There are many good recent publications you could refer to. There is even a relatively new journal dedicated to this materials process:

https://www.journals.elsevier.com/additive-manufacturing

Many of the articles in this journal are open access, so you should have no problem finding some places to start.

Andrew

Dear Olisa,

I can say that this topic seems to be "non-trivial" however rather non-novel, as there are already papers considering residual stress measurements in printed metallic and polymer parts. The used method is X-ray Diffraction.

However I think as a characterization tool for comparison, for example SLM and EBM manufactured components using residual stress investigation may be interesting.

All the best,

Residual stress is not good in terms of stress corrosion cracking, however compressive residual stress are useful in some applications. The consequences can be good and bad depend on the application.

The residual stress distribution may directly determine the success or failure of the additive manufactured part. The residual stress calculation and measurement are still a challenge since there could be phase transformation, complex thermal history, concomitant microstructrre evolution, etc involved. All of them can contribute to the lattice compatibility at sub-micro level which may induce local crack or failure.

This is also what I am interested in and I will continue to follow this discussion.

The large thermal gradient and high cooling rates during layers building process in all additive manufacturing technologies leading to the residual stress which can contribute to the built part failure. XRD test can be used to evaluate the residual stress also hole drilling method.

Hi Olisa,

I agree that residual stress for additive components is not a great label. From the 'additive' part, I assume you are concerned about additive tolerances. I have no experience with metal components, but for injection moulded glass fiber filled nylon, there are serious dimensional changes after moulding. The issue isn't internal stresses, but water. Nylon is hygroscopic to an extent. Moulding temperatures expel any water and the part will initially absorb water and change its dimensions. There are two general approaches to dealing with this. Either wait until the nylon absorbs atmospheric water or steam the parts to accelerate uptake.

George

Dear Olisa,

First of all, thank you for come up with this important question.

Understanding the residual stress problems in additively manufactured parts is more non-trivial than we are aware today. Furthermore, this phenomenon leads to various problems and we cannot limit those affected by this to only material scientists.

The following information are provided mostly for metal additive manufacturing processes and in particular powder bed fusion processes known with several commercial names such as SLM (Selective Laser Melting), DMLS (Direct Metal Laser Sintering) or EBM (Electron Beam Melting).

I totally agree with Wadea large thermal gradient and high cooling rates during layers building process is one of the main reasons for residual stresses. To improve this explanation, occurring thermal gradients causes cyclic thermal expansions which must not exceed elastic strain of the material. These cumulated strains can generate internal stresses in the part also known as residual stresses and cause distortions which may possibly lead failure by layer delamination or cracking.

Most of the time, detected problems of residual stresses and distortions are tried to be overcame on the final product using additional operations such as stress relieving and/or machining, adding extra costs on the product. On the other side, they are still insufficient to eliminate in-process risks such as delamination, cracking or job interruption due to part to re-coater rubbing.

Many techniques for measuring the residual stresses in a part are available. Usually a classification is made in two groups, whether the measurement is destructive or not. Destructive methods include the layer removal method, the crack compliance method, the contour method and the hole drilling method. Non-destructive methods include X-ray diffraction, neutron diffraction, ultrasonic and magnetic measuring methods.

Since there are hundreds of process parameters in additive manufacturing, it is not easy to refer all of them in conjunction with residual stresses. To name several of them I can say thermal properties of powder materials (Cp, k, CTE), energy input parameters of power, scan speed, hatch distance and layer thickness, scanning or hatching strategy, pre or post heating temperature, etc.

I would like to leave your last question to our colleague metallurgists/material scientists since I am a mechanical engineer (PhD).

I hope you will find these information useful.

Regards,

Dear Olisa Ezeh,

Please go through my papers for establishing a sound understanding of Residual Stress in SLM parts and its dependen on parameters.

These papers will hopefully give you a solid understanding of the underlying physics and as well as mechanisms for controlling the Residual Stress by parameters manipulation.

thank you

Dr Haider Ali

Residual stress build up is inherent in additively manufactured parts, particularly laser based. Largely, tensile stresses are unwanted as they lead to cracking and distortions. Distribution and magnitude of residual stress is dependent on many factors such as material thermo-physical properties and processing parameters. Several methods are available to measure these stresses, including diffraction methods such x-ray and neutron diffraction; and mechanical methods that include bridge curvature, contour, hole drilling, sectioning etc. Each of these methods has advantages and disadvantages, but x-ray and hole-drilling are some of the most commonly used.

This is quite challenging topic and many phenomena are involved.

For instance, in SLM, parts made of titanium residual stresses are high and you must do stress relief while others materials could live without it...

You can look in this publication https://www.mdpi.com/2504-4494/3/3/52

which as some tabular comparisons and a concise introduction to the process physics, hopefully it can help you building an answer on your topic.

Dear Ezeh,

Generally, residual stress values decreased when temperature is increased for additive manufacturing. Heat treating is used to remove stress or stress relief specially in the SLM process. However, distribution and magnitude of residual stress is dependent on many factors such as material thermo-physical properties and processing parameters . Thermal convection conductivity shall be decreased when enthalpy values are down with laser energy of 150-1000 watt are used for SLM if temperate differences are less in term of chamber temperature to laser melt temperature. Moment we increase the power in watt, temperature to heat the bed material shall also increase and metallic powder transforms in another phase.

It was found for the one of the matallic powder that increasing the bed temperature to 570 °C significantly reduced residual stress formation within components and enhanced yield strength and ductility. This pre-heating temperature enabled the decomposition of α′ materialistic micro structure into equilibrium α+β micro-structure. At 570 °C the yield strength and elongation of components was improved by 3.2% and 66.2% respectively.

Hope it help you out.

XRD is a is a precise and non - destructive technique for residual stress measurement. However, any method has its limits. As far as I know, this method is used only for crystalline materials and allows only near the surface measurement (about 0.01mm depth). In my opinion, the method with strain gauges, described in ASTM E837, is also accurate and much more versatile. It only involves removing a small amount of material. But it also allows the measurement of residual stress on the depth of the sample. In addition, the equipment is much cheaper (compared to XRD). These are the great advantages of this method. I strongly recommend to use this method, whenever is possible to do it.

Paul Barsanescu

"Gheorghe Asachi" Technical University of Iasi

Romania