Performing a 2D Microstructure Simulation...

Performing a 2D Microstructure Simulation(Beta)

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Additive User's Guide (Print and Science)

Table of Contents

1. Performing a Microstructure Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1. Using Microstructure Data to Evaluate Mechanical Properties ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2. Material Configuration .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3. Microstructure Configuration .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.4. Machine Configuration .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.5. Reviewing Results ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.6. Microstructure Validation Process .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111.7. Known Issues and Limitations .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141.8. Resolved Issues and Limitations .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

iiiRelease 2019 R3 - © ANSYS, Inc. All rights reserved. - Contains proprietary and confidential information


Chapter 1: Performing a Microstructure Simulation

For simulations with either Inconel 718 or Aluminum 357, given the meltpool characteristics, thermalinputs and all the process parameters – grain orientation of the material are output for three 2D planes(XY, XZ, and YZ) simulating typical results of EBSD laboratory tests. Also, grain size distribution andorientation angle distribution are output in graph form using a circle equivalence method and orientationmap for all three planes, respectively. Microstructure simulations are parametric so you can run severalpermutations to determine which process parameters will affect grain size the most. A Specific RandomSeed input parameter may be used to instigate the same nucleation pattern for a given set of processconditions so that comparison simulations are possible. Microstructure Simulation capability is a Betafeature at Release 2019 R3.

1.1. Using Microstructure Data to Evaluate Mechanical Properties

In a polycrystalline material, mechanical properties are controlled primarily by the material's inherentmicrostructure characteristics—grain size, grain shape, grain orientation, and composition of materials.These characteristics can change significantly with different processing methods and conditions, therebychanging the mechanical properties. Due to the complex nature of the additive manufacturing processwhere the material is subjected to repetitive heating cycles, the resulting microstructure is very inhomo-geneous and different for different process parameters, creating variability in mechanical properties.(See Akram, et al.) Using a Microstructure Simulation in Additive Science, you can predict the changesin microstructure—grain size, grain shape, and grain orientation—for a given set of process parameters.

Grain size can be used to predict the mechanical property of yield strength. The relationship betweenyield strength and grain size is described mathematically by the Hall-Petch equation:

where is the yield strength, is a materials constant for the starting stress for dislocation motion, is the strengthening coefficient (constant for a material), and is the diameter of a grain. This rela-

tionship shows that the grain size is inversely proportional to yield strength. The smaller the grain size,the higher the yield strength, and vice versa. This relationship holds true for most materials and is awidely accepted model to express the yield strength relationship with grain size. If you know the ma-terial constants, you can use the grain size obtained from a Microstructure Simulation to predict theyield strength of a material.

Grain orientation plays a major role in the directional nature of a part's mechanical properties. If thegrain size and orientation is uniform in all three orthogonal planes, this indicates the part's propertiesare isotropic. If grain orientation is predominant in one of the planes, this indicates that the printedpart will be highly anisotropic. As-built parts printed by AM are usually anisotropic, meaning differentbuild orientations give different mechanical properties. The grain orientation data obtained from a Mi-crostructure Simulation can provide insights into the grain orientation in the part produced using dif-ferent process parameters such as laser power, speed, scan pattern, hatch spacing, powder layer thickness,etc. Ultimately, the goal in AM simulation is to enable you to design a part based on your specific re-



https://www.sciencedirect.com/science/article/pii/S2214860417303998

quirements. For example, if your part requires anisotropy in one of the directions, you can design thepart by choosing the parameter set that results in the desired anisotropic microstructure.

1.2. Material Configuration

Materials validated for Microstructure simulations are Inconel 718 and Aluminum 357. See MicrostructureValidation Process (p. 11).

1.3. Microstructure Configuration

A Microstructure Simulation is essentially a two-part solution. A thermal solution of a cuboid geometryis performed first using the Thermal Solver to determine cooling rate, thermal gradient, and meltpoolsize (width and depth) for the material given a set of process parameters. Then the MicrostructureSolver is used on a smaller "cube" inscribed inside the cuboid to determine the microstructural inform-ation. The cube is actually a set of three 2D planes with width and height equal to Sensor Dimension.A 0.25 mm buffer from the edges on the top surface of the cuboid is required to ensure the microstruc-ture is obtained with a thermal meltpool that has reached steady-state. The thermal quantities ofcooling rate, thermal gradient, meltpool width and meltpool depth used by the Microstructure Solverare single, averaged values and are averaged from within the microstructure cube only.


Performing a Microstructure Simulation

Geometry Configuration

Geometry Width (mm) - The width of the cuboid, in millimeters, for the thermal solution. Valid inputvalues are positive real numbers between 1 and 10. Defaults to 1 mm.

Geometry Length (mm) - The length of the cuboid, in millimeters, for the thermal solution. Valid inputvalues are positive real numbers between 1 and 10. Defaults to 1 mm.

