Patent Publication Number: US-2019184494-A1

Title: Systems and methods for global thermal control of additive manufacturing

Description:
CONTRACTUAL ORIGIN OF THE INVENTION 
     This invention was made with government support under 70NANB14H012, awarded by the U.S. Department of Commerce, National Institute of Standards and Technology Center for Hierarchical Materials Design. The government has certain rights in the invention. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure generally relates to additive manufacturing systems and methods and, more particularly, to systems and methods for controlling temperature in additive manufacturing processes. 
     BACKGROUND 
     In certain additive manufacturing applications, it may be advantageous to control the temperature of the material and/or surrounding area when working with certain materials and/or build object geometries. Materials such as high carbon steel and titanium, for example, may be prone to cracking or other deformities when a high temperature gradient exists across the workpiece. As used herein, the term “high carbon steel” includes any steel having a carbon content equal to or greater than 0.2%. Alternatively, other materials may benefit from rapid cooling resulting which could result in smaller grain size or a desired material phase. Additionally or alternatively, the shape or geometry of the desired final build object may require a build strategy that may, for example, overheat thin features while forming adjacent thicker features. Furthermore, with uncontrolled cooling, in some applications, the additive manufacturing process can generate high residual stresses as a result of the geometry being built, the build strategy employed, use of materials with very different thermal expansions, or other factors. By controlling the cooling rate, these stresses can be gradually released as the part cools. 
     Current control and processing methods focus on phenomena local to the melt pool, and do not compensate for changing thermal gradients or cooling rates globally within the build. Although these current methods can improve build quality, it is only a partial solution toward achieving uniform microstructure, minimizing defects and residual stress, maintaining dimensional integrity, and optimizing mechanical properties. 
     The invention described herein establishes a mechanism and methodology for active thermal control to bound the thermal gradient and cooling rate within all areas of the additively manufactured build in progress that are above the critical temperature of the material. To enable this control methodology, new active temperature control functionality is added to the Direct Energy Deposition (DED) machine system utilizing a secondary heating source, cooling source and thermal measurement device. 
     A new control methodology is also included to calculate the position and intensity of the application of active heating or cooling. A dynamic thermal model of the build is developed that simulates the thermal history of the build and calculates the position and temperature of active thermal correction to be applied at each time step. The control system then commands and regulates the application of the thermal application at the specified temperature. This should enable active control of microstructure, geometry, porosity, material properties, residual stress and distortion. 
     SUMMARY 
     In a first aspect of the invention, a temperature control system is provided for controlling the temperature of a build during its fabrication in an additive manufacturing system. The temperature control system comprises a cooling source; and means for positioning the cooling source relative to the build. Preferably, but not necessarily, the temperature control system also includes a secondary heat source, separate from any heat source associated with the additive manufacturing system, for directing a heat flow to a selected portion of the build and means for positioning the secondary heat source relative to the build. 
     The secondary heat source may comprise one of a laser beam, an electron beam, mechanical engagement, an induction heater, a torch, a super-heated gas flow, or the like. In one non-limiting embodiment, the secondary heat source is a laser beam and the means for positioning the secondary heat source relative to the build includes a galvo-scanner. 
     The cooling source is configured to cool the build by one of forced convection, spray cooling, and via a material auxiliary to the build. The material auxiliary to the build comprises a cryogenic material. In one embodiment, the cooling source includes a nozzle for spraying a cooling liquid or gas, and the means for positioning the cooling source preferably comprises a robotic system. 
     The temperature control system may further comprising a heating and/or cooling plate that is applied to a substrate or previously built area of the build, with means for varying the temperature of the plate spatially and temporally being provided. 
     Preferably, but not necessarily, the temperature control system further comprises a programmable controller pre-programmed with a dynamic thermal model of a thermal history of the build for each time step and configured to control the secondary heat source and the cooling source to conform the temperature of the build to the thermal model. 
     One or more temperature sensing devices are provided for measuring the temperature of the build. Preferably, the temperature sensing device comprises one of an IR camera, a pyrometer, and a thermocouple. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic depiction of a DED manufacturing system utilizing a global thermal control system. 
         FIG. 2  is a positioning system for a cryogenic nozzle that may be used in the system depicted in  FIG. 1 . 
         FIG. 3  is a schematic depiction of an open loop active thermal control for use in the system of  FIG. 1 . 
         FIG. 4  is a schematic depiction of a closed loop active thermal control for use in the system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Direct energy deposition (DED) is an additive manufacturing technique that enables the creation of multi-material, functionally graded components, the repair or modification of existing components, and the addition of wear resistant coatings. This additive manufacturing method creates a 3D object by depositing powder directly into a small molten pool generated by a laser beam to create solid material layer-by-layer. As successive layers are deposited, the heat is conducted into the previously deposited layers and the substrate material and the thermal gradient within the component decreases. As a result, the use of constant process parameters—such as laser power, powder flowrate and processing speed—does not result in constant thermal history throughout the build. 
     The thermal history generated by the additive manufacturing process is determinant of the resulting microstructure, geometry, material properties, residual stress and distortion. Thermal control of the process is necessary to achieve constant microstructure and material properties in the final part as well as reducing residual stress and warping. 
     By way of the present application, a mechanism and methodology are provided for active thermal control to bound the thermal gradient and cooling rate within all areas of the additively manufactured build in progress that are above the critical temperature of the material. To enable this control methodology, active temperature control functionality is provided to the DED machine system. In one embodiment, the active temperature control functionality utilizes a secondary laser, a cryogenic nozzle and a pyrometer. 
