All posts by Håkan Hallberg

A possibility to publish your latest research on recrystallization in metallic materials!

I am guest editing a special issue of the journal Crystals together with Prof. M. Ristinmaa. The topic of this special issue is "Recrystallization of Metallic Materials" and it is open for submissions until August 1st. More information on the scope of the special issue and on submission procedures etc. can be found at the journal's special issue web page, following this link.

Crystals (ISSN 2073-4352) is an international, open access journal of crystallography, published monthly online by MDPI. The journal is indexed in, e.g., Scopus and Web of Science and the 2016 IF is 1.566.

New Paper on Laser Annealing of Cold Rolled Steel

A new paper is published in Metallurgical and Materials Transactions A, with the title Microstructure and Property Modifications of Cold Rolled IF Steel by Local Laser Annealing. The work investigates the possibilities in achieving very local microstructure and property modifications by laser annealing of cold rolled IF steel. It is shown that this is a promising method for making functionally graded steels with high precision.

Laser Processing of Metals

Lasers have been found to be a very competent and versatile tool in modifying the properties of metallic materials. Laser energy can in fact be used to make the work piece material either harder or more ductile. The high spatial precision of the energy input made possible by using a laser beam also makes it a very interesting tool for manufacturing of functionally graded metals, having mechanical properties that vary in predefined patterns.

Surface hardening by laser shot peening and improvement of the material’s ductility by laser annealing are two methods briefly outlined below

Laser annealing

During cold working of metallic materials – for example by rolling – the material will be subject to significant plastic deformations that greatly increase the strength and hardness of the metal, but which also makes it more brittle and prone to damage and fracture in subsequent manufacturing steps. To restore ductility, a heat treatment is usually performed following cold working. During annealing, the material is kept at an elevated temperature for some duration of time. During annealing, the cold worked material will undergo a sequence of recovery, recrystallization and grain growth, which eventually reduces the hardness and improves ductility. Annealing is, however, usually performed as a bulk process on entire steel components by placing them in a temperature-controlled furnace. However, as shown in Hallberg et al. 2017, significantly more precision can be achieved if a laser beam is used as the heat source. The accuracy of this method is in fact high enough to allow annealing of regions of millimeter size and in arbitrary patterns. This makes laser annealing a very promising tool for manufacturing of functionally graded steels.

Illustration of material property and grain size variation during cold working (left) and subsequent annealing (right) of a metallic material.

Laser Shot Peening (LSP)

The ability of metal components to withstand wear, fatigue and corrosion is of pivotal importance in many engineering applications. This ability can be significantly improved by introducing compressive stresses in the surface layer of the component. Shot peening is one well established method to achieve this, whereby abrasive metallic, glass or ceramic particles are allowed to impact the surface with great velocity. An alternative method is ultrasonic peening where the compressive stresses are caused by a sonic wave impacting with the surface. A third alternative is Laser Shot Peening (LSP). In LSP, short pulses from a high-energy laser beam are used to obtain the desired compressive stresses. All of these methods share common features in terms of them being purely mechanical – i.e. they do not interact thermally with the work piece – and in that they all cause a pressure on the work piece surface which is sufficient to cause plastic deformations and in some cases also martensitic (solid state) phase transformations. Following peening, relaxation takes place during which residual compressive stresses develop. These compressive stresses provide the desired improvement of wear, fatigue and corrosion resistance of the work piece surface. The residual stresses which develop due to plastic deformation and martensitic phase transformation during LSP are studied in Halilovic et al. 2016.

LSP has been found to be the peening process that provides the greatest penetration depth of the region holding the beneficial compressive residual stresses. In addition, the flexibility of working with a laser beam makes it possible to confine the peening to small areas and to peen surfaces in confined spaces - such as near holes and small notches – which are inaccessible by other peening methods.

Schematic illustration of Laser Shot Peening (LSP).