Geometry Height (mm) - The height of the cuboid, in millimeters, for the thermal solution. Valid inputvalues are positive real numbers between 1 and 10. Defaults to 1 mm.

Sensor Dimension (mm) - The dimension (width and length, in millimeters) of the 2D planes for themicrostructure solution. The sensor point is the intersection of all three planes and is always on the topsurface of the cuboid. Sensor Dimension must not exceed Geometry Width minus the 0.5 mm buffer(0.25 mm on each side) and must not exceed Geometry Length minus the 0.5 mm buffer (0.25 mm oneach side). Sensor Dimension must not exceed Geometry Height. Defaults to 0.5 mm.



Microstructure Configuration

Thermal Solution Configuration

A full thermal solution is performed on the cuboid geometry by default but the Additive applicationallows you to bypass the thermal solution if you know the material’s cooling rate, thermal gradient,and meltpool characteristics. Check this option to perform the microstructure solution only. (The LaserPower and Baseplate Temperature fields under Machine Configuration become disabled when you bypassthe thermal solution.)

Cooling Rate (°K/sec) - Must be between 1x105 and 5x106. Defaults to 1x106.

Thermal Gradient ( °K/m) - Must be between 1x106 and 1x108. Defaults to 1x107.

Melt Pool Width (mm) - Must be between 0.075 and 0.5. Defaults to 0.15 mm.

Melt Pool Depth (mm) - Must be between 0.015 and 0.35. Defaults to 0.1 mm.

Random Seed

If desired, you can specify a specific random seed to be used as a starting point for the nucleationpattern. Use an integer between 0 and 4294967295. This feature allows you to generate the same outputfor the same inputs to check for repeatability. If you do not use a random seed (default), the solver willassign a random number for the nucleation starting point and simulation results will not be repeatable.

1.4. Machine Configuration

Process parameters may be input with multiple values for each to set up parametric simulations. Thisallows you to study which parameters affect grain size and morphology the most.

Laser Power (W) - The power setting for the laser in the machine. Must be between 50 and 500. Defaultsto 195 Watts.

Scan Speed (mm/sec) - The speed at which the laser scans, excluding jump speeds and ramp up anddown speeds. Must be between 500 and 2500. The default value is 1000 mm/sec.

Baseplate Temperature (°C) - The controlled temperature of the baseplate. Must be between 0 and200. The default value is 80°C.

Layer Thickness (μm) - The thickness of the powder layer coating that is applied with every pass ofthe recoater blade. Must be between 10 and 100. The default value is 50 microns.

Hath Spacing (μm) - The distance between adjacent scan vectors when rastering back and forth withthe laser. Hatch spacing should allow for a slight overlap of scan vector tracks such that some of thematerial re-melts to ensure full coverage of solid material. Must be between 0.06 and 0.2 mm. The defaultHatch Spacing is 0.1 mm (100 microns).

Starting Layer Angle (°) - The angle at which the first layer will be scanned. It is measured from the Xaxis, such that a value of 0° results in scan lines parallel to the X axis. Must be between 0 and 180°. Thestarting layer angle is commonly set to 57° (default).

Layer Rotation Angle (°) - The angle at which the major scan vector orientation changes from layerto layer. Must be between 0 and 180°. This is commonly 67° (default).



1.5. Reviewing Results

A Microstructure Simulation produces one summary result page (the parent simulation) showing theset of permutations (child simulations) included within it. Selecting any of the permutations brings upa new page of individual permutation results. These results include:

• The microstructure in XY, YZ, and XZ planes available in both .avz format that can be viewed and expor-ted, and as .vtk files that can be exported. Use these data to view grain orientation and grain number.

• Grain size statistics available in both summary bar graph form that can be viewed and .csv files thatcan be exported.

Viewing Grain Orientation

The two primary result items in a Microstructure Simulation are grain orientation angle and grainnumber. Grain orientation angles from an individual permutation when viewed in the application areshown in the following figure. A unique grain number is assigned to one continuous grain. A uniquecolor is assigned for each orientation.



Reviewing Results

To view all three planes at once, use Paraview. Paraview is an open-source, multi-platform data analysisand visualization application. Within Paraview, extract the microstructure data by doing the following:

1. Open all three .vtk files corresponding to the three planes from one permutation and make themactive.

2. Select a data type to display, either grain number or orientation (in degrees). Usually you will want toview the grain orientation.

3. Be sure you are viewing a 3D view, under Layout.

4. Under Coloring, apply a preset color map; we recommend one called Jet (with 256 colors). This makesthe color bands more distinguishable.





Reviewing Results

These images are from version 5.4.1 of Paraview.

Viewing Grain Size Statistics

For each grain, the area and the area fraction of the total area are calculated. To do this, the diameterof an equivalent circle for each grain is calculated using the Circle Equivalence Method, as shown inthe following figure.