     The active heating or cooling implement may be positioned and controlled to achieve a desired temperature gradient or cooling rate. As used herein, the term “temperature gradient” is intended to encompass a desired rate of change of temperature spatially and the term “cooling rate” is intended to encompass a desired rate of change of temperature temporally. For example, the heating or cooling source may be controlled so that a selected area of the build object has a thermal gradient and or cooling rate maintained above and or below a desired threshold. 
     A schematic representation of the temperature control system in combination with an additive manufacturing system  10  is shown in  FIG. 1 . The system  10  comprises a primary heat source  12 , such as a laser, for delivering laser energy to a workpiece or build  14  through a primary processing nozzle  16 , the nozzle  16  being movable relative to the workpiece. The system  10  also includes sources of shield gas, metal powder, and carrier gas that are also flowed through the processing nozzle  16  to form a melt pool on the upper end of the workpiece  14 . The workpiece  14  is supported on an active base plate  18  that provides for heating and cooling. The system  10  also includes a melt pool sensor  20  for providing information regarding the temperature of the melt pool. 
     A secondary heat source (or sources)  22  is provided which is in addition to the primary heat source  12  used for the additive process. The secondary heat source  22  is also positionable relative to the build  14 . Any source of heating used may be used including an energy source such as a laser beam or electron beam as well as mechanical engagement to produce friction or an induction heater. For example, in the case of a secondary laser beam, beam position can be controlled by a galvo-scanner and independent motion system operating in parallel with the system for the primary heat source  12 . 
     In addition, a cooling source  24  is provided that is also able to be positioned relative to the build  14 . Sources of cooling may include forced convection, spray cooling, or conduction via material auxiliary to the build. Preferably the cooling source is an inert cryogenic gas, such as argon, that is delivered to the build through one or more nozzles  26 . An independent motion or robotic system may be used to position the cooling nozzle  26 . As noted previously, the cooling source may be used in the additive manufacturing system  10 , regardless of whether the system  10  includes a secondary heat source  12 . 
     With reference to  FIG. 2 , the nozzle  26  may be mounted to a telescoping arm  28  that is rotatably attached to a carriage  30 , the carriage  30  being movable about the workpiece  14  along a track  32 . Alternatively, a plurality of stationary nozzles may be provided that together provide for the delivery of a cooling liquid or gas, such as a cryogenic gas, to selected discrete areas of the entire workpiece. While not required, a pressure regulator is preferably provided that will allow increasing or decreasing the exit velocity of the cryogenic gas from the nozzle, to provide for a higher or lower cooling rate. 
     Some embodiments may include a heating and/or cooling plate applied to the substrate or previously built areas of the component. This plate may have the ability to vary temperature spatially and temporally, applying differing temperature profiles in space and time. 
     The temperature of the build object may be measured using a first temperature sensing device  34 , which may be provided as one or more of any number of devices including but not limited to an IR camera, pyrometer, or thermocouple. A second temperature sensor  36 , such as a pyrometer, is associated with the cooling nozzle  26  to measure the surface temperature of the workpiece as it is being cooled by the nozzle  26 . Temperature measurement may not be necessary for all embodiments; specifically those utilizing simulation or experience to plan intensity and position of applied heating or cooling. 
     The system  10  includes a programmable controller  38 . The controller  38  controls the operation of the DED system, including the intensities of the primary and secondary heat sources  12 ,  18  and the cooling source  24 , and the positions of the primary processing nozzle  16 , the secondary heat source  22 , and the cooling nozzle  26  relative to the build based on signals received from the temperature sensors  34 ,  36  and the melt pool sensor  20 . 
     In some embodiments, the controller  38  includes a closed loop ( FIG. 3 ) or open loop ( FIG. 4 ) control system to control the position or intensity of the applied heating or cooling to maintain the thermal gradient or cooling rate within a specified range. Position and intensity of applied heating or cooling could also be controlled manually, by a time based sequence, or based on a methodology prescribed by simulation, as described below, or empirically through experience. 
     A simulation is performed to create control system for the DED process that can prescribe an open loop control solution that includes melt pool temperature control, base plate temperature control, and active global temperature control. This simulation predicts the position and temperature applied by the active control to maintain a constant thermal gradient throughout the build. 
     The simulation is used to create simplified model the thermal profile of a DED thin wall deposition temporally and spatially in 2-D. The creation of a 2-D thermal model is described below for illustrative purposes. However, is should be understood that, depending on the geometry of the build, a 3-D model of the thermal profile of the build would be required. 
     The thermal simulation for the build is performed in two spatial dimensions, along the build height direction and along the scanning direction. For the purposes of this simulation, it is assumed that the workpiece can move with respect to the processing head in the build scanning direction, X, and the build direction, Z. The simulation is time accurate, with element birth as time progresses. A combined finite difference and finite volume scheme is used to update the temperature at each point with in the thin wall during each time step. The model uses a 2-D heat equation to update the temperature within the build due to conduction, and then a finite volume method is used to further update temperature due to radiation. The bottom of the build is maintained at a user-defined temperature or temperature profile, the melt pool is maintained at a set melt pool temperature (melt pool thermal control is assumed to be implemented), and the sides are assumed to radiate to a thermal sink at room temperature. 
     As previously discussed, it is desirable to maintain a specific thermal gradient for the build while the temperature remains above some critical temperature. This critical temperature could be the lowest temperature at which the phenomena to be controlled (such as phase change or grain growth) occurs. The purpose of the control model is to solve for the position and temperature to be applied in order to maintain the thermal gradient above the desired temperature and within the desired bounds. This model is open loop with respect to determining the appropriate position and temperature to maintain the desired thermal gradient. More ideally, a closed loop model would be used. The temperature application is preferably a closed loop process to ensure that the prescribed temperature is applied. A schematic for the open loop active control solution can be seen in  FIG. 3 . 
     In 1-D, the desired thermal gradient and critical temperature and can be used to calculate a critical depth from the top of the build at which the material will be below the critical temperature, 
     
