CN104792440B - Method of testing three-dimensional residual stress inside laser material increase manufacturing part - Google Patents

Abstract

The invention discloses a method for testing three-dimensional residual stress inside parts manufactured by laser additive manufacturing, and belongs to the technical field of residual stress testing. The method is based on the layer-by-layer drilling method, by establishing a compensation coefficient formula, calculating the compensation coefficient to compensate the measurement results, and improving the measurement accuracy of the layer-by-layer drilling method. Secondly, in order to measure the three-dimensional residual stress inside the part, two samples of the same size were printed by laser additive manufacturing under the same process conditions, and the residual stress was measured by layer-by-layer drilling method from two directions, and finally the measurement results were analyzed. Combined, the three-dimensional residual stress inside the laser additive manufacturing part is obtained. The three-dimensional residual stress test method proposed by the present invention can accurately detect the internal three-dimensional residual stress distribution state and change trend of the laser additive manufacturing parts, and provides a method and basis for controlling the internal stress of the laser additive manufacturing parts and preventing deformation and cracking of the parts.

Description

A test method for three-dimensional residual stress inside parts manufactured by laser additive manufacturing

technical field

The invention relates to the technical field of residual stress testing, in particular to a method for testing three-dimensional residual stress inside parts manufactured by laser additive manufacturing.

Background technique

Additive manufacturing technology is to convert designed products into 3D data through CAD (computer-aided design) software, and then use specific molding equipment (that is, additive manufacturing machines) to "manufacture" layer by layer with liquefied, powdered, and silky solid materials. "Out of the product. Additive manufacturing technology is also called "3D printing" or "rapid prototyping". The main molding methods of different additive manufacturing technologies according to the process include: stereolithography (SLA), layered solid manufacturing (LOM), laser selective sintering (SLS), fused deposition modeling (FDM) and metal near net shape. Different from traditional "removal" manufacturing, additive manufacturing technology can directly generate objects of any shape by adding materials based on computer graphics data without the need for original embryos and molds, so it can simplify the product manufacturing process and shorten the product development cycle. , improve efficiency and reduce costs.

However, in the process of laser additive manufacturing of metal parts, quality problems such as deformation and cracking often occur in the manufactured parts. These problems are restrictive problems that hinder the development and application of additive manufacturing technology. There are many factors that lead to these problems, but the most fundamental reason is that the temperature field distribution caused by the local heating of the laser beam leads to the concentration of residual stress in the part, which in turn causes problems such as "deformation and cracking". Therefore, it is a good way to solve the problem of "deformation and cracking" of laser additive manufacturing parts by testing the residual stress of laser additive manufacturing parts, analyzing its evolution law, and proposing residual stress control methods and criteria.

At present, the development of residual stress testing technology has reached dozens of related testing methods, but these methods mainly focus on testing the two-dimensional residual stress on the surface of the part, and there are not many methods that can test the three-dimensional residual stress inside the part. At present, only the short-wave X-ray diffraction method and neutron diffraction method can measure the three-dimensional residual stress inside the part. The test cost of these two methods is high, and they are not suitable for the measurement of residual stress of large parts on site.

Contents of the invention

Aiming at the current problem that it is impossible to measure the internal three-dimensional stress of laser additive manufacturing parts, the purpose of the present invention is to provide a method for testing the internal three-dimensional residual stress of laser additive manufacturing parts, which can accurately detect the internal stress of laser additive manufacturing parts. The three-dimensional residual stress distribution and change trend provide a method and basis for the internal stress control of laser additive manufacturing parts and the prevention of deformation and cracking of parts.

Technical scheme of the present invention is as follows:

A method for testing the internal three-dimensional stress of parts manufactured by laser additive manufacturing. The method first uses the layer-by-layer drilling method to measure the internal residual stress of the calibration sample block that has undergone stress-relief heat treatment, and then fits the compensation coefficient formula based on the measurement results. The results are compensated; then two parts of the same size are manufactured under the same process conditions by using laser additive manufacturing technology, and the residual stress is measured by layer-by-layer drilling method from two directions; finally, the compensation coefficient formula is used to calculate the residual stress of the part. The residual stress measurement results are compensated, and the compensated residual stress values are combined to obtain the three-dimensional residual stress inside the part. Specifically include the following steps:

(1) Select a piece of the same material as the part to be measured as a calibration sample, and perform stress-relief heat treatment on it to make its internal stress evenly distributed;

