An optimization design method for the composition gradient path of a functionally graded material

Abstract

The present invention discloses a method for optimizing the design of the composition gradient path of a functionally graded material. Aiming at the process of preparing FGM transversely by double-wire WAAM, the design method of the gradient path is optimized. By optimizing the gradient distribution function, curve fitting is performed on the gradient distribution function of the harmful phase segments, so that the harmful phase segments are smoothly transitioned without step composition points and the harmful phase region is reduced, thereby improving the overall performance and service life. The present invention uses a double-wire WAAM system to prepare FGM with a gradient transverse transition, avoiding step or non-smooth composition points in the gradient path and avoiding obvious interfaces, thereby improving the overall performance and service life of FGM.

Description

Technical Field

[0001] The present invention belongs to the field of arc additive manufacturing of functionally graded materials, and more specifically, relates to a method for optimizing the design of the composition gradient path of a functionally graded material. Background Art

[0002] Functionally graded materials (FGMs) refer to new composite materials with special functions in which elements such as the chemical composition and microstructure of the material change continuously or quasi-continuously from one side to the other along the thickness or length direction of the material, so that its physical, chemical and other properties change continuously in a gradient manner according to the design requirements. FGMs were initially used to alleviate thermal stress and were applied in high-temperature environments, especially in environments where there are large differences in the performance requirements or service temperatures on both sides of the material. Therefore, the uniformity and smoothness of the composition gradient transition of FGMs, as well as the composition components that are unfavorable to the material properties in some gradient paths, are important factors affecting the performance and service life of FGMs.

[0003] Additive manufacturing (AM) technology has been applied to the preparation of FGMs due to its flexibility. Currently, when using AM technology to prepare FGMs, the direction of the composition gradient transition mostly transitions in a gradient manner along the thickness direction (longitudinal direction) of the material. This transition method is quasi-continuous, and the uncertainty at the interlayer interface will affect the composition gradient transition, making the local composition change still not smooth. Therefore, changing the gradient direction to make it change continuously along the length direction (transverse direction) of the material can ensure uniform and smooth gradient transition. In addition, currently, laser additive manufacturing (LAM) and wire arc additive manufacturing (WAAM) are more commonly used in additive manufacturing technology. Compared with LAM, WAAM technology has a high material utilization rate because almost all of the wire is fed into the molten pool and the composition of the component can be precisely controlled. Therefore, by using a dual-wire WAAM system and changing the relative wire speed of the two wires during the forming process of a single layer, the preparation of FGMs with a gradient transverse transition may effectively control the composition change of the component. At the same time, during the process of preparing FGMs with a gradient transverse transition, in order to avoid obvious composition step interfaces, the composition gradient path can adopt a composition transition from 0 - 100%, that is, a complete transition from one material to another material is prepared. However, this also results in that in the entire gradient path, it is inevitable that there are some sections of harmful phase components, and the poor performance of the harmful phase component sections seriously affects the performance of the overall FGM. Therefore, it is necessary to optimize the gradient path of FGMs with a gradient transverse transition. Summary of the Invention

[0004] The purpose of the present invention is to overcome the deficiencies of the prior art, use a dual-wire WAAM system to prepare FGMs with a gradient transverse transition, aiming to avoid step or non-smooth composition points in the gradient path and avoid obvious interfaces, thereby improving the overall performance and service life of FGMs.

[0005] In the gradient path, inevitable tissues that are harmful to the overall performance will appear. These harmful phase components can be characterized through experiments and determined in combination with the phase diagram distribution. Therefore, this patent application proposes a design method for optimizing the gradient path in the process of transverse preparation of FGM by double-wire WAAM. The aim is to, without affecting the size of the transition section of the FGM, through optimizing the gradient distribution function, perform curve fitting on the gradient distribution function of the harmful phase components, enable the harmful phase components to smoothly transition without the occurrence of step composition points, and reduce the harmful phase region, thereby improving the overall performance and service life.

[0006] The technical object of the present invention is achieved through the following technical solutions.

[0007] A method for optimizing the composition gradient path of a functionally graded material is carried out according to the following steps:

[0008] Step 1, determine the distribution range of the harmful phase according to testing, observation or analysis;

[0009] Step 2, on the premise of ensuring that no step composition points appear in the entire gradient distribution path and the length of the transition section remains unchanged, optimize the gradient path function of the harmful phase components to obtain the gradient distribution function for different section paths;

[0010] Step 3, according to the gradient distribution functions of different section paths obtained in Step 2, control the relative speeds of each component in the preparation of the functionally graded material to achieve the purpose of reducing the aggregation degree of the harmful phase.

