Possible explanations for different surface quality in laser cutting with 1 micron and 10 microns beams

In laser　cutting　of　thick　steel　sheets,　quality　difference　is　observed　between　cut　surfaces　obtained　with 1 micron and 10 micron laser beams. This paper investigates physical mechanisms for this interesting and important problem of the wavelength dependence. First, striation generation process is described, based on a 3D structure of melt flow on a kerf front, which was revealed for the first time by our recent experimental observations. Two fundamental processes are suggested to explain the difference in the cut surface quality: destabilization of the melt flow in the central part of the kerf front and downward displacement of discrete melt accumulations along the side parts of the front. Then each of the processes is analyzed using a simplified analytical model. The results show that in both processes, different angular dependence of the absorptivity of the laser beam can result in the quality difference. Finally we propose use of radial polarization to improve the quality with the 1 micron wavelength

Copper-Triggered Aggregation of Ubiquitin

Neurodegenerative disorders share common features comprising aggregation of misfolded proteins, failure of the ubiquitin-proteasome system, and increased levels of metal ions in the brain. Protein aggregates within affected cells often contain ubiquitin, however no report has focused on the aggregation propensity of this protein. Recently it was shown that copper, differently from zinc, nickel, aluminum, or cadmium, compromises ubiquitin stability and binds to the N-terminus with 0.1 micromolar affinity. This paper addresses the role of copper upon ubiquitin aggregation. In water, incubation with Cu(II) leads to formation of spherical particles that can progress from dimers to larger conglomerates. These spherical oligomers are SDS-resistant and are destroyed upon Cu(II) chelation or reduction to Cu(I). In water/trifluoroethanol (80∶20, v/v), a mimic of the local decrease in dielectric constant experienced in proximity to a membrane surface, ubiquitin incubation with Cu(II) causes time-dependent changes in circular dichroism and Fourier-transform infrared spectra, indicative of increasing β-sheet content. Analysis by atomic force and transmission electron microscopy reveals, in the given order, formation of spherical particles consistent with the size of early oligomers detected by gel electrophoresis, clustering of these particles in straight and curved chains, formation of ring structures, growth of trigonal branches from the rings, coalescence of the trigonal branched structures in a network. Notably, none of these ubiquitin aggregates was positive to tests for amyloid and Cu(II) chelation or reduction produced aggregate disassembly. The early formed Cu(II)-stabilized spherical oligomers, when reconstituted in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) liposomes and in POPC planar bilayers, form annular and pore-like structures, respectively, which are common to several neurodegenerative disorders including Parkinson's, Alzheimer's, amyotrophic lateral sclerosis, and prion diseases, and have been proposed to be the primary toxic species. Susceptibility to aggregation of ubiquitin, as it emerges from the present study, may represent a potential risk factor for disease onset or progression while cells attempt to tag and process toxic substrates

Experimental investigation on fiber laser cutting of Ti6Al4V thin sheet

The high strength to weight ratio and good corrosion resistance of titanium alloys, have led to an increasing use of these materials, particularly in the aerospace industry. The laser cutting technique may be a promising tool in machining titanium alloy parts like those with subsequent welding requirement: in this case, surface quality of the kerf edges is of great importance. The low thermal conductivity and the high chemical activity of titanium alloys lead, in fact, to alterations of the surface properties of the machined zone. This paper presents the results of titanium alloy laser cutting using a 2 kW fiber laser. The cutting process was performed in continuous wave mode and using Argon as shear gas. Laser cuts were realized on titanium alloy Ti6Al4V sheets 1mm thick. Image analysis and microscopy, were carried out to examine the cutting edge quality features including thickness of the recast layer and heat-affected zone

Fiber laser cutting of Ti6Al4V sheets for subsequent welding operations: Effect of cutting parameters on butt joints mechanical properties and strain behaviour

The effect of laser cutting parameters on the mechanical behavior of laser butt welded joints whose edges were obtained by laser cutting was investigated. The paper aims to demonstrate that new high power solid-state fiber lasers not only represent a valid and reliable alternative to the most established CO2 and Nd:YAG laser sources, but also allow to obtain cuts having edges well suited for subsequent direct laser welding. First Ti6Al4V 1 mm thick sheets having edges machined by milling were laser welded. Once the optimal welding condition was determined, the mechanical characterization of sheets cut by fiber laser and then laser welded was performed. Comparative strain analysis performed by a digital image correlation technique highlighted the effect of the gap between the sheets resulting from the different cut edge quality. Experimental results showed that the correct selection of laser cutting parameters allows to obtain butt joints characterised by mechanical properties comparable with the ones obtained by milling. Cutting edge quality in the optimal range of gap values allows to obtain the best mechanical performances of the joint