(2) Take 5 measuring points on the surface of the calibration sample block, and use the layer-by-layer drilling method to measure it. The measured value of the residual stress value of the calibration sample block decays layer by layer, which needs to be compensated; The average value of the measured values is used as the standard residual stress value of the entire calibration sample block, and then the residual stress value measured from the second to the last layer is compared with the standard residual stress value of the sample block to obtain the compensation coefficient of each layer; the final basis For the relationship between the number of layers and the compensation coefficient, use the polynomial regression method to fit the empirical formula of the residual stress compensation coefficient in the x-direction and y-direction as follows:

y _αx = 0.0188x ³ -0.3174x ² +1.9333x-3.3181 (1)

y _αy = 0.0223x ³ -0.3607x ² +2.0799x-3.5101 (2)

In the formula: x is the number of layers of the drilled hole, y _αx is the residual stress compensation coefficient in the x direction, and y _αy is the residual stress compensation coefficient in the y direction;

(3) Laser additive manufacturing technology is used to manufacture two parts with the same shape and size under the same process conditions, and the stress distribution of the two parts is the same; the layer-by-layer drilling method is used to measure the stress of the two parts from two directions. Residual stress value, that is: measure the residual stress value of one part in the x and y direction at different depths (Z direction), and measure the residual stress value of the other part in the y and z direction at different depths (X direction);

(4) Apply the empirical formula of the compensation coefficient obtained in step (2) to compensate the residual stress values measured in step (3), and then combine the compensated residual stress values to finally obtain the three-dimensional residual stress inside the part.

The stress relief heat treatment process described in the above step (1) is to raise the temperature to 750°C for 4 hours, and then furnace cool to room temperature.

Compared with the prior art, the present invention has the following advantages:

(1) High measurement accuracy

There is a key problem in the layer-by-layer drilling method. Since this method uses the surface strain gauges to collect the strain release values of each layer, the measurement accuracy of the surface strain gauges will be affected as the hole depth increases. Therefore the measurement accuracy of layer-by-layer drilling method is not high, and the measurement method that the present invention proposes, proposes compensation coefficient formula according to a large amount of measurement results, application formula calculates compensation coefficient and compensates the measurement result of layer-by-layer drilling method, measurement accuracy higher.

(2) Low measurement cost, suitable for industrial applications

For the test of three-dimensional residual stress inside laser additive manufacturing parts, the most effective measurement methods at home and abroad mostly use short-wave X-ray diffraction method and neutron diffraction method, and the core equipment used by these two methods is very expensive. There are only a handful of research institutions for this kind of equipment. Therefore, the measurement cost of these two methods is relatively high, and the measurement method proposed by the present invention is only based on ordinary strain measurement equipment, and the selected strain gauge is also a relatively basic strain measurement consumable, so the cost and cost are relatively low. Low. However, the measurement method proposed by the present invention has good portability of the equipment used, and is particularly suitable for practical industrial applications.

(3) It can measure the three-dimensional residual stress inside the part

At present, there are many methods for measuring residual stress. According to whether the test method is damaged or not, it can be divided into two categories: physical non-destructive test method and mechanical destructive test method. Non-destructive testing methods include: X-ray diffraction method, magnetic method, ultrasonic method, neutron diffraction method, etc. Mechanical testing methods include: split full release method, layer-by-layer cutting method, electrochemical corrosion peeling method, drilling method and other methods. Moiré interferometry and holographic interferometry based on the drilling method. At present, except for the neutron diffraction method and short-wave X-ray diffraction method with high cost, none of the above methods can measure the three-dimensional residual stress inside the part, and the measurement depth of neutron diffraction method and short-wave X-ray diffraction method is also limited (50mm within). The measurement method proposed by the present invention can obtain the three-dimensional residual stress inside the laser additive manufacturing part by measuring from two directions and combining the measurement results.

Description of drawings

Figure 1 shows the laser additive manufacturing process of the sample.

Figure 2 is the laser additive manufacturing sample after pretreatment.

Figure 3 is the bonding diagram of strain gauges and terminals.

Figure 4 is a wire diagram for welding.

Figure 5 is a layer-by-layer drilling diagram.

Figure 6 is a schematic diagram of the layer-by-layer drilling method in both directions; in the figure: (a) X-direction drilling; (b) Y-direction drilling; (c) drilling position on the part.

Figure 7 is the experimental test site.

detailed description

Below in conjunction with accompanying drawing and embodiment the scheme of the present invention is described in further detail:

The layer-by-layer drilling method is to drill holes with a small feed rate each time, measure the strain value produced by each drilling hole, and obtain the stress value of different layer thicknesses through calculation. This method has two deficiencies. First, it uses surface strain gauges to collect the strain release value inside the part. As the hole depth increases, the measurement accuracy of the surface strain gauges will be affected. Even when a certain depth is reached, the surface strain gauges cannot The strain value is measured; secondly, the layer-by-layer drilling method measures the residual stress in the X and Y directions at different depths, and cannot measure the three-dimensional stress inside the part.

