The Effects of Laser Surface Modification on the Microstructure of 1.4550 Stainless Steel

In this study a stainless austenitic steel 1.4550 was laser heat treated with diode laser. The influence a gouache coating on remelted steel substrate was carry out. The cooling system during laser melted was analysis as well. Melted layers were manufactured with different laser beam power between 0.6 kW and 1.4 kW, constant scanning laser beam speed vl = 5.76 m/min and laser beam diameter equal dl = 1.2 mm. The surface was treated at room temperature and under CO2 cooling conditions and the results were compered. With the increase of the laser beam power, the dimensions of the laser tracks increase. The depth of laser tracks varies significantly than their width. The deepest melted layer was observed for a material that wasn’t coated by any of absorbent paste and when there wasn't cooling system.

Nowadays there is a tendency to manufacture products of higher quality in shorter time and with lower costs. To achieve some of mentioned objectives, it is necessary to use hard and resistant materials which usually have low machinability. Therefore, the objective of this research was to determine adequate parameters for laser assisted turning of commonly used austenitic steel.

Experiments
a Corresponding author: damian.przestacki@put.poznan.pl Specimens of stainless austenitic steel 1.4550 were laser heat treated using diode laser TruDiode 3006 produced by Trumpf company. In the research, influence of impact of applied coating -gouache of thickness equal to approximately 20 µm and of CO 2 cooling system during interaction of laser beam with the material. All significant parameters of laser heat treatment are shown in table 1. During laser heat treatment constant laser beam scanning velocity and laser beam diameter were applied, equal to respectively v l = 5.76 m/min and d l = 1.2 mm. On the other hand, laser beam power was variable and equal to P = 600 W, P = 1000 W i P = 1400 W. Specimens of dimensions 26 mm x 10 mm x 6 mm after laser heat treatment were included in thermosetting resin. Then, specimens were grinded with abrasive papers of degressive gradation, polished using alumina oxide suspension and etched with glycerin reagent. To carry out microscopic observations light microscope OPTA-TECH of LAB 40 series was used both with OptaViewIS software. Moreover microhardness testing with Vickers method was carried out, using load of 100 g (HV0.1) on Zwick 3212B microhardness tester. Figure 1 presents influence of laser beam power without applying initial coating and without applying cooling system during laser heat treatment. It can be seen that with increasing laser beam power, dimensions of laser tracks also increase, in particular their depth. With applied parameters of laser heat treatment without initial coating and without applying cooling system during laser heat treatment, obtained laser tracks are free of pores and cracks.   Figure 3 presents influence of laser beam power without applying initial coating but with applying CO 2 cooling system. Usage of increasing laser beam power results in obtaining larger dimensions of laser tracks. Especially changes can be seen if depths of laser tracks are analyzed. With applied laser heat treatment parameters without applying initial gouache coating and with applying CO 2 cooling system during the process obtained laser tracks are free of pores and cracks. Only near the surface fine crystals oriented perpendicularly to the surface can be seen and they were not visible on laser tracks obtained when cooling system was not applied.  Figure 4 presents influence of laser beam power with applied both initial gouache coating and CO 2 cooling system during laser heat treatment. It can be seen that with increasing laser beam power the relation is maintained and the higher laser beam power, the larger dimensions of laser tracks.

Results of experiment
If low laser beam power is applied (P = 600 W) with initial gouache coating and CO 2 cooling system, in obtained laser track cracks can be seen which are probably connected with high stresses caused by the cooling rate. Crack were not observed on laser tracks if laser beam power was increased to 1000 W or 1400 W. The influence of applied technological parameters on dimensions of laser tracks (depth, width) for 1.4450 steel after laser heat treatment are shown in table 2.  On the basis of carried out research it can be seen that as a result of laser beam impact microstructure in re-melted zone is dendritic which is susceptible to machining. In re-melted zone direction of dendritic growth can be clearly seen and it has its beginning on boundary between re-melted zone and grains of the substrate. Obtained results of microhardness does not show significant differences between core and re-melted material, therefore, in further research, attempts connected with manufacturing multiple laser tracks will be taken and these will be machined in the next step.

Conclusions
On the basis of the carried out investigations, the following conclusion are formulated: with increasing the laser beam power dimensions of laser tracks also increase. Depth of laser tracks changes significantly in comparison with its width.
The highest depth of re-melted zone was observed for material which was not covered with initial coating and if cooling system was not applied Microhardness in re-melted zone and in the substrate was similar