Morphology and Performance Characterizations of 316 Stainless Steel Additively Fabricated by Laser Thermal-Joule Heating Composite Process

: The Laser Thermal-Joule Heating Composite Process was studied by orthogonal tests based on an analysis of fabrication parameters such as the laser power, wire feeding speed, and electric current. Temperature profiles and the geometric morphology of deposited layers under different process parameters were analyzed, and the overlaps between the layers and the substrate were observed. Results show that when the temperature at the bottom layer of the additive manufacturing is higher than the melting point of the substrate, and the highest temperature at the top layer does not exceed the over-firing temperature, good morphology and close bonding with the substrate can be obtained. Finally, appropriate process parameters were identified and verified to print multiple layers continuously.

speed, and voltage on the single-layer fabricated coatings were investigated, which mainly adjusts the laser power and current value, simultaneously a CCD was adopted to monitor the temperature at different locations of the single-layer fabricated coatings in LT-JH CAM process. Third, based on the optimization of the process parameters of single-layer fabricated coatings, multi-track coatings were manufactured under different laser power and current values. Last, the density, tensile performance and industrial CT test of the fabricated coatings were investigated.

Alloy Composition and Material
The 316 stainless steel of wire and substrate are used in this study and chemical composition is given in table 1 as follows [16].

Forming principle and experimental device
The principle of this study is to use a pulsating wire feeding mechanism and a programmable heating power supply to short-circuit the metal wire and the substrate. The wire is melted by resistance heating, coordinated with the pulsating behavior and output current during the feeding process of the wire, to achieve quantitative melting of the ends of the wire, and to realize the additive manufacturing of parts during the laser-assisted heating process. The highspeed camera system CCD can monitor the wire heating process, which is shown as Figure 1. existing mainstream high-energy beam additive manufacturing technology, the method in this study has the following advantages of low equipment cost, high-energy conversion efficiency, high forming efficiency, and small deformation.
In addition, it can solve the problems of large stress, large deformation and easy cracking of many parts, which has great research value and application prospects.  because the molten pool is in a dynamic process, these detection methods currently have certain errors. Therefore, this study uses a combination of numerical simulation and experimental verification to achieve quantitative control of the molten pool temperature through temperature monitoring, thereby effectively improving the quality of the cladding.

The heat source model
According to the current proposed volumetric heat source, the double ellipsoid heat source can well describe the shape of the molten pool when the heat source moves in the depth direction, the similar semi-ellipsoidal heat source is used in this study. The difference between them is that due to the influence of the movement of the heat source, the heating area at the front of the heat source is smaller than that at the back. Therefore, the semi-axial length of the front hemisphere of the double ellipsoid is smaller than the semi-axial length of the rear hemisphere, while for the semiellipsoid heat source In other words, the front and rear hemispheres are completely similar. In this experiment, the moving speed of the platform is only 0.005m/s. When the heat source moves, the front and back heating areas of the heat source have little effect. Therefore, this article believes that the semi-ellipsoidal heat source can be used to replace the actual heat source [17]. As shown in Figure 3, suppose the semi-axes of the ellipse are , , , at the center of the heat source, that is, the origin of the coordinate system (0, 0, 0), the maximum heat flux density is , and the function expression of the heat flux distribution is: In the formula, A, B, C are the heat flow volume distribution parameters. Through a series of derivations, we can get: The parameters , and should be selected reasonably according to the actual measured penetration depth and penetration width of the bonding zone of the cladding layer.

Substrate heating heat source model
During the experiment, it is necessary to preheat the bottom plate in advance. The purpose of this is to reduce the temperature gradient during the printing process, so that the quality of the cladding molding can be guaranteed.
Because the bottom plate has been heated for a period of time at the beginning of the experiment, the substrate can be regarded as a constant temperature value during the numerical simulation. The treatment of this heat source is to load the bottom surface of the substrate with a constant temperature as a load.

Loading model
First the model into the transient temperature analysis module is imported, then the grids are divided, Enter the temperature of the environment, set the convection heat transfer method, and load it in the heat flux method. When the wire feeder sends the wire to contact with the substrate, the heat source is loaded on the head of the wire and moves over time, dividing the loading process into several loading steps. According to "time step = unit size/scanning speed", when the unit size is unchanged and the time step is increased, the scanning speed becomes slower. For example, according to the actual laser power is 600w, the light source radius is 0.001m, the platform moving speed is 0.005m/s, the effective thermal power value and formula (3)

Temperature field simulation model
In this study, the finite element software is used for analysis and hexahedral cells are used for meshing. Because the physical properties of the cladding layer and the substrate are different, the grid division of the cladding layer is refined, and a coarse grid is used at a position far from the scanning area to increase the calculation speed. Use formula (1)(2)(3)(4) to import the interface for definition to complete the loading of dual heat sources. At the same time, apply a constant temperature load to the lower surface of the substrate, and then observe the time for heat transfer to the upper surface of the substrate. Wait for this period of time before applying dual heat sources to simulate the temperature of the preheating process of the substrate [18].
The laser and electric composite additive manufacturing equipment is used. And the laser is irradiated on the substrate to form a molten pool. When the current passes through the wire and the substrate contact resistance, a huge amount of heat is generated. At the same time, under the control of the computer, the laser and the wire feeding head move to the preset direction. The mechanism feeds the wire at a certain speed so that it can quickly contact the substrate after melting and breaking. As the liquid metal solidifies again, an additive manufacturing layer is formed.