[0011] In the process of preparing the FGM with a gradient transverse transition, by controlling the relative wire speed change curve of the double wires, the gradient distribution function is obtained, and it is made to change linearly to achieve the gradient transition of 0 - 100% of the composition. However, in the entire section, harmful phase component sections will inevitably appear. Through experimental characterization and in combination with the phase diagram distribution, the harmful phase component sections are found, and then the gradient distribution function is optimized, as Figure 1 shown. Let the gradient distribution function be expressed as Equation (1), the length of the transition section be L, then x represents the relative distance from the composition point to the other side. Then f(0) = 0, f(L) = 1, which can represent the composition of the transition section to transition linearly from 0 - 100%.

[0012] F(x) = f(x) 0 ≤ x ≤ L (1)

[0013] Suppose the harmful phase distribution is around the position x0. Since the phase distribution is not concentrated at a certain absolute position but aggregated in a definite compositional region, that is, the harmful phase is segmented. Within this region, since the quantity and aggregation degree of the harmful phase are significantly higher than those in other compositional regions, this leads to stress concentration and ultimately reduces the performance of the FGM. Through experimental characterization and combined with the phase diagram distribution, suppose the harmful phase distribution is around the position x0. Since the phase distribution is not concentrated at a certain absolute position, the harmful phase segment is determined to be the compositional region segment from f(x1) = a to f(x2) = b, where x1 < x0 < x2 and 0 < a < b < 1. Optimize the gradient path function of this harmful phase segment. If directly avoiding the compositional points corresponding to this segment, step compositional points will appear, which is inconsistent with the principle of this method. Therefore, on the premise of ensuring that there are no step compositional points in the entire gradient distribution path and the length of the transition segment L remains unchanged, perform curve fitting on the a - b path segment, and the gradient distribution functions of the 0 - x1 and x2 - 1 path segments will also change accordingly. The optimized gradient distribution function is denoted as G(x), as shown in Equation (2).

[0014]

[0015]

[0016]

[0017] According to Equation (3), the function expressions of g1(x) and g3(x) can be obtained. The g2(x) segment needs to be curve - fitted, and the fitting principle is to ensure that each piece - wise function is continuously differentiable, and the fitting constraint condition is Equation (4). The comparison of the compositional gradient distribution function curves before and after optimization is as Figure 2 shown. The x - axis represents the relative distance of the compositional point to Material A, and the y - axis represents the compositional proportion of Material A in the FGM. Curve 2 is the optimized path curve. Since it is impossible to avoid the appearance of the compositional point x0, the distance of the compositional points in the harmful phase segment x1 - x2 is lengthened, that is, the compositional region segment from g1(x1) = a1 to g3(x2) = b1 in Figure 2 the attachment. In this way, the aggregation degree of the harmful phase can be reduced, its distribution can be made more dispersed, and the stress concentration caused by the aggregation of the harmful phase can be reduced, thereby improving the performance of the FGM.

[0018] The uniformity and smoothness of the FGM gradient transition are important factors affecting the performance and service life of gradient materials. Using a dual-wire WAAM system to prepare FGM with a gradient lateral transition can meet the uniformity and smoothness of the gradient change, achieve a complete transition between the two materials, that is, a 0-100% composition transition. The purpose of this invention patent is to provide an optimization method for the gradient path of preparing FGM with a gradient lateral transition using a dual-wire WAAM system. During the process of preparing FGM with a gradient lateral transition, since all composition ratios are involved, harmful phase component segments will appear. To avoid affecting the performance, when designing the gradient path, the component region should be minimized as much as possible and continuous transition should be ensured. During the lateral preparation process, the composition gradient transition is achieved by controlling the wire speed. Therefore, it is necessary to control the wire speed change curve, optimize the gradient distribution function, and optimize the gradient path. Furthermore, reduce the harmful phase region in the gradient path, improve the comprehensive performance of FGM, and extend the service life of the component. In addition, the method of this patent is not limited by the component size, has higher design flexibility, and can achieve more production applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is the schematic diagram of path selection (1) in the present invention.

[0020] Figure 2 It is the schematic diagram of path selection (2) in the present invention.

[0021] Figure 3 It is the schematic diagram of the FGM preparation system used in the embodiment of the present invention.

[0022] Figure 4 It is the SEM photo and EDS surface scan picture of the crack generated after preparation using the traditional method in the embodiment of the present invention.

[0023] Figure 5 It is the curve graph of the output voltage change of the board card in the embodiment of the present invention.

[0024] Figure 6 It is the distribution diagram of the FGM hardness change with position in the embodiment of the present invention, where (a) is the hardness change distribution diagram before gradient path optimization; (b) is the hardness change distribution diagram after gradient path optimization. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] The technical solution of the present invention will be further described below in conjunction with specific embodiments.

