Ultrasonic Waves for Analyzing Residual Stresses in Additive Manufactured Metal Parts

This Additive Manufacturing (AM) process produces metal parts layer by layer using a high energy laser beam to fuse metal powder particles. When each layer is complete, the build platform moves downward by the thickness of one layer, and a new powder layer is spread on the previous layer.

While this process is able to produce quality parts and components, residual stress is a major problem during the fabrication process. That's because large temperature changes near the last melt spot - rapid heating and cooling - and the repetition of this process result in localized expansion and contraction, factors that cause residual stress.

Residual stress (RS)

Residual stresses are self-equilibrating stresses which remain in the part after its manufacture even without supplementary thermal gradient and external forces. These stresses arise from misfits in the shape of parts of both different regions and different phases within a part, or even because of local variations in elastic constants, or thermal and mechanical properties. RS can result in several undesirable consequences on the part's properties, such as poor fatigue resistance, critical failure during operation, lower chemical resistance, lower magnetization, lower resistance to deformation, and diminished static and dynamic strength.

RS can be classified according to the length scale on which they occur:

Type I: Macro RS, which equilibrate over large distances and deform the part if boundary conditions are changed.

Type II: Micro RS, that occur over the grain scale because of the formation of different phases in the structure. They are frequently observed in a polycrystalline material because of the different grain orientation and the anisotropy of a crystal structure.

Type III: Micro RS inside a grain (over atomic dimensions), which occur because of defects such as dislocations, vacancies, or alien atoms distorting the crystal lattice in the crystal structure.

Ultrasonic Waves

Ultrasonic waves (UW) testing uses mechanical waves, which are oscillations of material particles that need a material medium to propagate. These waves are used in many fields of application: medical, mechanical structural measurement (NDT), sensors, sonars, and weapons. Typically, UW has a frequency higher than 20 kHz, corresponding to the upper limit of the human hearing range (high pitch). Its utilization in NDT has several advantages, such as good accuracy, high speed, reliability, repeatability, affordability, and capability to use in several environments such as liquids. Also, it is non-destructive and does not require direct contact with the part being tested, which makes it suitable for in-situ monitoring systems in AM. Finally, there is no limitation in terms of the type of material being tested or its geometric complexity (thin films or large components), and it is minimally influenced by temperature.

Analyzing Residual Stresses Ultrasonic Waves

Residual Stress measurement techniques.

Researchers from the Czech Republic and Brazil worked together to focus on the ultrasonic testing used for stress analysis of 3d printed metal parts and published a research article titled, “Residual stress analysis of additive manufacturing of metallic parts using ultrasonic waves: State of the art review”.

Process variables affecting the residual stresses

The authors have highlighted the potential of ultrasonic testing (UT) for measuring RS during and after construction. The use of ultrasonics has a long history when testing and characterizing materials, and may be very valuable when looking for problems such as deformation, delamination, or structural damage. As a non-destructive test method, UT involves sending short-pulse ultrasound to the material under test to detect internal defects.

The author proposes many benefits of the technology, including accuracy, speed, repeatability, and affordability, a large number of materials can be analyzed, and even this technology can be integrated with an automatic monitoring system and a 3D Printing system. However, it is more suitable for measuring RS in the whole part rather than in a specific area.

A typical configuration of the ultrasonic method using spatially resolved acoustic spectroscopy

Although the other RS measuring methods such as hole drilling (HD) and X-ray diffraction are still most commonly used measurement methods - provide accuracy and reliability for industrial applications, but many limitations are there such as small sample size, rough surfaces, destructive testing with numerous errors. 

The author highlights many research advances that are currently devoted to the use of UT for RS testing, including Spatially Resolved Acoustic Spectroscopy (SRAS), a technique that uses two lasers to inspect the surface and near-subsurface features, and various other laser-based methods. They suggested that although most machines today rely on X -rays, infrared cameras, and high-resolution cameras, these UT technologies can be incorporated into metal additive manufacturing systems to perform in-situ monitoring of parts.

Challenges 

The most remarkable challenge is that the optimization of the link between UT apparatus and the AM hardware is required. The complexity of AM parts brings another challenge to UT, since it is limited to simple geometries. Another limitation of using ultrasonics as a quality control system is the complex geometries of additive manufacturing parts, because UT is limited to simple geometries.

Commercial Systems 

Existing in-situ monitoring tools for commercial systems are quickly evolving: EOSTATE Meltpool is able to monitor the entire building process through four separate and independent modules (Base, PowderBed, Meltpool, Exposure OT), increasing control on machine parameters and part quality. Besides, in a recent development with MTU Aero, a promising contact UT device was created including high spatial resolution (40 μm) and capability to measure porosities. Furthermore, ARCAMLayerQam allows us to monitor porosity and defects up to 100 μm overall build chamber regarding before and after each layer. Another interesting tool is ARCAM xQam, an X-ray automated detector employed to calibrate the electron beam of EBM machines and it has the potential to be used in material characterization. 

What do you think about using ultrasonic waves for analysis of 3D Printed parts? Let us know in a comment below or on our Facebook page!