| HSLA-100 steel is widely used in modern construction of naval ships and submarines due to their high strength and good toughness.However,scrape,crack and corrosion are prone to generate in HSLA steel due to the special application environment and the steels are highly sensitive to those damage.Therefore,a restoration technique is urgently needed to extend its service life.Though the technique of traditional solution(Weld)is mature and at low cost,large heat affected zone(HAZ)size,induced distortion and high residual stresses impose restrictions on their wide applications.Laser additive manufacturing(LAM)as a new maintenance technology has the advantages of high energy density,controllable heat input and transformable components of powders,resulting in smaller heat affected zone,lower residual stresses,less deformation and higher geometrical accuracy.Those advantages make LAM a potential repair method.In this research,LAM was used to repair 20 mm HSLA-100 steel with pre-machined trapezoidal groove.The influences of processing parameters such as laser power,scanning speed,powder feeding rate and water depth on repair quality of specimen were investigated in air and underwater environment.Based on the previous experimental exploration,the optimized parameters were adopted such as laser power with 1500 W,scanning speed with 1000 mm/min and powder feeding rate with 7.8 g/min.For the underwater LAM repairing experiment,two different constructions of gas curtain nozzle(B and C)were designed to investigate the effects on the stability of local dry cavity and the repair quality.The microstructure and mechanical properties were analyzed.A serials of characterization tests including optical microscopy(OM),scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD),X-ray micro-computed tomography(micro-CT)and differential scanning calorimetry(DSC)were carried out to study the microstructure evolution.Besides,micro-hardness,nano-hardness and elastic modulus of the repaired specimens were studied.The tensile,bending and Charpy impact testing at-40? were done on the repaired specimens.Furthermore,the fracture mechanism was revealed by the SEM fracture analysis.The results showed that the phase of repaired zone(RZ)is martensite and tempered martensite.The morphology from bottom to top of the deposited layer is planar crystal layer band,columnar dendrite crystal and equiaxed grain.The micro-hardness and nanohardness of all repaired specimens are higher than that of substrate and the values increase with the increase of cooling rate.The tensile strength of specimens repaired in air or underwater with nozzle C are comparable to that of substrate(YS;about 770 MPa;UTS;about 830 MPa)and the elongation slightly decreases.However,the specimens repaired with nozzle B has a large decrease in tensile properties due to the defects in RZ.The impact absorbed energy of all repaired samples at the temperature of-40? are lower than that of the substrate and do not meet the requirement as per US military specification,MIL-E-22200/10A(-51?;61 J).In the present research,LAM can be used to repair trapezoidal groove of HSLA-100 steel successfully in air and underwater environment.The repairing process parameters,microstructure and mechanical properties are studied in detail.The repair process reported in this investigation can provide some theoretical and experimental guidance for repairing damaged parts in air and underwater environment. |
PDF Full Text Request