| Aluminum bronze alloy has a series of excellent properties such as good thermal conductivity,electrical conductivity,corrosion resistance and machinability.It has been widely used in electronics,power,chemical,shipbuilding and other industrial sectors,but there are also shortcomings of low strength.Low alloy and high strength steel has excellent mechanical properties,but poor corrosion resistance.Aluminum bronze and low alloy high strength steel bimetallic composites combine the advantages of the two alloys,have excellent mechanical properties and corrosion resistance,and play an important role in pressure vessels and ocean engineering.However,due to the large difference in thermal and physical properties between steel and copper,the weldability is poor,the connection is difficult,and the osmotic cracks are easy to occur at the steel/copper connection,which adversely affects the mechanical properties of the structural parts.To solve the above problems,based on the(Cold Metal Transfer,CMT)arc additive manufacturing technology,the additive manufacturing experiments of low alloy and high strength steel and aluminum bronze bimetal composite structures are carried out in this thesis.Firstly,the effect of welding speed and wire feeding speed on the macroscopic forming and size of single pass single cladding layer of aluminum bronze and high strength steel was studied.Based on the optimized process parameters,the effects of reciprocating and one-way additive paths on single-channel multi-layer wall forming are studied.The results show that the reciprocating additive method can effectively avoid the unilateral collapse of the wall,and the wall forming is better.The subsequent additive tests on steel/copper composite walls were carried out using the reciprocating additive path,and the effects of two different stacking sequences on the macro-forming,microstructure and mechanical properties of the composite walls were studied.The results show that the microstructure of the aluminum bronze side is mainly composed ofα-Cu matrix,residual martensiticβ’phase and iron-rich precipitated phaseк_Ⅳ.The microstructure of high strength steel is mainly composed of granular bainite and acicular ferrite,and there are a large number of iron-rich phases at the interface.With the distance from the interface,due to the weakening of iron diffusion,iron content decreases,the number and size of iron-rich phases decrease.There are osmotic cracks at the steel/copper interface,among which the number and size of osmotic cracks at the interface of aluminum bronze/high strength steel composite wall are large,which is related to the remelting of aluminum bronze layer.Through mechanical properties characterization,it is found that the impact absorption energy of the two groups of samples is low,only reaching about 50%of the standard absorption energy of aluminum bronze cladding metal.The stacking sequence has little effect on the impact property,and the interface bonding effect needs to be improved.In addition,the corrosion resistance of aluminum bronze and high strength steel additive samples was studied,and the corrosion mechanism of the two alloys was analyzed.It is found that the formation of osmotic crack is mainly related to the wetting and osmotic effect of liquid copper on the grain boundary of steel.Liquid copper wets and penetrates the grain boundaries of steel and expands under stress,resulting in the formation of osmotic cracks.The suppression mode of osmotic crack is mainly to reduce the diffusion of Cu element by changing the composition of interfacial elements.Alternatively,by changing the composition of interface elements,the diffusion of Cu elements can be reduced.Based on the above research,this thesis adopts the method of adding transition layer to regulate the connection behavior of steel/copper interface,and designs four different transition layers:Cu-Ni,iron-Ni,Cu-Cr and nickel-Cr.The results show that the addition of transition layer reduces the diffusion and permeability of copper,and no osmotic crack is found at the interface joint.According to the mechanical properties,the tensile fracture of the four groups of samples after the addition of transition layer occurs on one side of the aluminum bronze matrix,indicating that the steel/copper interface bonding effect is good,and the impact toughness of the four groups of samples is improved.The impact absorption energy of the samples with Fe-nickel transition layer is the highest,which is about 41.0J,and the impact absorption energy of the samples without transition layer is increased by 52%.Taking Cu-Ni transition layer as an example,the corrosion resistance of aluminum-bronze layer near the interface of samples with transition layer and without transition layer is compared.The results show that the transition layer weakens the diffusion of iron elements to a certain extent,resulting in the reduction of iron-rich phase content in the aluminum-bronze layer near the interface of samples,and the corrosion resistance is improved.In conclusion,adding transition layer can effectively improve the bonding effect of steel/copper interface,and obtain the high strength steel/aluminum bronze bimetallic composite structure with excellent properties. |
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