Evaluation of the strain behaviour of butt joints on AZ31 magnesium alloy thin sheets welded by Nd:YAG laser

In this work, the mechanical and technological behaviour of AZ31 magnesium alloy laser welded joints is investigated. The forming behaviour of the joints is analysed by both tensile and biaxial stretch tests. Each test is monitored using a digital image correlation system in order to acquire the complete strain field during the whole test. Both in tensile and biaxial stretching tests, the strain maps reveal that the weld bead makes the strain path experienced by the welded specimen more critical than the one experienced by the base material, and this can be related to morphological defects of the weld bead

Blow forming of AZ31 magnesium alloy at elevated temperatures

In this work, the forming behaviour of a commercial sheet of AZ31B magnesium alloy at elevated temperatures is investigated and reported. The experimental activity is performed in two phases. The first phase consists in free bulging test and the second one in analysing the ability of the sheet in filling a closed die. Different pressure and temperature levels are applied. In free bulging tests, the specimen dome height is used as characterizing parameter; in the same test, the strain rate sensitivity index is calculated using an analytical approach. Thus, appropriate forming parameters, such as temperature and pressure, are individuated and used for subsequent forming tests. In the second phase, forming tests in closed die with a prismatic shape cavity are performed. The influence of relevant process parameters concerning forming results in terms of cavity filling, fillet radii on the final specimen profile are analysed. Closed die forming tests put in evidence how the examined commercial magnesium sheet can successfully be formed in complicated geometries if process parameters are adequately chosen

Gas forming of an AZ31 magnesium alloy at elevated strain rates

In this work, the gas forming of AZ31 magnesium alloy 0.75-mm-thick sheets at elevated strain rates (fast gas forming) is investigated through an experimental-numerical approach. First, free inflation tests were carried out to find the conditions, in terms of temperature and forming pressure, able to give the best compromise between the alloy formability and the forming time. The analysis was successively moved to a closed die forming application with a stepped geometry case study in order to analyse the real forming process. Both an axisymmetric model of the free inflation test and a 3D model of the closed die forming process were built to correlate the results from free inflation tests (in terms of optimal strain rate values) to the closed die forming test: Numerical simulations were run to find the pressure value to be applied in gas forming tests. Experimental gas forming trials were finally conducted in order to support the approach and to analyse post-forming characteristics of the formed parts. Results showed that very small fillet radii can be reached on a commercial Mg alloy sheet setting very short forming times (few seconds). The choice of the forming temperature and of the corresponding optimal strain rate strongly affects the grain growth and the cavitation phenomena. Even if the alloy is prone to a strong static and dynamic grain growth at elevated temperatures, a small mean grain size value can be reached in the formed component due to the short forming times

A strain-dependent model for the coefficient of friction in the tool-blank interaction in superplastic forming

Superplastic forming (SPF) is an advanced sheet metal forming process adopted mainly in the aerospace and automotive industries, as well as for biomedical applications, thanks to its capacity to replicate very complex geometries. Since it basically consists in blowing a metal sheet with a gas pressure into dies at elevated temperature, the study of the tribological issues plays a fundamental role in the understanding of the forming technology. In particular, the evaluation of the coefficient of friction (COF), which takes into account friction phenomena between the sheet and the die surface, is essential to obtain qualitatively satisfying final results preserving tools lifetime. Although a constant COF is usually assumed in SPF modelling, the determination of the real values of COF is very challenging because of its dependence on several parameters, and available test methods are often inadequate to properly assess all those dependences. For these reasons, a new test method is proposed and the evaluation of strain-dependent values of COF through a mathematical model is attempted in this work. Both experimental results, acquired from specifically designed closed-die forming tests, and numerical ones, from finite element modelling of the SPF process, are employed for the calibration of the mathematical model through the evaluation of the thickness distribution in the formed specimens

Laser hardening of AISI 52100 bearing steel with a discrete fiber laser spot

Surface hardening with discrete laser spot treatment is an interesting solution since the adoption of a single pulse allows the treatment of different surface geometries avoiding the effect of back tempering. The aim of this work is to find a suitable process window in which operate to get best results in terms of hardness, diameter and depth of the treated region. A single pulse out of a fiber laser source impinging on a bearing hypereutectoid steel was used using different power values, pulse energy and defocussing distances, in order to get the optimal process parameters. The dimensions of the hardened zone and its hardness were then acquired and related to the laser process parameters, to the prior microstructure of the steel (spheroidized and tempered after oil quenching) and to the roughness on the specimen before the laser treatment. Experimental results highlighted that both the surface condition (in terms of roughness) and the initial steel microstructure have a great influence on the achieved hardness values and on the dimension of the laser hardened layer. The pulse energy and power strongly affected the dimension of the hardened layer, too