New application progress of full two-dimensional surface residual stress analyze

Research background

Direct laser deposition (DLD), as a highly reliable manufacturing method, is recognized as one of the effective metal additive methods. DLD is a common processing technology under complex working conditions such as emergency maintenance and outer space operation. Especially in recent years, DLD has become more and more important in the repair and manufacturing industry of high-value parts, complex parts and multi material composite parts, and has been widely used in the field of precision manufacturing. However, its process parameters (such as laser power, scanning graphics, etc.) tend to lead to changes in dimensional accuracy and structural characteristics. At present, many methods have been proposed to analyze the principle of deformation and residual stress, but it is still difficult to evaluate the influence of temperature and stress during rapid melting and solidification.

Research progress

Recently, researchers from Northeastern University, Institute of Metals of Chinese Academy of Sciences and CRRC Qingdao Sifang Locomotive and Rolling Stock Co., Ltd. have established a three-dimensional finite element model based on the thermal mechanical relationship in the multi-layer DLD, and applied the temperature and residual stress of continuous laser deposition (CLD) and pulsed laser deposition (PLD) (small and light portable X-ray residual stress analyzer is used in this paper- μ- X360s) and numerical model [1]. It was found that PLD process, including continuous pulse laser deposition (CPLD) and interval pulse laser deposition (IPLD), could improve the uniformity of temperature and residual stress more effectively than CLD process. This research is expected to be used for deformation control and microstructure consistency of multilayer deposition parts. The detailed research shows that:

1. The temperature results obtained from the proposed simulation model show reasonable consistency with the experimental data collected from the thermocouple (the error of the results of the three measuring positions is within 10%, and the error of the beginning and end of the experiment is 20%);

2. At the end of each deposition layer, the temperature field shows (note: CPLD and IPLD) that PLD process cools faster than CLD process. This indicates that the pulse interval reduces the heat accumulation in the sedimentary layer;

3. The IPLD process adopted shows a more uniform distribution of residual stress, which is due to the small changes in temperature field and thermal stress;

4. With the increase of the number of deposition layers, the high temperature of the three processes (note: CLD, CPLD, IPLD) gradually increases; Among them, the temperature of CLD process increases rapidly. Due to heat conduction, the temperature in the middle area of each layer is higher than that on both sides. At the beginning of deposition, the cooling rate of IPLD process is fast, but the cooling rate values of the three processes gradually decrease with the deposition process.

The results reported in this paper will be helpful for the analysis of controlled heat accumulation and stress concentration in DLD process. The IPLD process is applicable to thin-walled parts that are greatly affected by deformation. For larger parts, the efficiency of the deposition process still needs to be considered.

Graphical Abstract

 
Schematic Diagram of DLD Technical Principle

Comparison data between simulation and experimental results of residual stress

Distribution Comparison Diagram of Temperature Field and Residual Stress Field of Different Processes (a) CLD (b) CPLD (c) IPLD

Equipment introduction

The residual stress testing equipment used in this article is the model of the Japanese Pulstec company μ- X360s X-ray residual stress analyzer. Based on circular full two-dimensional detector technology μ- X360 series X-ray residual stress analyzer equipment has the characteristics of advanced technology, high test accuracy, mini size, light weight, high portability, etc. It can not only be used in the laboratory, but also be easily carried to various workshops under non laboratory conditions for on-site or outdoor residual stress measurement in situ. We expect that this equipment can help more users at home and abroad to do excellent scientific research!

Source: Yangtze River Delta Laser Alliance