In LSP, the surface of the work piece is covered with an ablative layer which may consist of a layer of black paint or a metallic foil. A laser beam – typically with an intensity in the range between 1 GW/cm2 and 20 GW/cm2 - is focused on the peening region, causing the ablative layer to vaporize. The temperature of the vaporized material increase very rapidly to temperatures in the order of 10 000K which results in ionization and plasma formation. The laser pulse is usually very short (3-30 ns) and the plasma continues to absorb the laser energy during the duration of the pulse. The temperature increase in the plasma gives rise to a hydrodynamic pressure shock wave which propagates into the work piece. To further enhance the process, the work piece and the ablative layer is frequently confined beneath a layer of water or glass. This layer is transparent to the laser beam and reduces the expansion of the plasma away from the target. This confining layer can increase the magnitude of the pressure wave going into the work piece by a factor of five or more. The LSP process can provide a pressure of up to 10 GPa in a spot measuring a few millimeters in radius. The pressure wave propagates into the work piece and cause plastic deformations as the affected material is confined by the surrounding material, compressive residual stresses will develop.

WCCM 2016 mini-symposium on "Numerical methods for front tracking problems at the microscale"

The 12th World Congress on Computational Mechanics (WCCM XII) will be held in Seoul, Republic of Korea, 24-29 July 2016. At this event, I am co-organizing a mini-symposium on Numerical methods for front tracking problems at the microscale. The conference (and the mini-symposium) is open for abstract submissions. More information can be found on the conference web pages:

Uranium Dioxide (UO2)

Different kinds of oxide fuels are used in nuclear power plants, most commonly used – and for the longest time – is Uranium Dioxide (UO2). A solid understanding of the fuel performance is central to control in-service performance, properties and degradation of the fuel as well as for safe handling of it.

As fuel material, UO2 is usually sintered into small cylindrical pellets, measuring about 10 mm in diameter and similar in length. These cylinders are stacked in fuel rods inside a Zircalloy cladding. The small radial gap between the pellets and the cladding is usually filled with a pressurized gas such as Helium. A number of such fuel rods are mounted in fuel assemblies together with control rods with a high capacity for neutron absorption. The fuel assemblies are then used as heat source in fission power plants.

As the fuel pellets are “burnt” in the reactor, fission processes take place and the degree of irradiation of the fuel pellets is usual measured in terms of the “burnup”, that is the fraction of the initial material that has undergone fission.

Under common in-service conditions, the core of the fuel pellets can be maintained at a temperature of 2000K while the pellet surface is at around 800K (the melting point of UO2 is approximately 3140K). The outside temperature is maintained by a constant flow of coolant through the fuel assemblies. Under such extreme thermal gradient conditions, the fuel material undergoes drastic changes. These changes have a strong influence on fuel performance and properties such as the thermal conductivity and structural rigidity. The grain structure will have different morphologies in different regions. This is schematically illustrated below.

Cross-section of a UO2 fuel pellet, showing characteristic microstructure variations.

The extreme thermal gradients will also cause so-called “hourglassing” of the fuel pellets along with cracking – both radially and circumferentially – due to thermally induced stresses and swelling due to solid fission products.

By the release of fission gasses (e.g., Xe, Kr, I and Cs), gas-filled pores or voids will form in the microstructure. The gas bubbles form in the grain interiors and migrate by diffusion to coalesce along the grain boundaries.

Formation of gas-filled bubbles, pores and voids in the grain structure of UO2 during irradiation.

The presence of gas bubbles can cause swelling and cracking of the fuel pellet and the gas can also be released inside the Zircalloy cladding, lowering the heat conduction capacity of the Helium that surrounds the pellets. In either case, the integrity of the Zircalloy cladding is compromised.

In Hallberg & Zhu (2015), the stability of grain boundary texture under grain growth in UO2 is studied through level set modeling, taking anisotropic grain boundary properties into account. The characteristic morphologies of faceted voids in UO2, due to heterogeneous interface energies, is studied in Zhu & Hallberg (2015) by 3D phase field simulations.