       
         
           
             
               h 
               crit 
             
             = 
             
               
                 T 
                 crit 
               
               dTdx 
             
           
         
       
     
     where T crit  is the critical temperature, and dTdx is the desired thermal gradient. It is assumed that the spatial thermal gradient is proportional to cooling rate. To determine the amount of heating or cooling that would be required to achieve this critical temperature at the critical depth, a similar method is used as when updating the temperature due to radiative heat transfer. After the radiation update step, the amount of energy required is equal to 
         q   req =( T crit −T ( i ))*ν*ρ* c   p  
 
     If the energy is positive, heating is required at the critical depth. If it is negative, cooling is required. After the energy is calculated, T(i) is set to T crit . If the total build height at a given time step is less than the critical depth, the substrate temperature is set to a higher temperature than room temperature. The substrate temperature then is: 
     
       
         
           
             
               T 
               sub 
             
             = 
             
               
                 T 
                 crit 
               
               + 
               
                 
                   ( 
                   
                     
                       T 
                       add 
                     
                     - 
                     
                       T 
                       crit 
                     
                   
                   ) 
                 
                 * 
                 
                   
                     h 
                     build 
                   
                   
                     h 
                     crit 
                   
                 
               
             
           
         
       
     
     where h build  is the current height of the total additive manufacturing build. To expand this solution to 2-D, h crit  must first be solved for each time step. This is the distance from cells that are at the upper critical temperature to the critical points at which heating or cooling must be applied to maintain the desired thermal gradient above the critical temperature. h crit  is given by: 
     
