Study On Microstructure And Properties Of2024 Aluminum Alloy By Stationary Shoulder Frictional Stir Additive Manufacturing

Additive manufacturing is a new manufacturing method developed in the late1980 s.Now,additive manufacturing technologies have made great progress in the fields of aerospace,machinery and electronics,and medical,and have rapidly developed into a research hotspot in the field of manufacturing.The stationary shoulder FSAM technology was used for additive manufacturing of2024 aluminum alloy,and the additive material(AM)was subjected to subsequent heat treatment.The microstructure of the AM was observed by OM and SEM.The content and morphological characteristics of the second phase in the AM was analyzed by XRD and TEM.Combined with the microhardness of the AM,studied the strengthening mechanism of the stationary shoulder FSAM on 2024 aluminum alloy.The results show that the stationary shoulder FSAM technology can produce well-formed,non-macro-defective 2024 aluminum alloy additive material when the stirring rod rotation speed is 600r/min and the walking speed is 30mm/min,and the horizontal additive rate is 89.3%,and the vertical additive rate is 80.5%,nearly achieving the near-net-shape of aluminum alloy.The microstructure of AM is a recrystallized fine equiaxed grain,the grain size is significantly reduced relative to the base material(BM).The precipitated phase content decreases relative to the BM,and its shape changes from a rod to an ellipsoid,reduced in size,coherent stain reduction.With the increase of heating at the bottom of the AM,The grain size of the AM gradually decreases from the bottom to the top,the content of the precipitated phase gradually increases,the size gradually decreases,and the coherent strain gradually increases.The microhardness of the AM increases gradually from the bottom to the top,the maximum hardness is 99 HV,still lower than the BM.This is because the gradual disappearance of the coherency strain between the second phase by the FSAM,resulting in over-aging of the additive structure.After a single aging treatment,the AM is still fine equiaxed crystals,the second phase content increases,the size increases,appearing the second phase agglomeration phenomenon,and a large number of metastable semi-coherent second phases are transformed into stable non-coherent second phase,reduced coherent strain.Aftersolution and aging treatment,the AM is a massive block,and a large number of coherent secondary phases are re-evaluated,the coherent strain increases,and the second phase agglomeration phenomenon did not appear.Changes in grains and second phase result in the decrease of the microhardness of the AM after single aging,and gradually increases from the bottom to the top along the additive direction.The maximum hardness value at the top of the AM is 85HV;the microhardness of the AM after solution and aging treatment increases,and is evenly distributed along the additive direction.The average hardness value is 136 HV,which basically reaches the hardness value of 2024 aluminum alloy in T4 state.The second phase strengthening plays a major role in the subsequent heat treatment of the additive material.