The present invention aims at the problem of measurement error in the layer-by-layer drilling method, by measuring the internal residual stress of the calibration sample block after stress-relief heat treatment, and fitting the compensation coefficient formula according to the measurement results, the measurement results are compensated, and the layer-by-layer drilling is improved. The measurement accuracy of the hole method, while measuring the three-dimensional residual stress inside the part, using the laser additive manufacturing method to print two samples of the same size under the same process conditions, and using the layer-by-layer drilling method to measure the residual stress from two directions Measurement, and finally combine the measurement results to obtain the three-dimensional residual stress inside the laser additive manufacturing part.

Example 1:

(1) Preparation of samples

The experiment was carried out on a 5KW CO ₂ cross-flow laser, and the measurement samples were prepared in a vacuum environment. Both the laser additive manufacturing powder and the substrate material used are TA15 titanium alloy. Before the experiment, the substrate was ground and polished to remove the surface oxide layer and increase its surface finish, and then it was further cleaned with acetone. The TA15 titanium The alloy powder is dried in a vacuum environment at 120°C. The specific process parameters used in the experiment are shown in Table 1.

Table 1 Main process parameters of laser additive manufacturing

laser power scanning speed Powder feeding speed scanning method

P/W v/ mm s ^-1 f/ g min ^-1 3500 7 0.7 interlaced scanning

The overall size of the prepared sample is 56mm×24mm×30mm. The sample preparation process and the laser additive manufacturing sample after pretreatment are shown in Figure 1 and Figure 2.

(2) Residual stress measurement

The sticking technology of the resistance strain gauge is relatively complicated, and the sticking quality has a great influence on the reliability of the measurement. Therefore, it is a very critical link. The following steps are strictly followed in the test.

1) Design layout plan: The distribution of strain gauges in the test is consistent with the distribution of strain gauges in the calibration sample, the diameter of the drilled hole is 1.5mm, and the distance between strain gauges is 18mm to avoid interference between holes.

2) Sheet selection: first check the appearance of the strain gauge, remove the strain gauge with shape defects in the sensitive grid and the strain gauge with bubbles, mold spots, and rust spots in the sheet, and then use the bridge to measure the resistance value of the strain gauge, and select the resistance value .

3) Grinding: Grinding the positions of the points to be measured on the surface of the test piece to make the surface smooth.

4) Line drawing: Accurately draw a cross line with a steel needle at the measured point for positioning.

5) Cleaning: Clean the surface of the part to be tested with cotton wool soaked in acetone to remove grease and dust and keep it clean.

6) Paste: Apply a layer of adhesive evenly on the back of the selected strain gauge, the thickness of the adhesive layer should be moderate, then align the cross line of the strain gauge with the cross line of the test piece to be tested, and gently correct the direction , then cover with a piece of cellophane, roll the strain gauge with your fingers in one direction, squeeze out air bubbles and excess glue, and ensure that the glue layer is as thin and uniform as possible, and then paste the lead terminals with the same adhesive (as shown in Figure 3 ).

7) Curing: Dry naturally for more than 4 hours after patching.

8) Inspection: Check the resistance of the strain gauge and the insulation of the lead wire of the strain gauge with a multimeter.

9) Fix wires: Solder the two lead wires of the strain gauge to the terminal, and then lead the connecting wire out of the terminal (as shown in Figure 4).

10) Use the layer-by-layer drilling method to measure the internal residual stress of the sample. When drilling, the alignment device is used to align the drilling position through the microscope, and then the holes are drilled layer by layer at a constant speed (Figure 5) to the set depth. Here, a total of 5 layers are drilled, and the thickness of each layer is 1mm.

(3) Compensation coefficient fitting formula

Select a calibration sample (TA15) made of the same material as the part to be measured, and perform stress relief heat treatment (750°C for 4 hours + furnace cooling) to make the internal stress evenly distributed. Take 5 points on the surface of the calibration sample block, and use the layer-by-layer drilling method to measure them. It is found that after the layer-by-layer drilling reaches the second layer, the measured value of the residual stress value continues to attenuate, which needs to be compensated. Here, the average value of the measured values of the first layer of the 5 measurement points is used as the standard residual stress value of the entire calibration sample block, and then the residual stress value of the 2-8 layer is compared with the standard residual stress value of the sample block to calculate the residual stress value of each layer Compensation factor. Finally, according to the relationship between the number of layers and the compensation coefficient, the polynomial regression method is used to fit the empirical formula of the residual stress compensation coefficient in the x and y directions, as shown in formula 1 and formula 2.

y _αx = 0.0188x ₃ -0.3174x ² +1.9333x-3.3181 (1)

y _αy = 0.0223x ₃ -0.3607x ² +2.0799x-3.5101 (2)

Where x is the number of layers drilled, y _αx is the residual stress compensation coefficient in the x direction, and y _αy is the residual stress compensation coefficient in the y direction.