       
         
           
             
               h 
               crit 
             
             = 
             
               
                 ( 
                 
                   
                     T 
                     add 
                   
                   - 
                   
                     T 
                     crit 
                   
                 
                 ) 
               
               dTdxdy 
             
           
         
       
     
     The control model will output points and temperatures; the global thermal control system will traverse these points and apply the prescribed temperature as the build progresses. The acceleration and feedrate of the heating and cooling elements will need to be taken into consideration as these points of traverse are determined to ensure that the hardware system does not lag far behind the simulation solution in execution. Assuming that the heating/cooling positioning devices can move much faster than the additive process, it is reasonable to expect that the heating or cooling can be applied to the h crit  curve as it moves with the process. 
     The open loop prediction of the energy to be applied may be unreliable due to unknown laser absorption and convection coefficients. Thus, the closed loop control for the temperature is preferred to ensure that the prescribed temperature solution is applied. The closed loop control shown schematically in  FIG. 4  takes as an input the prescribed temperature solution as predicted a-priori by the simulation described above. The error between the prescribed temperature and the actual temperature will be measured by a multicolor pyrometer. The control will then decide based on whether the temperature is predicted and detected to be above the desired temperature or below to apply heating, cooling, or nothing to the area of interest. If heating is required, the secondary laser will be activated. If cooling is required the cryogenic nozzle will be activated. 
     The cooling rate of a component is controlled to result in specific properties in the finished component. Properties such as tensile strength, fatigue life, creep rate, fracture behavior can be determined empirically, via micromechanics, and computationally if the structure of the component including details on the matrix, precipitates, phases, voids, grainsize and residual stress are known. This structure can be determined via a grain growth model and a solid phase transformation model from the thermal history including solidification rate, cooling rate, fluid flow, vaporization, and solute transfer. The thermal history can be determined from the process parameters via a part-scale heat conduction model, a melt pool scale thermos-fluid model, and a powder-scale thermos-fluid model. Once an ideal thermal gradient range is determined for a specific alloy and geometry via this simulation hierarchy, it can be input into the global thermal control model to prescribe a solution for that cooling rate. 
     The process is related to the thermal history via three models: a part scale heat conduction model, a melt pool-scale thermo-fluid model, a powder scale thermo-fluid model. An analytical or FEM model can be used to model part-scale heat conduction. A thermo-fluid model can be used to model the melt pool. Thermo-fluid model can be utilized to model powder-scale phenomena. 
     The thermal history is related to the structure via the combination of a grain growth model and a solid phase transformation model. A cellular automation technique can be used for the modeling of grain growth based on a known thermal history. Alternatively, the Monte Carlo model can be used to simulate grain growth. This grain growth model can be coupled with a solid phase transformation model, such as Computer Coupling of Phase Diagrams and Thermochemistry which represents thermodynamic properties for various phases, and a phase field model which can be applied to the solidification dynamics. 
     The structure is related to the properties empirically, via micromechanics, and computationally. Empirical methods include defining the Hall-Petch relationship for the material being built—which can predict yield strength based on grain size, creating a damage model to predict the fracture mechanics of the material, determining the Larson-Miller parameter to relate rupture life to temperature, and predicting creep damage via the Murakami method. Furthermore, within the realm of micromechanics, homogenization can be used to predict the response of the material based on the properties of the individual phases, a GTN model can be used to predict ductile fracture, a Dyson model can be used to predict creep damage based on dislocation structure, and a Fatemi-Socie model can be used to estimate multi-axial fatigue life. Finally, computational methods can be used to predict final properties by resolving grains, pores, inclusions and precipitates. 
     By quantitatively explaining via the models above how solidification rate and cooling rate affect final component properties, solidification rates and cooling rates can be chosen to create a component with ideal properties. 
     EXAMPLE 