[0026] Build a dual-wire WAAM lateral preparation FGM system, as shown in the appendix Figure 3As shown in the figure. The equipment used mainly includes a Trans TIG 5000 Job G / F welding machine designed and manufactured by Fronius Austria, an ABB IRB 2600 six-axis robot, two independent wire feeding mechanisms, an Advantech_USB-4711A data acquisition card, a preheating device, and a shielding gas tail hood, etc. The maximum DC welding current of the welding machine is 350A, and the allowable shielding gas flow rate is 5–30L / min. At the same time, the pulse frequency, duty cycle, peak current, etc. can also be adjusted. The welding machine communicates with the robot through a DeviceNet data cable. After successful communication, the ABB robot can control the welding machine, including arc starting, arc extinguishing, gas supply, gas stop, etc. The wire speed range of the two wire feeding mechanisms is 0-500mm / min, and the wire speed error is less than 3mm / min; the board communicates with the PC through USB, controls the output voltage change curve of the board, thereby controlling the wire speed change, and realizing the preparation of FGM.

[0027] For Material A, stainless steel ER308L with a diameter of 1.2mm is selected, and for Material B, nickel-based alloy Inconel625 with a diameter of 1.14mm is selected. The material composition content is shown in Table 1. The substrate is selected as a stainless steel substrate. Before the test, the substrate is polished, cleaned, the surface oxide film and contaminants are removed, and it is fixed on the workbench. The FGM preparation process parameters are shown in Table 2.

[0028] Table 1 Material composition content (wt.%)

[0029] Material C P Si Mn S Ni Cr Mo Nb Ti Fe Inconel625 0.012 0.003 0.04 ＜0.01 0.001 Bal. 22.7 8.7 3.56 0.21 0.21 ER308L 0.024 0.019 0.42 1.85 0.01 9.5 20.2 - - - Bal.

[0030] Table 2 FGM preparation process parameter settings

[0031]

[0032] Prepare an FGM with a composition gradient of 0-100% transition and a transition section length of 80mm. Before optimizing the path, the starting wire speed of stainless steel ER308L is 0mm / min, and it starts to accelerate to a wire speed of 200mm / min at the end of the arc as the arc moves; Inconel625 is the opposite, and the two wires change according to a uniform acceleration law, realizing a 0-100% composition gradient transition from stainless steel ER308L to nickel-based alloy Inconel625.

[0033] The specimens prepared by the above method were tested, and it was found that when the proportion of stainless steel ER308L component was 80%, the precipitation and aggregation of harmful phases such as Laves phase, MC phase, M 23 C6 led to crack generation. No crack was found in other positions. The specimens were ground, polished, electrolytically etched with 10% oxalic acid solution at 6V voltage for 15s, and the crack morphology was analyzed by scanning electron microscope SEM (model: JSM-7800F) and energy dispersive spectrometer EDS, as attachedFigure 4 As shown. It can be found that the second phase is Nb, Mo precipitates, Laves phase and carbides. As the Ni content increases, the solubility of C in the matrix austenite decreases, resulting in an increased tendency for carbide precipitation. At the same time, Nb and C have a high affinity. When the addition amount of IN625 is less than 20%, NbC is formed first. In addition, Mo can promote the precipitation of the main carbide (Nb, Mo)-carbide (MC) formed in the later stage of solidification. Due to the high Cr content and low Al and Ti contents, there may be potential carbides M7C3 and M 23 C6, which leads to the generation of cracks. It is thus determined that the composition ratio of 80% of stainless steel ER308L is the harmful phase composition point. Furthermore, it is determined that the composition ratio (a) of ER308L ranging from 75% to 85% is the harmful phase component segment, and the corresponding position points are in the region from x1 = 60 mm to x2 = 68 mm relative to the stainless steel.

[0034] The forming time of the transition section is calculated to be 40 s according to the welding speed. To ensure that the dimensional specifications remain unchanged, the forming time after optimizing the path is also 40 s. Therefore, according to the optimized gradient path distribution function, by choosing the forming time as the x-axis and the wire speed as the y-axis, the relative wire speed change function of the twin wires can be obtained. The method for controlling the wire speed in this system is to control the voltage value of the wire feeding mechanism through an industrial board card. After testing for parameter matching, the relationship between the wire speed and the voltage value of the wire feeding mechanism in this system is that a wire speed of 100 mm / min corresponds to 1 V voltage. After calculation and data matching, the output voltage change curve of the board card is finally obtained, as shown in the appendix Figure 5 shown. The shaded part in the figure corresponds to the harmful phase component segment. Using the calculated output voltage change curve, the voltage output of the board card is controlled through PC-side software programming, and finally the gradient path optimization is realized.