Experimental methods
In this study, the numerical analysis parameters, include the length of the layer (58mm), the scanning speed (5mm/s), the voltage (10V), the laser power (250W, 200W and 150W respectively) and the current value (4A, 16A, 25A and 32A respectively). The parameters are shown in Table 2.This article focuses on the characteristics of forming parts under different laser powers and currents. After deposition, the as-deposited blocks were characterized to examine the morphology and the density. The laser power is 250w, and it is found that under the process parameters shown in Figure 6(a), the maximum temperature of point A, C, and D exceeds 1200℃, but the maximum temperature of point B is lower than 1200℃. It shows that although the wire is melted at this time, the transition zone of the additive manufacturing layer and the substrate may not be sufficiently dense, which is the "critical point" to be found. When the current is increased, the parameters at each point can meet our requirements. If the maximum temperature is too high when the current is increasing to 16A or 25A, the over burning will occur, so the process parameters shown in (b) are reasonable. As shown in Figure 8a, b and c, the temperature at point B is lower than 1200°C, which cannot form a molten pool with the substrate and thus cannot be well fused with the substrate. When the current is increased to 32 A, the temperature at point B is higher than 1200°C, which can form a molten pool with the substrate and fuse together well.
Therefore, when the laser power is 150w and the current is 32A, it is a better process parameter. according to the single-layer additive manufacturing obtained above, the optimization of the process parameters (as shown in Table 3)are selected, that is, when the scanning speed is 5mm/s , substrate temperature is 250℃,wire feeding speed is 540 mm/min, voltage is 10V, the laser power is 250w,200w,150w, the current is 16A,25A and 32A respectively, On the basis of single-layer parts, the process parameters are used for multi-layer parts, as shown in table   3.

Morphology characterization
When the laser power is 250w and the current is 4A, the wire cannot be well bonded with the substrate, which can be seen from table 4(1). The current from 4A to 16A gradually increases, but only the morphology of forming layer in Table 4 (2) is best, and the wire can be firmly bonded to the substrate. The thickness of  When the laser power is 200w and the current is 4, 16, 25 and 32 A respectively, current increases successively as shown in Table 5 (1)-(4), in which Table 5 (3) is the best, the thickness in Table 5(1) and (2) is uneven, are formed due to the high current point. When the laser power is 150w and the current is 4, 16, 25 and 32 A respectively. The morphology of the layers are shown in Table 6. It can be seen that the wire cannot be completely melted and the wire is broken, as shown in Table 6 (1), (2) and (3). But when the current is increased to 30A, the wire is firmly bonded to the substrate and can be combined with the substrate well, as shown in Table 6 (4). This is because during the forming process, the temperature at the bottom of the forming layer is relatively low. Therefore, when the laser power is 150w and the current is 32A, it is a better process parameter. according to the single-layer additive manufacturing obtained above, the optimization of the process parameters (as shown in Table 7)are selected, that is, when the scanning speed is 5mm/s , substrate temperature is 250℃,wire feeding speed is 540 mm/min, voltage is 10V, the laser power is 250w,200w,150w, the current is 16A,25A and 32A respectively, On the basis of single-layer parts, the process parameters are used for multi-layer parts, as shown in table 8. In the case of other parameters unchanged, when the laser power is 250w, 200w, 150w, the current is 16A, and 25A and 32A respectively, the good performance of the cladding layer can be obtained. That is the optimal resistance heat, which can make full use of resistance heat and reduce the dependence on laser without causing the welding wire to fuse outside the molten pool. So that it can gradually reduce the laser power and apply current to find the process parameters of composite heating.

Density
The density of the part is an important index to measure the internal quality of the metal part. Density strengths along the horizontal and vertical directions of the samples built by three different processes are presented in Table 7.
Each reported value is an averaged value of three test results. After degreasing and cleaning, the mass of the samples in air and the water have used a balance to weigh respectively, and then according to the formula (5) obtain the density of the sample.
Where, m1 is the mass of the sample to be tested measured in air; m2 is the mass of the sample to be tested fully immersed in water; ρ is the density of the tested sample and ρ2 is the density of water.
When the laser power is 150w and the current is 32A, the density is 97.7%. When the laser power is 200w and the current is 25A, the density is 97.8%. And when the laser power is 250w and the current is 16A, the density is 98.1%. It can be seen that the compactness of the part is very high, and the layers can be fully fused under the better process parameters to realize the metallurgical bonding between the overlapping layer materials.

Tensile performance
Tensile strengths along the horizontal and vertical directions of the samples (shown in It also can be seen from Table 9 that the strength of the 316L stainless steel specimens prepared under the three different processes is higher than the standards of castings and forgings, and the tensile strength of the specimens' increases with the increase of laser power

Industrial CT test
The parts shown in table 10 were tested by industrial CT. When the laser power is 150w and the current is 32A, the defect volume ratio is 4%. When the laser power is 200w and the current is 25A, the defect volume ratio is 2.4%.
And when the laser power is 250w and the current is 16A, the defect volume ratio is 1.7%. (As shown in table 10).
Defects basically appear on both ends of the part, at the start and stop positions of printing. The main reason is that at the beginning of printing, the wire has been preheated to a certain temperature and melted instantly under the high energy of the laser. The air path is opened at the same time, and part of the oxygen is drawn in, causing the parts to have pores. When printing stops, the laser and the gas circuit are closed at the same time. At this time, the metal has not completely solidified, and there will be oxygen inclusions to form pores.

Conclusions
(1) When the laser power is 250w, 200w, 150w, and the current is16A, 25A, 32A respectively, the results of meet the requirements can be obtained. The results of multi-layer additive manufacturing show that each printing layer will affect the subsequent printing, which is equivalent to the effect of preheating. Each layer printed up will also affect the previous additive manufacturing layer, but this effect will become smaller and smaller, so the wave peaks appear at regular intervals but show a gradual decline. .