(4) Three-dimensional residual stress measurement inside laser additive manufacturing parts

Two samples were manufactured under the same process parameters, and the stress distribution state of the two samples should be basically the same. The residual stress values of the two samples were measured from two directions using the layer-by-layer drilling method (Fig. 6). Residual stress values in y and z directions at different depths (X direction). Then apply the compensation coefficient formula to compensate for it. Finally, the measured values are combined to obtain the three-dimensional residual stress inside the sample, as shown in Table 2, and the test site is shown in Figure 7.

Table 2 Three-dimensional residual stress measurement value (unit: MPa)

Note: σ _x is the residual stress value in the x direction, σ _y is the residual stress value in the y direction, and σ _z is the residual stress value in the z direction.

Analysis of Table 2 shows that, inside the sample, σ _z is generally much larger than σ _x and σ _y , and it is compressive stress, and the residual stress at both ends is larger than that in the middle. Moreover, the internal residual stress of the sample manufactured by laser additive manufacturing is smaller than that of the surface layer. As the drilling depth increases, the internal residual stress of the sample gradually decreases. Moreover, the surface residual stress is mainly tensile stress, and it gradually turns into compressive stress after going deep into the 6 layers (6.6mm) of the sample.

Claims (3)

1. A method for testing the internal three-dimensional residual stress of laser additive manufacturing parts, characterized in that: the method first adopts the layer-by-layer drilling method to measure the internal residual stress of the calibration sample block through the stress relief heat treatment, and fits according to the measurement results Compensation coefficient formulas were developed to compensate the measurement results; then two parts with the same size were manufactured under the same process conditions by using laser additive manufacturing technology, and the residual stress was measured by layer-by-layer drilling method from two directions; finally The compensation coefficient formula is used to compensate the residual stress measurement results of the part, and the compensated residual stress values are combined to obtain the three-dimensional residual stress inside the part; the method includes the following steps: (1) Select a piece of material that is the same as the part to be measured as a calibration sample, and perform stress-relief heat treatment on it to make its internal stress evenly distributed; (2) Take 5 measurement points on the surface of the calibration sample block, and measure it by layer-by-layer drilling method. The measured value of the residual stress value of the calibration sample block decays layer by layer, and compensation is required; the first layer of 5 measurement points is The average value of the measured values is used as the standard residual stress value of the entire calibration sample block, and then the residual stress value measured from the second to the last layer is compared with the standard residual stress value of the sample block to obtain the compensation coefficient of each layer; the final basis For the relationship between the number of layers and the compensation coefficient, use the polynomial regression method to fit the empirical formula of the residual stress compensation coefficient in the x-direction and y-direction as follows: y _αx =0.0188x ³ -0.3174x ² +1.9333x-3.3181 (1) y _αy =0.0223x ³ -0.3607x ² +2.0799x-3.5101 (2) In the formula: x is the number of layers of the drilled hole, y _αx is the residual stress compensation coefficient in the x direction, and y _αy is the residual stress compensation coefficient in the y direction; (3) Laser additive manufacturing technology is used to manufacture two parts with the same shape and size under the same process conditions, and the stress distribution of the two parts is the same; the layer-by-layer drilling method is used to measure the stress of the two parts from two directions. residual stress value; (4) Apply the empirical formula of the compensation coefficient obtained in step (2) to compensate the residual stress values measured in step (3), and then combine the compensated residual stress values to finally obtain the three-dimensional residual stress inside the part. 2. The method for testing the internal three-dimensional residual stress of laser additive manufacturing parts according to claim 1, characterized in that: the stress relief heat treatment process in step (1) is to heat up to 750 ° C for 4 hours, and then furnace cool to room temperature. 3. The method for testing the internal three-dimensional residual stress of laser additive manufacturing parts according to claim 1, characterized in that: said in step (3) adopts the layer-by-layer drilling method to measure the residual stress of two parts from two directions respectively. The stress value refers to measuring the residual stress values of one part in the x and y directions at different depths, and measuring the residual stress values of the other part in the y and z directions at different depths.