ARIS WEAVE NCN N2-0328: The influence of the thermal history on the microstructure and mechanical properties of additively manufactured materials

 

1.       Summary of the Research Project

 

Additive manufacturing is considered the technology of the future for the lean manufacturing of high-quality machine parts made of plastics, ceramics and metallic alloys. Unlike subtractive manufacturing, it should be highly efficient in terms of material and energy consumption. Unlike casting and forging, it should be flexible enough to allow very rapid manufacturing of complex parts without investment in tooling, moulds and dies. In recent years, the number of parts produced by additive manufacturing has grown significantly, reaching a level that some experts believe heralds the new industrial revolution. Additive manufacturing is now being used not just for prototyping (as it once was), but for manufacturing of fully functional parts in the automotive, energy, biomedical, food, tooling and even aerospace industries. One of the most advertised features of additively manufactured metallic parts is the homogeneity of structure and mechanica properties as opposed to, say, castings. Supposedly, this is achieved by layer-by-layer manufacturing (with layer thicknesses of about 0.1 – 2 mm) and only local remelting and similar cooling conditions. However, this is only true for very small parts and in most cases the final inhomogeneities in the properties of additively manufactured parts are greatly underestimated. This is mainly because the microstructure is usually very similar throughout the volume of the part and therefore the part is considered homogeneous. However, for most types of engineering metal alloys, the microstructure (understood as grain size, shape, type) does not play the most important role. The most important property is the phase composition, which depends not only on the chemical composition of the alloy, but mainly on the heat treatment of the alloy. In additive manufacturing, the material undergoes an in-situ heat treatment in which it is heated cyclically (up to liquidus temperature) and cooled rapidly, with the amplitude decreasing with time and progress in manufacturing the part. The character of these thermal cycles depends largely on the processing parameters, such as linear heat input (i.e. laser/electron beam/electric arc power), deposition head speed, layer thickness, and also on the thermal conductivity of the alloy being processed, as well as the type of shielding gas and deposition atmosphere. Since most alloys used in industry are heat treatable (most steels, nickel alloys, titanium alloys, copper alloys, aluminium alloys), the problems caused by the very specific heat treatment conditions must be fully understood in order to improve the properties of additively manufactured products and avoid the need for subsequent heat treatment. This not only increases the price of the parts, but can also lead to degradation and deformation of parts, or is simply impossible in the case of repaired parts, especially those with functionaly graded properties.

 

This project aims to investigate the influence of thermal history on the microstructure and properties of additively manufactured materials. For this reason, a non-obvious method based on observation with an infrared camera will first be developed to determine the thermal prehistory. Later, numerical modelling of the additive manufacturing process will be performed in order to determine temperature and stress fields. The influence of the processing parameters on the thermal history of the samples will be studied, and later the influence of the thermal history on the microstructure and properties of the additively manufactured parts. Finally, if necessary, post-processing heat treatment will be proposed and the results will be verified. Two direct energy deposition (DED) techniques will be compared – LENS (Laser Engineered Net Shaping) at Military University of Technology, Warsaw Poland and WAAM (Wire Arc Additive Manufacturing) at University of Ljubljana to determine the main diferences in building of medium to large size parts.

 

Project value: 299.897,34 €

Project duration: 1.1.2024 – 31.12.2027

 

2.       Project team members

 

Project team members https://cris.cobiss.net/ecris/si/en/project/21073

 

Project leader: Damjan Klobčar

0782  University of Ljubljana, Faculty of Mechanical Engineering

0795  University of Maribor, Faculty of mechanical engineering

1555  University of Ljubljana, Faculty of Natural Sciences and Engineering

 

Foreign project team member

Project leader: Marek Krzysztof Polański

Military University of Technology (www.wojsko-polskie.pl/wat/)

 

3.     Research tasks and realisation

 

The research project investigates the influence of thermal history during metal additive manufacturing on the microstructure and mechanical properties of materials. The research focuses on two additive manufacturing technologies LENS (Laser Engineered Net Shaping) – MUT and WAAM (Wire Arc Additive Manufacturing) – University of Ljubljana (UL).

 

The project is organized into several work packages (WP) covering experimental research, numerical modelling, and the analysis of microstructure and mechanical properties.

 

WP1 – Temperature Measurement Methodology

Status: completed

In this work package, an experimental methodology for temperature measurement during additive manufacturing processes was developed.

Main activities included:

·       establishing laboratory measurement setups for both LENS and WAAM systems

·       developing data acquisition and measurement control systems

·       measuring temperature fields using:

o   thermocouples

o   infrared cameras

o   visible-spectrum cameras

·       determining material emissivity as a function of temperature and processing conditions

The main result of this work package is a reliable temperature measurement system capable of measuring temperatures in the range of approximately 200–1800 °C, providing a solid experimental basis for further project research.