Grain area and diameter values for all the grains are aggregated into a Grain Size Distribution bar chartshowing the area fraction of grains whose diameter falls in discrete ranges indicated on the chart. Thisbar chart is displayed in the application when you click the Graph link as shown in the following figure.While this particular bar chart is not exportable, the full data can be exported as a .csv file. The .csv filedoes not contain the same data as the aggregate bar charts but contains full data for each grain witharea fraction and equivalent circle diameter such that you can produce similar graphs on your own. Asecond bar chart shows a similar aggregate of Orientation Angle.

From the charts below, this particular result set shows the majority of grains have a diameter between30 and 35 microns and are oriented between 80 and 90°.



Reviewing Results

How Porosity is Revealed in Results

It is always wise to examine your results to be sure they make logical and intuitive sense and to enhanceyour understanding of the microstructural phenomena occurring in the simulation. Instances of lack-of-fusion porosity, in particular, are important to discern and are, in fact, easy to recognize in microstruc-



ture results. A grain number of 0 indicates a location of unmelted material, or lack-of-fusion porosity.Orientation and grain number maps will show gaps at these locations, as shown in the following figure.This example was run as an exaggerated case to show lack-of-fusion porosity by defining inputs suchthat the Meltpool Depth was less than Layer Thickness, and Meltpool Width was less than Hatch Spacing.

1.6. Microstructure Validation Process

Validation of the microstructure solver for each material is performed by comparing experimental andsimulation results for various sets of process parameters. Several cubes are printed using a SelectiveLaser Melting method with various process parameters. After analyzing porosity data, samples are se-lected that result in high density. These samples are prepared using the standard metallography pro-cedure for EBSD (Electron Back Scattered Diffraction) analysis for grain size and grain orientation estim-ation, which consists of grinding and polishing the surface of the cubes up to 0.05 µm surface finish.The prepared samples are subjected to SEM (Scanning Electron Microscopy) equipped with an EBSD



Microstructure Validation Process

detector to generate the grain orientation and grain size data. Simulation results are generated byrunning the simulation end-to-end from thermal to microstructure using the same process parametersas in the experiments.

Example of Validation Data

A representative example of validation data is provided here for the IN718 alloy. Similar work is donefor every material that is available for Microstructure simulations. Note that microstructure results maychange as solver refinements are made in subsequent releases.

For the IN718 alloy, seven samples were selected. The following table lists the comparison data betweenexperimental and simulated grain size. The grain size obtained from EBSD analysis was from one of thevertical planes of the cube, therefore, the simulation result is compared with one vertical plane of thecube.

Simulated Grain Size(Vertical Plane)

Experimental Grain Size(Vertical Plane)

Process Parameters

32.96 ±2.58 µm32.2 µmPower = 165 W Speed = 800 mm/sec






30.47 ±0.50 µm33.34 µmPower = 375 W Speed = 2000 mm/seca

a Note that microstructure results may change as solver refinements are made in subsequent releases.

In addition to grain size validation, the grain morphology is also validated by comparing simulationresults with experimental results. Specifically, we examine how the grains are oriented in a sample frameof reference. An example comparison of grain orientation is shown below. (The experimental resultsimage is shown upside down to ensure consistent viewpoints; they are both aligned with the builddirection.)





Microstructure Validation Process

1.7. Known Issues and Limitations

None

1.8. Resolved Issues and Limitations

Several known issues and limitations in the previous Beta release have been resolved in this release.This is not a comprehensive list of all resolved bugs; rather a subset of important ones with broad impact.

• An error related to the grain-orientation reference location has been fixed. The grain orientations aredetermined from a reference that starts at 0 degrees and increases counter-clockwise to 180 degrees.



The reference is different, depending on the plane of interest. Absolute orientation angle should berelative to the positive horizontal-axis in a plane (x in XY, x in XZ, and y in YZ) in the direction the heatsource is moving; the longitudinal direction. Previously, an incorrect reference was used. This affectsgrain orientation values. Grain size is essentially unaffected. (122871)

• An error associated with the direction of laser travel for scan lines parallel to the vertical planes hasbeen fixed. This affects grain orientation values. Grain size is essentially unaffected. (108857)

• Previously, areas lacking fusion/melting contributed to the overall area in the calculation of area fractionresults. This has been fixed so that now these pockets of porosity are not included in area fraction results.Simulations run with the exact same inputs may yield larger area fraction values compared to the lastrelease (with more noticeable changes apparent in simulations with more porosity). (142671)

• Certain rare combinations of meltpool depth, layer thickness, and geometry height resulted in unex-pected exception errors, showing "Activity microstructure-solver error" messages. This has been fixed.(122813)



Resolved Issues and Limitations

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