Forming Processing And Properties Of Copper-tin Alloy In Selective Laser Melting

Copper-tin alloy,as the oldest traditional alloy,possess good mechanical properties,excellent wear and corrosion resistance,as well as outstanding electrical and thermal conductivity,and is widely used in electronics,machinery,aerospace,defence,navigation and other industrial fields.With the development of industrial technology,performances of traditional copper-tin alloy are difficult to meet the requirements of emerging industries and major engineering in our country.Selective laser melting(SLM),as an additive manufacturing technology,has attracted increasing attention and provided the possibility of developing high-end copper-tin alloy products due to a wide range of advantages including fast market response,opening design freedom,high material utilization and forming efficiency,the forming potential of highly-complicated shapes and obtaining excellent comprehensive performance,etc.Thus,in this paper,a study on forming process and properties of copper tin alloy in SLM was carried out.The Latin square and response surface method were employed to optimize and discuss the SLM processing parameters of densification and surface roughness for copper-tin alloy.Meanwhile,the phase,microstructure,mechanical and tribological properties as well as heating processing of copper-tin alloy were further analysed and discussed in detail.Some important research results can be drawn as follows:(1)The effect of tin content on the SLM forming properties was researched by comparing Cu-4Sn with Cu-15 Sn.It is found that tin content has a great influence on the densification of copper-tin alloy in SLM,which is attributed to both the increase of absorptivity at laser and decrease of heat conductivity with the increase of tin content.The optimized densification of 93.68% for Cu-4Sn is obtained using the parameter set of laser power 190 W,scanning speed 150 mm/s and hatch space 100 ?m.The optimized densification of 99.8% for Cu-15 Sn is obtained using the parameter set of laser power 187 W,scanning speed 185 mm/s and hatch space 170 ?m.Meanwhile,it can be detected that the laser power is a highly significant factor for densification of copper-tin alloy.(2)For the poor and inconsistent surface quality of Cu-15 Sn in SLM,the influences of processing parameters on surface roughness of top,upper and lower surfaces were systematically investigated.For the top surface,an optimized route is put forward to acquire good surface quality.The laser power is firstly adjusted to get a large processing window of scanning speed and hatch space.Then,the proper hatch space is chosen to guarantee full melting of powder.Finally,scanning speed is optimized according to fabrication efficiency and performance requirement.For the upper and lower surfaces,the staircase effect caused by forming angle has the most significant effect on the surface quality.Furthermore,the influence of staircase effect on the upper surface quality can be weakened by increasing the laser power.The lower surface quality can be improved by increasing scanning speed to reduce adhesive powder and balling phenomenon.(3)Based on the X-ray and electron diffraction techniques,phase components and its structural features of SLM Cu-15 Sn were characterized and analyzed.Meanwhile,the microstructure and its evolution were analyzed and discussed.It is found that SLM can significantly refine the microstructure and reduce macro-segregation.The characteristic microstructures that columnar crystal of epitaxial growth in the building direction and dendrite and cellular crystal in the top surface are formed.Thanks to the inhibition effect of rapid solidification on the peritectic transformation,a shell structure with ? hard brittle phases wrapped around ? phases can be obtained.A large number of crystal defects including dislocation pile-up,twin crystals can be detected in the Cu-15 Sn,proving the production of greater thermal stress in SLM.(4)The macro-and micro-texture,tensile properties,hardness and friction properties of SLM Cu-15 Sn were investigated and analysed.Compared with the traditional drawing QSn15-1-1,the mechanical properties of SLM Cu-15 Sn with tensile strength of 661 MPa and yield strength of 436 MPa have enhanced beyond about 50%,but the elongation at break is basically the same.Furthermore,the Vickers hardness of 212 HV of SLM Cu-15 Sn is also improved compared to that of QSn15-1-1(150 HV).Besides,the SLM Cu-15 Sn possesses the good friction performance and its friction coefficient presents the trend of decreasing firstly and then increasing with the increase of sliding speed.(5)The effects of different heat treatment on phase,microstructure,mechanical and friction properties of SLM Cu-15 Sn were researched.It is found that annealing heat treatment can homogenize the microstructure but coarsen the grains of SLM Cu-15 Sn.The strengthening mechanism of Cu-15 Sn bronze has transformed from the refine grain strengthening to the solid solution strengthening after annealing heat treatment.The SLM Cu-15 Sn after heat treatment possess tensile strength of 545 MPa and yield strength of 328 MPa,which are still superior to those of QSn15-1-1 specimens with tensile strength of 444 MPa and yield strength of 256 MPa.Furthermore,it is noted that plastic property has been greatly improved and the elongation increases from about ~7% to >20%.However,the Vickers hardness decreases to 160-173 HV,resulting in the poor friction property.As a whole,the comprehensive performance of SLM Cu-15 Sn is enhanced after annealing heat treatment.