Abnormal interfacial bonding mechanisms of multi-material additive-manufactured tungsten-stainless steel sandwich structure

Tungsten(W)and stainless steel(SS)are well known for the high melting point and good corrosion resistance respectively.Bimetallic W-SS structures would offer potential applications in extreme environments.In this study,a SS→W→SS sandwich structure is fabricated via a special laser powder bed fusion(LPBF)method based on an ultrasonic-assisted powder deposition mechanism.Material characterization of the SS→W interface and W→SS interface was conducted,including microstructure,element distribution,phase distribution,and nano-hardness.A coupled modelling method,combining computational fluid dynamics modelling with discrete element method,simulated the melt pool dynamics and solidification at the material interfaces.The study shows that the interface bonding of SS→W(SS printed on W)is the combined effect of solid-state diffusion with different elemental diffusion rates and grain boundary diffusion.The keyhole mode of the melt pool at the W→SS(W printed on SS)interface makes the pre-printed SS layers repeatedly remelted,causing the liquid W to flow into the sub-surface of the pre-printed SS through the keyhole cavities realizing the bonding of the W→SS interface.The above interfacial bonding behaviours are significantly different from the previously reported bonding mechanism based on the melt pool convection during multiple material LPBF.The abnormal material interfacial bonding behaviours are reported for the first time.

Microstructure and thermal properties of dissimilar M300–CuCr1Zr alloys by multi-material laser-based powder bed fusion

Multi-material laser-based powder bed fusion (PBF-LB) allows manufacturing of parts with 3-dimensional gradient and additional functionality in a single step. This research focuses on the combination of thermally-conductive CuCr1Zr with hard M300 tool steel.Two interface configurations of M300 on CuCr1Zr and CuCr1Zr on M300 were investigated. Ultra-fine grains form at the interface due to the low mutual solubility of Cu and steel. The material mixing zone size is dependent on the configurations and tunable in the range of0.1–0.3 mm by introducing a separate set of parameters for the interface layers. Microcracks and pores mainly occur in the transition zone.Regardless of these defects, the thermal diffusivity of bimetallic parts with 50vol% of CuCr1Zr significantly increases by 70%–150%compared to pure M300. The thermal diffusivity of CuCr1Zr and the hardness of M300 steel can be enhanced simultaneously by applying the aging heat treatment.

Multi-material additive manufacturing-functionally graded materials by means of laser remelting during laser powder bed fusion

Many processes may be used for manufacturing functionally graded materials.Among them,additive manufacturing seems to be predestined due to near-net shape manufacturing of complex geometries combined with the possibility of applying different materials in one component.By adjusting the powder composition of the starting material layer by layer,a macroscopic and step-like gradient can be achieved.To further improve the step-like gradient,an enhancement of the in-situ mixing degree,which is limited according to the state of the art,is necessary.In this paper,a novel technique for an enhancement of the in-situ material mixing degree in the melt pool by applying laser remelting(LR)is described.The effect of layer-wise LR on the formation of the interface was investigated using pure copper and low-alloy steel in a laser powder bed fusion process.Subsequent cross-sectional selective electron microscopic analyses were carried out.By applying LR,the mixing degree was enhanced,and the reaction zone thickness between the materials was increased.Moreover,an additional copper and iron-based phase was formed in the interface,resulting in a smoother gradient of the chemical composition than the case without LR.The Marangoni convection flow and thermal diffusion are the driving forces for the observed effect.

Additive manufacturing of copper-stainless steel hybrid components using laser-aided directed energy deposition

