Metrology for Multi-Physics AM Model Validation

Summary

Multi-physics models are necessary to simulate, study, and optimize metal additive manufacturing (AM) processes, such as powder bed fusion (PBF) and directed energy deposition (DED). Before these models can be used to design manufacturing processes or qualify parts for medical and aerospace applications, they must first be validated. In fact, the ANSI Additive Manufacturing Standardization Collaborative (AMSC) specifically identified AM model verification and validation as a key gap (Gap D9) in the Standardization Roadmap for Additive Manufacturing (2017). Unfortunately, for many with the capability and expertise to model metal AM process, the expense of empirical studies to validate their models is prohibitive. Manufacturers with the means to perform extensive empirical studies keep their results propriety. While published research is available for modelers to validate their efforts, the available data is often limited due to the scope of the study or metrological challenges. These challenges include the negative impact that melt pool dynamics and plume formation have on the optical measurement of melt pool shape or temperature, the variable emissivity that is necessary to calculate true temperature and cooling rates of the rapidly evolving surface, and the inability to measure internal temperature, stress, or phase evolution during the process. Because of these challenges, AMSC has identified NIST as a key organization to help the AM industry validate models.

Description

Objective

To provide reference data for the validation of multi-physics models of metal additive manufacturing processes to enable improvement of AM process models and more rapid and predictable process development for AM production by manufacturers.

What is the Technical Idea?

The Metrology for Multi-Physics Model Validation Project has two primary objectives to fill Gap D9 identified by AMSC. The first objective is to provide reference data to validate models of metal AM processes. This will be accomplished in-part through the Additive Manufacturing Benchmark Test Series (AM-Bench, www.nist.gov/ambench). AM Bench allows modelers to test their simulations against rigorous, highly controlled AM benchmark test data, which is generated at NIST. This data includes in-situ temperature and cooling rate measurements during the PBF process and post-process distortion, stress, and microstructure characterization. In addition to the AM Bench, reference data (thermal history, stress, distortion, microstructure) will be generated from highly controlled builds of varying geometry that are executed using both PBF and DED using industrially relevant super alloys.

The second objective of the project is to develop the metrology and analysis techniques, and the standard guidelines necessary to measure temperature, stress, and phase evolution for model validation of PBF and DED processes. Numerous systems will be utilized to accomplish these measurements. The NIST Additive Manufacturing Metrology Testbed (AMMT) will be used to achieve traceable true temperature measurement of the surface of the melt pool and the immediately adjacent material. This is accomplished through in situ characterization of the emissivity and reflectivity of the surface as it undergoes phase transformation during the PBF process. For DED process model validation, metrology systems will be implemented on the Optomec LENS MR7 for traceable temperature, melt pool formation, and track surface topography of DED processes.

While surface temperature and topography measurements are valuable, the layer-by-layer nature of metal AM processes causes rapid re-heating and cooling of the material several layers below, resulting in a temperature history that cannot be measured from the surface. The final microstructure that is often observed through post-process measurements is a result of this complex thermal history. For metal AM material models to be accurate, simultaneous measurement of the temperature, stress, and phase evolution is required for adequate model validation. Currently, no system exists that is capable of measuring in situ the temperature and phase evolution of material processed using laser AM technologies. To meet this need, a new PBF testbed will be developed under the AM Machine and Process Control Methods for Additive Manufacturing Project to use synchrotron X-ray as the tool to measure these properties during controlled builds. Once this testbed is developed by FY’21, it will provide data correlating temperature history, residual stress, and phase evolution to validate models beyond what can currently be achieved.

What is the Research Plan?

To achieve its objectives, the project focuses on the following key products and deliverables:

Publicly accessible datasets to validate the following types of process models:

• Part-scale models: Investigate thermal history, distortion, and residuals stress in parts
  • Datasets consist of thermal history and post-process distortion and residual stress
  • 1-2 datasets per year (FY19-FY23), each data sets consists of multiple parts/geometries
• Melt pool scale models: Mass-energy balance to predict track formation and process efficiency
  • Datasets consist of temperature, topography, microstructure
  • 1-2 datasets per year (FY19-FY23), each data sets consists of multiple scan conditions
• Material models that predict microstructure and phase evolution from thermal history
  • Datasets consist of simultaneous internal temperature and phase evolution measurements of the material several layers below the surface
  • Dependent on the construction of the necessary testbed
    • 1 dataset per year (FY20-FY23) of fundamental temperature and time dependent phase transformation
    • 1 dataset per year (FY21-FY23) acquired during the manufacture of a part

Development of metrology and analysis techniques to populate the datasets listed above:

• Design and implementation of metrology systems for Directed Energy Deposition (DED)
  • Designed and integrated in existing Optomec LENS MR7 system for the following:
    • Traceable temperature measurements for Directed Energy Deposition DED processes (FY19-FY20)
    • High-speed imaging system to measure powder capture, melt pool dynamics, and track formation (FY21-FY22)
• Improvement of existing test platforms to improve metrology capabilities and accuracy
  • Additive Manufacturing Metrology Testbed (AMMT)
    • Development and installation of new or improve sensors and hardware capabilities (FY19-FY23)
  • Commercial (EOS M270) thermography platform
    • Expand measurable temperature range for higher temp. alloys (FY19-FY20)
    • Increase detection area