[0035] The pre-optimization gradient distribution function F A (x) of stainless steel 308L of material A, and the pre-optimization gradient distribution function F B (x) of nickel-based alloy Inconel625 of material B are as follows:

[0036] F A (x) = 1.25x, F B (x) = -1.25x 0 ≤ x ≤ 80

[0037] Through mechanical property characterization, it is determined that the harmful phase component segment is in the range of 75%-85% of the composition of stainless steel ER308L. Stretching this interval of the composition segment can avoid the aggregation of harmful phases and reduce stress concentration. Based on this optimization idea, the optimized gradient distribution functions G A (x), G B (x) are obtained, and the two are in an opposite relationship.

[0038]

[0039] Among them, g2(x) can be curve-fitted in software according to the constraint conditions shown in the following formula, and finally a set of data points can be obtained to generate the gradient distribution function curve.

[0040]

[0041] For the FGM prepared after optimization, not only the size specifications are not affected, but also the harmful phase aggregation area is reduced, crack occurrence is avoided, and the mechanical properties of the FGM are improved. The microhardness of the FGM was tested using a microhardness tester (model: HVS-1000ZCCD) with a load of 500 g and a dwell time of 10 s. Figure 6 It shows the microhardness change diagram tested on the FGM every 1 mm, where the hardness of the non-gradient regions on both sides is compared with different colors. (a) is the distribution diagram of hardness changing with position before the gradient path optimization. The data shows that the average hardness of 100% SS308L is 164.86 HV 0.5 ; with the addition of IN625, the microhardness shows a trend of first decreasing and then increasing. When the mass fraction of IN625 is 10%-20%, the hardness is lower than the average hardness of the non-gradient regions on both sides, which can be defined as the weak mechanical property region, and the lowest reaches 140.07 HV 0.5 ; subsequently, when the mass fraction of IN625 is 100%, the hardness reaches the highest value of 224.1 HV 0.5 . (b) is the distribution diagram of hardness changing with position after the gradient path optimization. It can be found that with the addition of IN625, the hardness shows a linear increasing trend, and no weak mechanical property region is found in the range of 10%-20% IN625 content. Therefore, it can be considered that the optimization of the gradient path improves the aggregation of harmful phases in the weak mechanical property region and improves the mechanical properties.

[0042] The above makes an exemplary description of the present invention. It should be noted that without departing from the core of the present invention, any simple deformation, modification or equivalent replacement that can be made by those skilled in the art without creative labor falls within the protection scope of the present invention.

Claims

1. A method for optimizing the design of the composition gradient path of a functionally graded material, characterized in that, Proceed as follows: Step 1: Determine the distribution range of the harmful phase according to tests, observations or analyses. Step 2: Optimize the gradient path function of the segmented harmful phase on the premise of ensuring that there are no step component points in the entire gradient distribution path and the length of the transition section remains unchanged, so as to obtain the gradient distribution function for different section paths. Step 3: Control the relative speeds of the various components in the preparation of the functionally graded material according to the gradient distribution functions of different section paths obtained in Step 2, so as to achieve the purpose of reducing the aggregation degree of the harmful phase. In the process of preparing the FGM with gradient transverse transition, by controlling the relative wire speed change curve of the double wires, the gradient distribution function is obtained, so that it changes linearly to achieve the gradient transition of 0-100% of the composition.

2. The optimized design method for the composition gradient path of a functionally graded material according to claim 1, characterized in that The gradient distribution function is expressed as Equation (1). If the length of the transition section is L, then x represents the relative distance from the composition point to the other side, and f(0) = 0, f(L) = 1, which can represent the composition of the transition section to be gradient transition from 0-100%. F(x) = f(x) 0 ≤ x ≤ L (1). Suppose the harmful phase is distributed around the x0 position, that is, the harmful phase component section. Determine that the harmful phase component section is the composition region section from f(x1) = a to f(x2) = b, where x1 < x0 < x2, 0 < a < b < 1. Optimize the gradient path function of this harmful phase component section. On the premise of ensuring that there are no step component points in the entire gradient distribution path and the length of the transition section remains unchanged at L, perform curve fitting on the a-b path section, and the gradient distribution functions of the 0-x1 and x2-1 path sections will also change accordingly; the optimized gradient distribution function is expressed as G(x), as shown in Equation (2). Obtain the function expressions of g1(x) and g3(x) according to Equation (3). The g2(x) section needs to be curve-fitted, and the fitting principle is to ensure that each segmented function is continuously differentiable. The fitting constraint condition is Equation (4).

3. A method for optimizing the composition gradient path of a functionally graded material according to any one of claims 1-2, characterized in that Use a double-wire WAAM system to prepare the FGM with gradient transverse transition.

4. The gradient transverse transition FGM prepared by using a method for optimizing the composition gradient path design of a functionally graded material as described in any one of Claims 1-2.

5. The gradient transverse transition FGM according to claim 4, wherein Reduce the aggregation degree of the harmful phase, make its distribution more dispersed, reduce the stress concentration caused by the aggregation of the harmful phase, thereby improving the performance of the FGM and extending the service life of the component.

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