-9	0.80
   0.8	0.05	5.36e5	1.09e-8	2.16e-9	0.81
   1.0	0.05	5.50e5	1.03e-8	2.06e-9	0.81
   1.2	0.05	5.15e5	1.07e-8	2.14e-9	0.81

4. ANALYSIS OF THE INFLUENCE OF CLADDING PROCESS PARAMETERS ON RESIDUAL STRESS

   Based on the residual stress values calculated using the energy-based method from Tables 5 through 7, the influence trends of laser power, scanning speed, and powder feed rate on the residual stress of the cladding layer were obtained. As shown in Fig 6(a), the residual stress reached a maximum value of 201 MPa at a laser power of 1800 W and decreased to a minimum of 85 MPa at 1000 W. Fig 6(b) illustrates the significant effect of scanning speed on residual stress. At a scanning speed of 5 mm/s, the stress value was 228 MPa; when the speed was increased to 13 mm/s, the stress decreased to 154 MPa. Overall, the residual stress decreased markedly with increasing scanning speed. Furthermore, Fig 6(c) presents the relationship between powder feed rate and residual stress. At a powder feed rate of 0.4 r/min, the residual stress reached its maximum of 217 MPa. As the powder feed rate increased to 1.2 r/min, the stress decreased to a minimum of 135 MPa, exhibiting a gradual decreasing trend with increasing powder feed rate.

   Fig 6. Residual Stresses in Clad Layers (a) Under Different Laser Powers (b) Under Different Scanning Speeds (c) Under Different Powder Feed Rates

5. CONCLUSIONS

This study experimentally investigated the residual stress in laser cladding layers based on nanoindentation and examined the influence of process parameters. The main conclusions are as follows:

1. Single-track laser cladding experiments were conducted using Cr12MoV die steel as the substrate and Ni60 alloy powder as the cladding material. Specimens were successfully fabricated under varying laser powers (1000-1800 W), scanning speeds (5-13 mm/s), and powder feed rates (0.4-1.2 r/min).

2. Residual stresses on the cladding layer surface were measured using nanoindentation technology with an energy-based correction method. The variation of residual stress with different process parameters was obtained.

3. Analysis of the results indicates that the residual stress increases with increasing laser power, decreases with increasing scanning speed, and exhibits a decreasing trend with increasing powder feed rate.

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