Defets In Titanium Alloy Processed By Seletive Laser Melting And Its Influence On High-cycle Fatigue Properties

Selective Laser Melting technology is one of many additive manufacturing methods.Its principle is to allow a high-energy laser beam to scan a pre-coated powder according to a pre-scheduled scanning path to melt it completly,which is made connected layer by layer after solided.It can form complex structural components and is widely used in many fields.In this paper,laser power and scanning speed were selected as experimental variables,The range of laser power was from 100 w to 250 w,and the range of scanning speed was from 300 mm/s to 1500 mm/s.30 groups of samples were processed.The research on the sample peocessed by different process parameter is that:(1)density and relative density.The minimum value of the density in the 30 samples is 4.2118 g/cm3,the corresponding relative density is 94.674%,the maximum value is 4.4198g/cm3,and the corresponding relative density is 99.321%.(2)The distribution of the defects in the horizontal and vertical sections of the sample made by metallographic method shows that:there are a large number of defects at the area of high power low scanning speed and low power high scanning speed in the process parameter table;the defect ratio is at least 0.01%,corresponding to the number and size of defect is 4 and 67 ?m,respectively;(3)The effects of laser power and scanning speed on the proportion and number of defects are explored by control variate method.the laser power is controlled to remain unchanged.When the laser power(100 w,130 w)is low,as the scanning speed increases,the proportion of defects and the number of defects in the samples'cross-section and vertical section will show an upward trend;When the laser power(160 w,190 w,220 w,250 w)is high,as the canning speed increases,the proportion of defects and the number of defects in the samples' cross-section and vertical section will show a decreasing trend;the scanning speed is controlled to remain unchanged.and when the scanning speed(300 mm/s,600 mm/s)is low,as the laser power increases,the proportion of defects and the number of defects in the samples' cross-section and vertical section will show an upward trend;at higher scanning speeds(900 mm/s,1200 mm/s,1500 mm/s),as the laser power increases,t the proportion of defects and the number of defects in the samples' cross-section and vertical section will show a decreasing trend.(4)This section also studies the effect of energy density on the law of defects in shaped samples with different process parameters.In the sample with energy density ranges 50 J/mm3 and 140 J/mm3,Both the values of cross-section defects and the number of defects in the sample are small.In this region,the size of the defect in the sample is large(less than 80 ?m).In the sample with energy density less than 50 J/mm3 and greater than 140 J/mm3,Both the values of cross-section defects and the number of defects in the sample are large.In these two regions,the size of the defect in the sample is large(100 ?m-200 ?m).The study of defects found that there are mainly two kinds of defects in the samples under different process parameters.One is an irregular sharp-angled defect that appears in the low power and high scanning speed area,and the other are nearly-circular regular holes that appears in the high-power scanning speed area.The irregular sharp-angled defects are caused by the spheroidization phenomenon because of the lack of laser energy,which increases the thickness of the powder coating and leads to unfused in the second laser scanning.nearly-circular regular holes are due to the steam bubble which caused by excessively high energy laser beams can not escape from melting pool before solidification.;the shape and size of the two types of defects will change with the respective process parameters.The size of the irregular sharp-angled defects is varing between 50 ?m and 500 ?m,and the size of the nearly-circular regular holes are varing between 50?m and 200 ?m.The high-cycle fatigue limit of optimum process parameters sample is about 390 MPa;the defects in fatigue sources and crack growth zone will reduce the fatigue life,and the defects in the transient fracture region have a small effect on the fatigue life.