Research On Brazing Process And Mechanism Of ZrO₂ Ceramic To Ti Alloys

ZrO₂ ceramic has many excellent features,such as high strength,high toughness and good high temperature resistance.The joining of ZrO₂ ceramic and Ti alloy is widely used in aerospace field.Brazing is one of the most common methods to achieve the effective joining of ceramics and metal alloys.However,there is a huge challenge to obtain a brazed joint with high quality due to the mismatch in their physical and chemical properties.Conventional ceramic/metal brazed joints have the disadvantages such as interfacial metallurgical incompatibility and high residual stress.In this study,reliable brazing of ZrO₂ ceramic and TC4 alloy or Ti Al alloy was achieved by using acive brazing method.The main controlling factors of the joint properties were revealed.Composite filler,interlayer and ceramic surface roughening were used to optimize the microstructure and improve the mechanial properties of the joints.The interfacial reaction characerization of Ag-Cu-Ti/ZrO₂ ceramic system were studied by wetting ZrO₂ ceramic with Ag-Cu-Ti filler metal.The reaction layers next to ZrO₂ ceramic was characterized as Ti O+Cu3Ti3O layers by using FIB/TEM analysis.The formation mechanism of reaction layers along the ZrO₂ ceramic was discussed from the perspective of thermodynamics.Effects of tempperature and Ti content on the interfacial reaction characerization along the ZrO₂ ceramic was studied.When the temperature reached 910°C,Cu3Ti3 O layer was separated from Ti O layer.As Ti content reached 6wt.%,Ti-Cu intermetallic compounds?IMCs?were formed adjacent to reaction layers.Then,Ag-Cu filler metal was used to braze ZrO₂ ceramic and Ti alloy.The interfacial microstructure of the joints was investigated.The Al element from Ti alloy was involved in the interfacial reaction along the ceramic to form Ti O+?Cu,Al?3Ti3O layers.Effects of brazing parameters on the microstructure and mechanical properties of the joints was studied.The relationship between interfacial microstructure and mechanical properties was established.The main controlling factors of the joint properties were the large residual stress in the joints and the domination of brittle Ti-Cu IMCs in the brazing seam.Based on the main controlling factors of the joint properties,nove l Ag-Cu+WB composite filler was developed to braze ZrO₂ ceramic and Ti alloy.During brazing,WB particles could react with the active Ti to in-situ form Ti B whiskers and W particles.Whisker and particle co-enhanced microstructure in the brazing seam was obtained.Effects of brazing parameters and WB content on the microstructure and joint properties was investigated.With the increase of brazing temperature,the reaction between WB particles and active Ti was facilitated.As the WB content increased,the reaction of WB particles and active Ti was restrained.Ideal microstructure reinforced by Ti B whiskers and W particles in ZrO₂/TC4 joint could be obtained at 870°C for 10 min with 7.5wt.% WB content.The maximum shear strength of the joints was 83 MPa,which was about 59.6% increase over the joints without WB addition.The highest shear strength of ZrO₂/Ti Al joints was 67 MPa when brazed at 880°C for 10 min with Ag-Cu+7.5WB composite filler,increased by 39.6% compared with the joints without WB addition.The strengthening mechanism was mainly due to the relieving of the residual stress in the joints and the dispersion of brittle intermetallics compounds in the brazing seam.The use of composite filler would cause the problems,such as agglomeration of the added particles and segregation of the reinforcing phases.To avoid the above problems,novel nano Cu O modified Cu foam interlayer?abbreviated as N-Cu foam intelayer?was fabricated by chemical oxidation method on Cu foam to braze Zr O 2 ceramic and Ti alloy.At oxidation time of 2h,uniform and fine Cu O nanostructure was formed on the Cu foam.A lot of fine particles were distributed uniformly in the brazing seam of the joints brazed with N-Cu foam interlayer.FIB/TEM analysis results indicated that these fine particles were?Cu,Al?3Ti3O particles.Effects of brazing parameters on the microstructure and mechanical properties of the joinits was studied.At the lower brazing temperature,particle phase tended to aggregate near to ZrO₂ ceramic.As the brazing temperature increased,particle phase diffused toward Ti alloy gradually and distributed uniformly in the brazing seam.The maximum shear strength of ZrO₂/TC4 joints was 96 MPa at 870°C for 10 min,which was about 84.6% increase.The highest shear strength of ZrO₂/Ti Al joints was 87 MPa when brazed at 900°C for 10 min,increased by 81.3%.The reaction layers adjacent to ZrO₂ ceramic were the weak zone of the joints.Femtosecond laser technology was employed for ZrO₂ ceramic surface roughening.Effects of processing parameters on the morphology and dimension of the grooves was studied.With the increase of pulse energy,the width of the grooves increased gradually,and the depth first increased and then stabilized.As the repetition frequency increased,the depth of the grooves increased gradually,but its width did not change much.When the processing speed increased,the depth of the grooves decreased and the width did not change much.Then,ZrO₂ ceremic after femtosecond laser roughening was brazed to Ti alloy.Effects of processing speed on the microstructure and mechanical properties of the joints was studied.The microstructue of the joints at different processing speeds was similar.The maximum shear strength of ZrO₂/TC4 and ZrO₂/Ti Al joints at processing speed of 200?m/s was 102 MPa and 98 MPa,respectively.Combining ceramic faying surface roughening and N-Cu foam interlayer,the mechanical properties of ZrO₂/TC4 and ZrO₂/Ti Al joints could be further improved to 126 MPa and 118 MPa,which displayed an increasement of ¹62.5% and ¹45.8%,respectively.