Rafal Jendrzejewski

Laser treatment of the 38HMJ steel surface in a liquid nitrogen environment

Samples of the 38HMJ steel kept in a liquid nitrogen bath (LN2) at 77 K are treated by a 1 kW CO2 laser beam of intensity of about 1.3x105W/cm2 on the sample surface. For thermal processing by means of the 10,6 micrometers radiation the transparency window of LN2 below 0.3 eV is utilised. The irradiation effects are compared with results obtained for the reference samples of the same material under standard gas shielding conditions. Measurements of microhardness HV 0.1 indicated on reduction of the heat affected zone for the LN2 case. A similar effect is observed for higher sample velocities and a value of about 700 HV is obtained in a 30 micrometers surface layer. Surface analysis by means of Auger electron spectroscopy confirms the local nitrogen element of the laser treated areas depending on the processing conditions.

Multilayer laser cladding of metal powders for fast prototyping

Volumetric structures consisting of multilayer clads made of the bronze B10 and also of stellite SF6 alloys are prepared by laser remelting of the metal powder of the original particle size of ca 60 micrometers injected into the processing zone through a conical nozzle. For remelting with a cw CO2 laser beam of intensities around 105 W/cm2 and sample feed rates not exceeding 2.5 cm/s an efficient formation of the structures is observed and their geometry can be controlled. A fine grained, dendritic microstructure of the samples is observed by means of the optical and scanning microscopes. In the AFM scans the characteristic dimension of ca 60 nm reproduced in the periodic structures of the SF6 surface corresponds most probably to crystallines formed due to segregation after the liquid-solid phase transition. Similarly to SF6 the powderized B10 material is well suited for a fast prototyping.

Laser stereolithography by multilayer cladding of metal powders

3D-structures obtained by means of laser cladding of the metal alloy powders: bronze B10 and stellite 6 and the process parameters are studied experimentally. The structures are made trace-on-trace by remelting of the metal powder injected into the focusing region of the 1.2 kW CO2 laser beam. For the powder and sample feeding rates of 8-22 g/min and 0.4-1.2 m/min, respectively, and the applied beam intensities not exceeding 2 X 105 W cm-2 the process is stable and regular traces connected via fusion zones are produced for each material. The thickness of these zones does not exceed several per cent of the layer height. The process results in the efficient formation of multilayer structures. From their geometry the effect of energy coupling and interaction parameters are deduced. Moreover, the microanalysis by means of SEM- and optical photographs of samples produced under different experimental conditions confirms the expected mechanical properties, low porosity and highly homogenous structure of the multilayers. In addition to the known material stellite 6 the bronze B10 is originally proposed for a rapid prototyping.

Temperature and stress fields during laser cladding of stellite protective coatings

The time dependent temperature and strain-stress fields are numerically calculated for the laser cladding of protective stellite SF6 coatings on the X10Cr13 chromium steel. The temperature dependent material properties and also effects of the base preheating and time delay between cladding of the consecutive layers are taken into account. Results of the model calculations indicate that the micro-cracking susceptibility of the coating is smaller for the preheated base. In the case of the substrate initial temperature of 600 °C the calculated strain stresses lie below the limit of the stellite tensile strength. This is confirmed in experiment by the crack-free coatings while for the samples cladded on the non-preheated substrate cracks are produced. An increase of the cracking susceptibility with prolonged time delay between preparation cycles of the consecutive layers is evidenced by modelling, which agrees with experiment, too.

Laser dispersing of ceramic powders into Al-alloys

The general objective of the work was formation of highly wear resistant metal matrix composite (MMC) surface layers on aluminium based Al 6061 alloy by means of laser dispersing. The surface of the substrate is locally melted by the high power diode laser beam and simultaneously powder particles are injected into molten material. The optimal process parameter window for the laser dispersing of SiC in Al 6061 has been found. The measured values of the wear rates of the sample with dispersed SiC particles are about seven times lower than that of the reference Al-substrates. Results show that laser dispersing is highly promising technology to improve the surface, mainly wear properties of light metals. However the possibilities of industrial application are still limited due to considerable laser beam power and preheating temperature applied as well low productivity because of low scanning speed, and therefore further investigations are required.

Organic semiconductor rubrene thin films deposited by pulsed laser evaporation of solidified solutions

Organic semiconductor rubrene (C42H28) belongs to most preferred spintronic materials because of the high charge carrier mobility up to 40 cm2(V·s)-1. However, the fabrication of a defect-free, polycrystalline rubrene for spintronic applications represents a difficult task. We report preparation and properties of rubrene thin films deposited by pulsed laser evaporation of solidified solutions. Samples of rubrene dissolved in aromatic solvents toluene, xylene, dichloromethane and 1,1-dichloroethane (0.23-1% wt) were cooled to temperatures in the range of 16.5-163 K and served as targets. The target ablation was provided by a pulsed 1064 nm or 266 nm laser. For films of thickness up to 100 nm deposited on Si, glass and ITO glass substrates, the Raman and AFM data show presence of the mixed crystalline and amorphous rubrene phases. Agglomerates of rubrene crystals are revealed by SEM observation too, and presence of oxide/peroxide (C42H28O2) in the films is concluded from matrix-assisted laser desorption/ionization time-of-flight spectroscopic analysis.

Spatial power distribution of a high-power CO2 laser beam: experimental study

Experimental studies of the spatial power distribution in the radiation beam from the industrial, transverse - flow cw CO2 laser working in the range up to 1.5 kW are reported. A beam diagnostic system utilizing a rotating pinhole scanner was developed and used to record and measure the spatial distribution of the power density in the laser processing region. The beam dimensions for locations at various distances to the focal plane were derived from the recorded profiles analysis agreed with those obtained by the knife-edge technique implemented in previous experiments.