Study On The Process And Properties Of Laser Melting Deposited TC4 Titanium Alloy And 30CrNi2MoVA Steel

The powder-feeding laser melting deposition process has become one of the most concerned additive manufacturing technologies with its advantages of high flexibility and high efficiency without size limitation.However,most of the current studies on LMD forming TC4alloy use a laser power of not more than 1000 W.The systematic study on the LMD TC4 alloy at a higher power is essential to further improve the forming efficiency.In addition,as a special material,30Cr Ni2Mo VA high-strength steel has never had related research on its LMD process.The LMD forming performance of TC4 titanium alloy and 30Cr Ni2Mo VA steel is mainly studied in this dissertation,and a new laser energy concept and its empirical equation were proposed based on the molten pool temperature.The evolution of the microstructure and properties of the materials is also analyzed.Accordingly,this paper further discusses the feasible route to obtain Ti/Fe dissimilar metal joints of two materials by LMD process.The single factor variable method was used to study the influence of different process parameters on the temperature of TC4 molten pool firstly.The results show that the temperature will increase with the increase of the laser power,but will decrease with the increase of the scanning speed and the powder feeding rate.Among these parameters,the laser power has a dictating influence in the temperature.The concept of effective laser energy E_e is proposed based on this,which can better reflect the influence of different process parameters.The optimized process window of LMD TC4 is obtained through single-track orthogonal experiment,which is:laser power 1000-2000 W,scanning speed 8-12 mm/s,powder feeding rate 7-14 g/min,laser spot diameter 3 mm,overlap rate 50%.Afterward,the microstructure characteristics,mechanical properties and their evolution of LMDed TC4 alloy under high laser power were studied.The results show that due to the layer-by-layer nature of LMD,the microstructure of the samples is susceptible to solid phase transition due to the effects of cyclic heat.In addition,when the forming process is laser power 2000 W,scanning speed 12 mm/s,powder feed rate 10.5 g/min and effective laser energy 7.98×10~5.With the proper thickness of the layer and the thermal cycle effects,the non-equilibrium needle-like??phase inside can fully decompose into equilibrium lath?+?microstructure.Thus,the sample shows an excellent comprehensive mechanical performance,whose tensile strength can reach 1024 MPa,and the elongation after break is close to 14.9%.The applicability of the E_e concept in the LMD forming 30Cr Ni2Mo VA steel system was also verified by collecting real-time molten pool temperature data.The optimized process window of LMD 30Cr Ni2Mo VA steel is also obtained through the single-track orthogonal experiment,which is:laser power 1500-2500 W,scanning speed 10-15 mm/s,powder feeding rate 10.2-20.4 g/min,laser spot diameter 4 mm,overlap rate 50%.The structural characteristics and mechanical properties of the 30Cr Ni2Mo VA steel sample were studied.The results show that the increase of the effective laser energy will raise the transition temperature of supercooled austenite,which will facilitate the transition of the structure from lamellar bainite to equiaxed bainite.The microstructure is mainly composed of strip-shaped bainite at a low E_e(1.20×10~5?1.60×10~5),such samples have a tensile strength exceeds 1250 MPa and an elongation near12%benefit from the dislocation substructure inside.When E_e increases(1.70×10~5?2.32×10~5),the microstructure of the sample becomes mainly composed of equiaxed transitional bainite,the dislocation substructure inside could be eliminated to a certain extent,thus the tensile strength slightly reduce to 1200 MPa,but the elongation will increase to nearly 14%under the effect of fine grain strengthening.Due to the difference in physical and chemical properties of Ti and Fe,it is readily yield hard and brittle Fe-Ti intermetallic compounds,when the two elements are mixed,which facilitate delamination fracture.When pure Cu or V element is used as the intermediate transition layer,the thin transition layer cannot effectively prevent the diffusion of Ti and Fe.However,the thicker transition layer can easily generate brittle Cu-Ti intermetallic compounds and Fe-Cr-V(?phase).In addition,using a V+Cu composite transition layer to join the two materials,the Fe-Ti intermetallic compounds is successfully suppressed and the formation of harmful Cu-Ti phase and?phase is well controlled,thus obtaining a strong Ti/Fe dissimilar metal joint(?_b:166 MPa).