Study On Laser Welding And Nanosecond Laser Ablation Of Al-Si Coating Hot Stamping Steel

In order to prevent oxidation and decarburization in the hot stamping process,the surface of the Usibor 1500 steel is coated with Al-Si coating.The presence of Al-Si coating affects the properties of the welding seam.Therefore,the Al-Si coating must be removed before laser welding.But how to remove the coating and the influence of the retained state of the coating on the microstructure and properties of the hot stamping steel is not clear.In this paper,the microstructures and properties of laser tailor-welded blanks(TWBs)welds with different Al-Si coating removal conditions are investigated.A series of phenomena occurred in the process of laser ablation of Al-Si coating were studied,and the mechanism of nanosecond laser ablation was discussed.Firstly,according to the upper and lower surface coating of welding groove of Usibor 1500 laser TWBs,nine kinds of different combination of Al-Si coating removal states of laser TWBs are designed.The metallographic features of welded joint after welding and hot stamping of Usibor1500 are investigated.The mechanical properties of welded joint after hot stamping are tested by room temperature tensile test.The results show that the heat affected zone of the welded blank is divided into coarse grained zone,fine grained zone and incomplete crystalline zone after laser welding.After hot stamping,the base metal and heat affected zone both transformed into plate martensite.The existence mode of the coating has no effect on the HAZ structure,but the weld zone structure changes with the existence of the coating.After laser welding,the weld zone is composed of coarse lath martensite and inhomogeneous second phase,which is white under the optical microscope.The white phase decreases with the increase of the removal coating edges.After the hot stamping,the original coarse lath martensite transformed into a more fine grain lath martensite,and the white phase increased.The fracture pattern of the specimen is related to the distribution of the white phase in the welding seam.In the case of the upper surface coating are present,the lower surface coating to remove or not has a slight influence on the property of the tailor-welded blank.The removal of upper surface coating can significantly improve the performance of weld joint.In the case of upper surface coating are removed,the weld joint performance was significantly improved with the number of the lower surface coating removal edges increase.Secondly,the traditional ways to remove the coating are mechanical friction,sand blasting and chemical reagent.However,these methods are either too low accuracy or pollution of the environment.Therefore,it is necessary to explore a fast,efficient and green way to remove the coating.The influence of nanosecond pulse laser parameters,including pulse energy,pulse frequency,pulse number,on the ablation performance and mechanism of Al-Si coating are studied in this paper.The experimental results show that with the pulse frequency increasing,the pulse number required to remove the coating is decreased.The single-shot ablation threshold of Al-Si coating is 4.4 J/cm2.Due to the accumulation effects,the ablation threshold of the coating decreases with the increase of pulse number and pulse frequency.For nanosecond ablation,evaporation and phase explosion are the two primary ablation mechanisms.These two mechanisms are mainly determined by laser frequency.In order to remove the Al-Si coating completely,the pulse frequency must be greater than 5 kHz.Due to the phenomenon of phase explosion,the coating removal accuracy will be reduced,therefore,the phase explosion should be avoided during laser ablation,which means the pulse frequency should below 100 kHz.The Hirschegg model is used to analyze the evolution of ablation depth.As the pulse number and frequency increase,the ablation depth increases and then keep constant.