Customer Spotlight: Centre for Additive Manufacturing, University of Nottingham

Simulating Selective Laser Melting (SLM)

SLM is an Additive Manufacturing (AM) technology for producing end-use components by melting metallic powders using high density energy flux created by a moving laser source.



The team at Nottingham had clear motivation for researching what happens during the manufacturing process as the large thermal gradients result in transient and permanent residual stresses that may cause:

  • Failure and/or distortion during manufacture
  • Distortion upon removal from the build platform
  • Overall reduction in part performance (e.g. reduced fatigue resistance)

Post Build Cracking

By simulating the process, it will identify the heat transfer during manufacture as well as the the distribution and magnitude of residual stress.

Modelling SLM however is very challenging and computationally intensive due to the transient and non-linear nature of the process both across time and space. This is because the model requires to capture the exposure of the fine laser (70 micron size) which raster across each layer (40 micron thickness) across a relatively large part. Due to the extremely high temperature gradients created by the exposure of the laser, an extremely fine mesh resolution is required. 

The capabilities offered in Marc in particular adaptive meshing, time-stepping and extensibility, and the parallel solver performance were vital for studying the process under different scenarios.


Feedback received from the researchers detailed that Marc features around material models and the documentation of the user subroutines were powerful tools to model these specific manufacturing problems. The post-processing capability offered in Python scripting language (PyPost) was also important to analyse the results with higher efficiency. 

To summarise: Marc was chosen as the software to investigate this problem for a variety of reasons:

  1. Competency dealing with highly non-linear manufacturing problems
  2. Scalability (DDM, parallel performance)
  3. Well documented and extensive list of User Defined Subroutines
  4. Fairly user friendly pre/post processor via Mentat
  5. Excellent technical support from local MSC UK office
With Marc chosen the simulation could take into consideration:
  • Weakly coupled transient thermo-mechanical analysis
  • State variable model to track material properties for (powder, liquid ,solid)
  • Thermal dependent thermal and mechanical properties + Plasticity Modelling
  • Layer and element activation
  • Adaptive time-stepping
  • Local adaptive meshing
  • Direct laser scan path and parameter modelling
  • Extensive use of Fortran subroutines and additional C++ coupling for extra functionality
The end results speak for themselves with clear temperature and stress distributions outputs providing rich information to investigate different scanning strategies to mitigate problem areas.
Temperature Distribution

State Variable Tracking 
Powder = -1, Liquid = 0, Solid = 1
Von Mises Stress Distribution

But wait there's more!
They further investigated the effects of adding the next powder layer ontop for a 'real' 3D analysis:
Temperature Distribution

Von Mises Stress Distribution

Once the process setup was defined, more detailed analyses were defined for higher resolution scanning strategies. The figure below shows the highly transient behaviour of the process with the development of residual stress from the localised heat input.
Thermo-mechanical behaviour for 3x3mm area for Ti-6Al-4V

 The transient animation showing the local adaptive remeshing that was required to fully capture the highly localised effects for a 5x5mm patch:
Temperature

Von Mises Stress

The investigation further progressed into the effect of lattice structures on the generation of residual stress. 

Comparison of Steady State Effective Heat Transfer

Von Mises Stresss
Thermal History and State Variable Tracking


Lastly to end with the ultimate combination of all the analysis development from the above processes:

For more details, please take a closer look at the research paper here:

 For more details contact:

Luke Parry

Centre for Additive Manufacturing

University of Nottingham


More to come!

Popular posts from this blog

TIPS: MSC Nastran Notepad++ Language Filter

TIPS MSC Nastran Notepad++ Language Filter Notepad++  As a lot of us use Notepad++ for deck editing, so colleagues have created an MSC Nastran format xml language file that can be implemented to highlight keywords, comments etc. Download the file from: userDefineLang.xml To use, simply copy to your AppData/Roaming folder typically found in: C:\Users\Name\AppData\Roaming\Notepad++ By default the filetypes are set to BDF and DAT files. Or alternatively you will find it in the Language menu under NASTRAN. There is also format for Patran PCL code as well as Patran session file format. More to come!
Read more

THEORY: Implicit vs Explicit - Introduction

Image
Implicit Structural Solutions   Development of the finite element method began in earnest in the middle to late 1950s for airframe and structural analysis. By the late 1950s, the key concepts of stiffness matrix and element assembly existed essentially in the form used today. NASA issued a request for proposal for the development of the finite element software NASTRAN (developed by  MSC.Software) in 1965. Steady State or Static Equilibrium (ΣF = 0) Force  = Stiffness x Displacement In actual practice, inverting the stiffness matrix to solve the system of equations for displacement is highly inefficient. MSC Nastran uses a more efficient matrix decomposition procedure rather than the matrix inversion method. It is necessary to iterate the solution to be able to solve non-linear problems (using methods such as Newton-Raphson shown below, etc.) for many real-world problems. These nonlinearities can be associated with: Contact Material Behaviour ...
Read more

Customer Spotlight: Midlands Simulation Group

Image
Hierarchical  Structure Analysis Comparing an I-beam or box section girder with a solid beam of same dimension shows that the girder is much lighter - but just as stiff. Similarly, a sandwich panel of differing core thickness, but identical skin provides equally light – but ~40 times more stiff. It turns out that making an I-Beam girder out of lots of little girders improves the efficiency further - and most likely better able to withstand impact. This is a "hierarchical" or cellular structure. The Eiffel Tower is a classic example of a 3rd degree hierarchical structure. Even with its relatively poor quality iron, it is more resource efficient than many a modern skyscraper. An attractive manufacturing route is that of bonding of layers (laminating), forming to shape and then inflating to stiffness. The layers are not bonded everywhere: on inflation, the un-joined regions separate and form internal struts or membranes (miniature honeycomb or I-beams) where the outer layer...
Read more