Combining dissimilar materials in a single component is an effective solution to integrate diverse material properties into a single part.Copper-stainless steel hybrid components are attracting more and more attention since the high thermal conductivity of copper can greatly enhance the thermal performance of stainless steel,which benefits its applications in many industries.However,direct joining of copper and stainless steel such as SS316 L is challenging since they preserve significant dissimilarities in physical,chemical,and thermo-mechanical properties.This paper aims to fabricate well-bonded copper-SS316 L hybrid parts using a laser-aided directed energy deposition(DED) process.A nickel-based alloy Deloro22(D22) is introduced between copper and SS316 L to address the detrimental issues in copper-SS316 L direct joints.Using this technique,defect-free interfaces are achieved at both the D22-SS316 L and copper-D22 transition zones.Tensile testing of Cu-D22-SS316 L and D22-SS316 L hybrid parts shows the fracture occurs at pure copper and SS316 L region,respectively,indicating an excellent bonding at the interfaces.Ascending in the building direction,a transition of grain structure is observed.A significant diffusion zone is obtained at both the D22-SS316 L and the Cu-D22 interfaces.The large diffusion distance results in a smooth variation in microhardness over the dissimilar materials.The microhardness increases from SS316 L to D22 with the highest value of 240 HV and then decreases from D22 to Cu with the lowest value of 63 ± 4 HV.Testing of thermophysical properties of the Cu-D22-SS316 L system indicates there is a ~300 % increase in thermal diffusivity and a ~200 % increase in thermal conductivity when compared to pure SS316 L.The significant increase in thermal diffusivity and conductivity validates the enhanced thermal performance of SS316 L when it is joined with pure copper.

Simultaneously improving fabrication accuracy and interfacial bonding strength of multi-material projection stereolithography by multi-step exposure

Multi-material additive manufacturing(MMAM)takes full advantage of the ability to arbitrarily place materials of addi-tive manufacturing technology,enabling immense design free-dom and functional print capabilities.Among MMAM technologies,projection stereolithography(PSL)exhibits a great balance of high resolution and fast printing speed.However,fabrication accuracy of multi-material PSL is hin-dered by large overcure used to strengthen interfacial bond-ing weakened by chemical affinity and material-exchange process.We present a novel multi-step exposure method for multi-material PSL process to overcome this shortcoming.Firstly,the whole layer is moderately exposed producing over-cure of single-material PSL level to generate geometries.Then weakened interfaces are strengthened individually with addi-tional steps of exposure.The multi-step exposure is integrated into the already efficient materials printing order of multi-material PSL process.Curing depth and overcure of photocur-able resins are modeled and characterized.Exposure required to achieve sufficient interfacial bonding of single-material interfaces built through material-exchange process and multi-material interfaces with altering materials printing order is determined with tensile tests.Microfluidic channels are used to compare fabrication accuracy of traditional single-step exposure and our multi-step exposure method.This method can be widely applied in multi-material PSL to improve fabri-cation accuracy in a variety of applications including micro-fluidic devices.

Direct 4D printing of functionally graded hydrogel networks for biodegradable,untethered,and multimorphic soft robots

Recent advances in functionally graded additive manufacturing(FGAM)technology have enabled the seamless hybridization of multiple functionalities in a single structure.Soft robotics can become one of the largest beneficiaries of these advances,through the design of a facile four-dimensional(4D)FGAM process that can grant an intelligent stimuli-responsive mechanical functionality to the printed objects.Herein,we present a simple binder jetting approach for the 4D printing of functionally graded porous multi-materials(FGMM)by introducing rationally designed graded multiphase feeder beds.Compositionally graded cross-linking agents gradually form stable porous network structures within aqueous polymer particles,enabling programmable hygroscopic deformation without complex mechanical designs.Furthermore,a systematic bed design incorporating additional functional agents enables a multi-stimuli-responsive and untethered soft robot with stark stimulus selectivity.The biodegradability of the proposed 4D-printed soft robot further ensures the sustainability of our approach,with immediate degradation rates of 96.6%within 72 h.The proposed 4D printing concept for FGMMs can create new opportunities for intelligent and sustainable additive manufacturing in soft robotics.

激光多材料增材制造技术研究进展和展望

以航空航天为代表的极端使役零部件工作环境严酷,结构复杂,功能集成。多材料增材制造(Multi-material additive manufacturing,MMAM)能够满足特殊区域的宏观性能定制化和结构—功能一体化,在核能、军事、航空和医疗等领域具有极大的应用潜力。重点阐述了MMAM技术影响界面结合强度的主要因素及消除缺陷的方法;介绍了目前MMAM技术不同的送粉方法和每种送粉方法的优缺点;讨论了工艺参数对MMAM技术的影响规律;最后总结MMAM技术目前的瓶颈性难题,展望了未来的主要研究方向。