| Wire + arc additive manufacturing（WAAM）is one of the most common additive manufacturing processes in metal parts manufacturing.WAAM uses an electric arc as a heat source to melt the welding wire,and adopts layer-by-layer deposition to achieve rapid forming of parts.However,due to the phase transformation stress of multiple thermal cycles and the thermal stress of large gradient temperature,the additive manufacturing structure will cause residual stress and forming defects,such as pores and hardened particles.Therefore,it is of great significance to explore the microstructure,the initiation and propagation of small fatigue cracks of additive manufacturing components to ensure the reliability and safety of additive components.In this paper,a combination of experimental and theory is used to study the following contents of 5356 aluminum alloy arc additive parts:First,the optimal process parameters of 5356 aluminum alloy arc additive parts were selected for forming and manufacturing.The metallographic observation showed that the 5356 aluminum alloy arc additive parts exhibited periodic structural changes,which were divided into interlayer two regions:interlayer and intra layer.A transmission electron microscope（TEM）test was carried out on the 5356 aluminum alloy arc additive manufacturing parts with the transmission electron microscope of JEOL JEM2100.It is found that the metal structure of the aluminum alloy additive manufacturing parts in the stacked state is mainly composed of white α（Al）phase matrix.And black β（Al3Mg2）phase composition.The dislocation density analysis was carried out for the cross section of the original specimen and fracture surface of the fatigue failure specimen.It is found that the dislocation density of the interlayer is less than the dislocation density of intra layer,and the dislocation density at the fatigue fracture is significantly higher than the dislocation density before fatigue fracture.Second,the MTS-858 electric servo fatigue testing machine was used to carry out continuous fatigue test and interrupted fatigue test of constant amplitude loading of 5356 aluminum alloy arc additive parts under stress ratio of-0.3,and the fatigue life and fatigue life under different stress levels were obtained.Surface crack growth morphology.It is found that the aluminum alloy arc additive fatigue specimens belong to multi-crack propagation.When the stress ratio is-0.3,the initial crack length is about 70μm,and about 80% of the cracks originate at the hardened particles.It is found on the micrograph of the specimen that the main crack originated at the edge or the secondary edge of the specimen.As the cyclic load increases,both ends of the crack expand at the same time,and the width of the crack increases unchanged.It is found from the macrograph of the test piece that the final cracks are generated at the edge of the test piece,causing damage to the entire test piece and causing fatigue fracture.In addition,ultrasonic nondestructive testing technology is used to characterize the fatigue damage of 5356 aluminum alloy arc additive manufacturing parts.The ultrasonic nonlinear parameters under different fatigue damage conditions at the same location and under the same fatigue damage condition at different locations are analyzed respectively.Under different fatigue damage states at the same position,the nonlinear coefficient increases with the increase of fatigue damage.In the same fatigue damage state at different locations,the nonlinear coefficient at the main crack area is the maximum compared with the nonlinear coefficients in other areas.Therefore,the nonlinear coefficient can be used as the identification and location of fatigue cracks.Finally,considering the influence of grain size and hardness in the interlayer and intra layer,the effective stress intensity factor is corrected based on the Elber crack closure effect.A fatigue life prediction formula suitable for aluminum alloy arc additive parts is established. |
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