Wetting Spreading And Brazing Interface Microstructure Of Different Filler Metal On Selective Laser Melting TC4 Substrates

Titanium alloy has excellent low density,high structural strength,excellent corrosion resistance,high fracture toughness and good biocompatibility,and it is extensively used in aerospace,automobile manufacturing,biomedicine and other fields.It has become one of the most common metal materials in 3D printing technology.TC4 is widely studied in SLM field because of its excellent print quality and important practical application value.However,due to the constraints of SLM equipment size and process,SLM technology can only meet the manufacturing of small and medium-sized complex components,and connecting small SLM parts into larger parts needs further research.Among all the joining methods,brazing is one of the most commonly used methods for joining dissimilar materials.In the brazing process,the wettability of molten filler metal on the base metal is the decisive factor for the quality of welded joint.Therefore,it is necessary to quantitatively model and predict the wetting phenomenon of molten metal on solid base metal.In this paper,the TC4 substrate formed by SLM was selected.According to the unique phase transition temperature of TC4,two kinds of filler metal(AgCuTi and BNi2)were selected(the former melting point is lower than the phase transition temperature,the latter is higher than the phase transition temperature).The wetting spreading process was observed and recorded in real time under the protection of high purity argon by high temperature hot stage microscope.The influence of temperature on the wettability of different filler metal on the substrate was studied and analyzed The wetting kinetics and wetting mechanism are discussed.The results show that temperature has a great influence on the wettability of AgCuTi/SLM TC4 system,but has limit influence on BNi2/SLM TC4 system.The results show that the final wetting area of the two wetting systems increases with the increase of temperature,and the precursor film appears in AgCuTi/SLM TC4 system,and the spreading speed of the precursor film is very fast with the increase of temperature.At 850°C and 900°C,the wetting process of AgCuTi filler metal on TC4 substrate is similar,which can be divided into four stages:1)initial spreading stage;2)rapid spreading stage;3)precursor spreading stage;4)progressive equilibrium stage.However,at 950°C,the increase of temperature promotes the wetting of AgCuTi filler metal on TC4 substrate,which can be divided into four stages:1)initial spreading stage;2)rapid spreading stage;3)slow spreading stage and precursor spreading stage(two stages simultaneously);4)progressive equilibrium stage.The wetting spreading process of BNi2/SLM TC4 wetting system is similar at three temperatures,which can be divided into three stages:1)initial spreading stage;2)main spreading stage;3)asymptotic equilibrium stage.The wetting kinetics of AgCuTi/SLM TC4 system can be analyzed by using the R~n?t model.The n values of the rapid spreading stage at three temperatures are all 1,which belongs to the reaction limited wetting.Substrates with different surface morphologies were prepared in AgCuTi/SLM TC4 system,and the effect of substrate surface morphology on the wettability of the system was studied.The results show that the roughness of substrate surface morphology can promote the wetting of filler metal,and the small grooves can guide and promote the spreading of filler metal.The final spreading shape of filler metal is affected by the substrate morphology:on the substrate with directional grinding,the spreading shape of filler metal is oriented along the grinding direction;on the substrate with matrix morphology,the spreading shape of filler metal is rhombic in two vertical grinding directions;on the polished substrate,the resistance of filler metal spreading is small,and the spreading area is large,and the final spreading shape is round.The corresponding welded joints at different temperatures were prepared by vacuum brazing furnace,and then the strength test was carried out to explore the influence of temperature on the strength of the joint.The results show that the mechanical properties of brazed joint are improved with the increase of temperature,but the mechanical properties of brazed joint are decreased with the increase of temperature.At 900°C for 5 min,the maximum strength of AgCuTi/SLM TC4welded joint is 172 MPa;at 1080°C for 5 min,the maximum strength of BNi2/SLM TC4 welded joint is 155 MPa.In order to compare the wettability of TC4 substrate formed by SLM and rolling TC4,two kinds of filler metal were used to carry out wetting spreading experiments on cast TC4 under the same conditions.The results show that although the phases of the substrates of the two systems are different before wetting(TC4 formed by SLM is mainly composed of??,but rolling TC4 is mainly composed of?+?),the element composition of the two kinds of substrate(both Ti-6Al-4V)and the internal structure after wetting are consistent,which leads to the wetting spreading phenomenon on SLM formed TC4 similar to that on conventional rolling TC4.