     The purpose of this study was to relate the combined effects of solidification time and cooling time of the built material to its final ultimate tensile strength. Cooling time was defined as the time from when the location of interest last passes through 1,200° C. to when it reaches 400° C. Nine locations on a laser deposited IN718 thin wall were studied in detail to understand the effect of cooling rate on tensile strength. Tensile samples were machined at these locations. The thermal histories of the locations of interest were compared with build geometry and the ultimate tensile strength of that location. An inverse proportional relationship was seen between the distance of the location of interest from the substrate and the cooling time. A trend was also seen linking increased surface temperature and increased solidification time. Weighted Cooling And Solidification Time (WCAST) was defined as the sum of weighted normalized solidification time and the normalized cooling time. Ultimate tensile strength was seen to decrease as WCAST increased. Optical microscopy images of the build microstructure confirm that longer cooling and solidification times lead to coarser microstructures, which may cause the lower tensile strengths measured. 
     For this experimental study, a hybrid additive and subtractive five-axis machine tool, a DMG MORI LaserTec 65 3D, was used. The machine includes a direct diode laser with a maximum power of 2,500 W at a wavelength of 1,020 nm. The beam is focused by a lens with a 200 mm focal length to a 3 mm spot size. Gas atomized super alloy INCONEL 718 powder of particle size 50-150 μm was used. Argon gas with a flow rate of 7 L/min was used as the shield gas and the conveying gas to deliver the powder coaxially to the melt pool. 
     An IN718 thin wall was deposited on a stainless steel 304 substrate. The wall was 120 mm in length, 60 mm in height, and one track thick, or approximately 3 mm. A thin wall was chosen so that temperature can be assumed constant through the wall thickness. The programmed layer height was 0.5 mm. The wall was created using a zig-zag tool path with no dwell between layers. Between layers, the laser was off while the motion system traversed vertically to the next layer. The laser tool path was reversed at the end of the substrate, so acceleration and deceleration occurred within the tool path during the process. The laser power used was 1800 W, the powder flow rate was 18.0 g/min, and the scanning speed was 1,000 mm/min. 
     During the deposition, temperature measurements were performed using a digital infrared camera (FLIR A655sc). After deposition, the resultant thin wall was heat treated per the procedure in the ASM 5664 standard and miniaturized ASTM E8 pin-loaded tensile specimens in the vertical (build) orientation were cut from the thin wall using EDM. The specimens were tested under displacement control using a miniature screw-actuated load frame under a quasi-static, pure tension loading at a nominal (crosshead) strain rate of 5e −4 /sec. 
     The thermal histories of the locations of interest were compared, and a relationship was identified between the distance above the substrate and the cooling time. Specifically, the cooling time was longer for locations lower in the build because each subsequent layer added reheats the location, slowing the cooling. However, as the build height increases as new layers of material are deposited, heat is conducted into the substrate, which acts as a heat sink. As this heat accumulates, the thermal gradient decreases, and conduction is slowed. The effect of this is increasing temperature of the surface that each subsequent layer is deposited onto, because each previous layer cannot dissipate as much heat as the one before it. This results in increased solidification times for locations near the top of the build because the deposition surface temperature is higher. This is the opposite trend from that observed for cooling time. 
     The effect of proximity to the side edges of the build was also investigated. Dependent upon the toolpath used, proximity to the edge of the side wall does may have an effect on cooling time. More extreme thermal cycling resulted from a zig-zag toolpath used for the deposition. Such a toolpath allows for approximately equal dwell time between the deposition of subsequent layers in the locations in the center of the build, but the samples near the edge of the wall experience the deposition of two layers in quick succession and then a longer pause while the process moves to the other edge of the build. Since the passes for areas in the center of the wall are equally spaced, the surface of the previous layer is able to cool before the next layer is deposited. However, for areas near the edge, the first of the paired passes at the edge of the wall heats the deposition surface significantly without time to cool before the second pass. The result is that the deposition surface for the second pass of edge areas is significantly hotter than the surface of center areas resulting in slower solidification times. 
     Links between cooling time, solidification time and ultimate tensile strength (UTS) were examined. While no clear trend was seen between UTS and cooling time, or between UTS and solidification time, when the combined effect of both solidification time and cooling time was compared with UTS, a trend was observed. The solidification time was found to have a greater (1.7 times) the effect of cooling tome on the UTS. 
     From the experimentation, the following was observed: An inverse proportional relationship was seen between the distance of the location of interest from the substrate and the cooling time; A trend was observed linking increased surface temperature with increased solidification time; Ultimate tensile strength was seen to decrease as the Weighted Cooling And Solidification Time (WCAST) increased; and Longer cooling times and longer solidification times lead to coarser microstructures. 
     Accordingly, the systems and methods described above would allow for varying the thermal conditions in DED to create builds that have uniform or gradient mechanical properties.