 

Additionally, to monitor the thermal history of materials produced using the LENS® system, experimental setup was constructed, consisting of three infrared cameras: OPTRIS PI640i with a temperature range of 0–250 °C, OPTRIS PI640i with a temperature range of 200–1500 °C, and OPTRIS PI05M with a temperature range of 900–2450 °C. The use of three cameras enables the recording of temperature variations in a wide range from approximately 0 to 2500 °C on the side surface of the built component.

In addition, an experimental setup was equipped with an Optris CTratio 2MH1 pyrometer with a temperature range of 550–3000°C, mounted on the laser head and used to monitor the melt pool temperature. The entire monitoring system was placed in the working chamber of the LENS® device, enabling in-situ measurements and recording of thermal history during the additive manufacturing process.

 

 

Figure 1 Experimental setup.

 

WP2 – Numerical Modelling of Temperature Fields

Status: ongoing

This work package focuses on the development of numerical models of heat transfer during additive manufacturing.

The modelling work includes:

·       simulations of LENS and WAAM processes

·       use of advanced simulation tools:

o   Abaqus

o   Flow-3D

·       prediction of temperature fields and thermal cycles

·       validation of simulation results using experimental temperature measurements

The goal is to develop models capable of predicting temperature conditions during the manufacturing of complex geometries.

 

In addition, the Simufact Welding software is also used for numerical modeling of AM processes. The numerical model is validated by measuring temperature distributions during the process using a type B thermocouple welded into the building specimens (Figure 2) as well as by measuring substrate displacement with the DIC technique (Figure 3).

 

Figure 2 Example comparison of temperature distribution obtained in the experiment and the numerical model for laser power of: (a) 400 W and (b) 500 W.

Figure 3 Substrate displacement results from: a) numerical model, b) experiment.

 

 

WP3 – Influence of Material and Process Parameters

Status: completed

This work package investigated the influence of material properties and processing parameters on temperature fields during additive manufacturing.

The analysed parameters included:

·       thermal conductivity of materials

·       material deposition speed

·       heat source power

·       shielding gas atmosphere

·       geometry and size of samples

·       number of simultaneously manufactured samples

The results provide a better understanding of:

·       thermal cycles during the manufacturing process

·       solidification behaviour

·       phase transformations in materials

This knowledge is essential for optimizing process parameters and manufacturing strategies.

 

Using the experimental setup from DS1, preliminary research on the influence of process parameters (such as layer thickness and laser power) on the temperature fields of samples produced by the LENS process was conducted. Each sample was monitored during manufacturing using infrared cameras. Ten measurement areas were identified in the captured images/videos for each sample, and their local thermal histories (temperature versus time) were analysed (Figure 4)

 

Figure 4 Thin-walled sample with analysis areas marked and thermal history (temperature–time) determined for individual areas.

 

Based on the recorded thermal histories for individual areas of each sample, the temperature range (Figure 5) and thermal exposure (Figure 6) were analysed in three characteristic zones of the sample: the upper, middle and lower parts of the element.

 

Figure 5 The temperature range for three selected areas.

Figure 6 Thermal exposure for three selected areas.

 

WP4 – Microstructure Analysis

Status: ongoing

This work package studies how temperature fields during additive manufacturing affect the microstructure of materials produced by LENS and WAAM.

Research activities include:

·       analysis of grain growth

·       determination of phase composition

·       investigation of local microstructural changes

Special attention is given to:

·       the influence of in-process vibrations during WAAM

·       the influence of post-process heat treatments

Preliminary results indicate that vibrations during the process can:

·       reduce grain size

·       reduce material anisotropy

·       decrease porosity

 

WP5 – Mechanical Properties of Materials

Status: ongoing

This work package investigates how thermal history during additive manufacturing affects the mechanical properties of materials.

The following properties are analysed:

·       tensile strength

·       yield strength

·       elongation at fracture

·       hardness

·       fracture mechanics behaviour

·       fatigue resistance

The research also examines how process vibrations and post-process heat treatment can improve the homogeneity of microstructure and mechanical performance.

 

International Collaboration

The project is carried out through collaboration between:

·       University of Ljubljana (UL) – research on the WAAM process

·       MUT – research on the LENS process

The comparison of these two additive manufacturing technologies enables a deeper understanding of:

·       temperature evolution during manufacturing

·       microstructure formation

·       resulting mechanical properties of additively manufactured materials.

 

4.       Bibliography

 

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