93W-7Ni激光粉末床熔融增材制造研究

以W颗粒和Ni颗粒预先混合的93W-7Ni粉体为原料,采用激光粉末床熔融（Laser powder bed fusion, LPBF）制备了钨镍合金试样。研究了不同线能量密度的钨镍合金试样的相对密度、显微组织及显微硬度。结果表明：钨镍合金试样的缺陷主要为不规则孔隙,不存在明显可见的裂纹缺陷,且提高线能量密度能够明显减少缺陷。当线能量密度为1.5J/mm时,钨镍合金试样相对密度达98.04%。LPBF制备的钨镍合金试样的显微组织主要由W单质相和Ni-W基体相组成,并伴有大量细小的枝/颗粒状钨晶粒。试样的显微硬度高达529.83HV0.5,且基本不受线能量密度的影响。     

粉末床激光熔融304L不锈钢的热处理工艺

采用激光粉末床熔融(LPBF)技术制备了 304L不锈钢,并对其分别进行了 600、800、1000 和1300℃保温2 h 的热处理,研究了热处理温度对其显微组织的影响.结果表明:样品的晶粒尺寸随温度升高呈长大趋势.当热处理温度为1300℃时,样品的晶粒明显粗化.由于LPBF 的快速凝固使304L 不锈钢中生成了 6...     

基于机器学习的Ti-6Al-4V合金激光粉末床熔融工艺优化

基于高斯过程回归(GPR)模型,对激光粉末床熔融Ti-6Al-4V合金的致密度和表面粗糙度观测数据进行了机器学习,得到了高致密度合金样品的激光功率-扫描速度的工艺优化窗口,并探讨了激光功率-扫描速度对表面粗糙度的影响。结果表明：获得高致密(≥99.5%)合金的激光功率-扫描速度工艺窗口呈梨形,扫描速度比激光功率对致密度影响更大,且高功率条件下适宜的扫描速度范围较宽。降低激光功率和提高扫描速度会单调增加表面粗糙度,且在低激光功率和高扫描速度下该影响更显著。同一激光能量密度下打印的合金致密度取决于具体的扫描速度和激光功率,但表面粗糙度基本相同。优化工艺窗口下样品的表面粗糙度小于10μm。实验证明GPR预测的优化工艺窗口是可靠的,该方法可拓展应用到其他合金增材工艺优化设计中。     

Ni含量对激光粉末床熔融成形NiTi形状记忆合金显微组织和力学性能的影响

激光粉末床熔融(laser powder bed fusion,LPBF)成形NiTi合金由于Ni元素的蒸发导致成分偏离粉末设计成分,而NiTi合金的形状记忆效应、超弹性等性能受Ni含量影响极大。因此有必要对不同Ni含量NiTi合金的LPBF成形性、显微组织以及力学性能开展研究。采用真空电极感应熔炼气雾化技术制备了Ni_50.8Ti、Ni_51.0Ti以及Ni_51.5Ti(原子分数,%)3种预合金粉末,研究其在不同工艺参数下的冶金缺陷、显微组织及力学性能的演化规律。结果表明,高Ni含量NiTi合金在LPBF成形过程中容易产生垂直于建造方向的裂纹,成形性较低Ni含量NiTi合金差。Ni_51.5Ti合金室温下的临界应力可达476 MPa,但断裂伸长率仅为2%;Ni_50.8Ti合金临界应力仅为122 MPa,断裂伸长率可达8%。     

95W-5（Ni／Fe／Co）合金高温变形及影响因素

通过温度、应变速率对95W-5（Ni／Fe／Co）合金流变应力曲线的影响，分析了该合金的变形特点。结果表明：该合金在200℃～600℃温度范围内变形时，初始加工硬化十分显著。宏观上表现为合金的屈服强度较高。95W-5（Ni／Fe／Co）合金在700℃～1300℃的流变应力曲线有一明显的特点就是曲线的斜率变小，而且出现短暂稳定态的流动。这是由于粘结相发牛动态回复的结果。此外该合金的高温塑性亦随应变速率的增大而增加。扫描电镜分析结果从微观上验证了该合金高温塑性变化的原因。     

电子束粉末床熔融技术发展综述

电子束粉末床熔融技术是一种有着广泛应用前景的金属增材制造技术,低应力、高能量利用率等特点使其在高温合金、高熔点金属成形等方向有着巨大优势。近年来,随着更多研究人员及企业进入该领域,电子束粉末床熔融装备有了诸多进展,且在新材料及工艺开发方面也取得了重要突破。本文对电子束粉末床熔融技术原理、近年来进展进行综述并对其今后的发展方向进行展望。     

粉末粒度对FGH95合金热变形行为的影响

本文研究了3种粉末粒度组成的FGH95合金热变形行为,结果表明粒度组成对FGH95合金等温压缩变形行为有较大影响,粗粉FGH95合金变形能力明显低于全粉和细粉,而全粉和细粉则差别不大.     

激光粉末沉积温度场和应力场的有限元数值模拟

基于激光粉末沉积的特点,建立了激光粉末沉积过程的有限元数学模型和物理模型,使用ANSYS有限元软件对激光粉末沉积过程的温度场和应力场进行模拟,获得了沉积过程中温度和应力在时间和空间的分布和变化规律.采用X射线衍射方法对残余应力进行测试,结果表明数值模拟结果和实验结果相吻合.     

激光粉末床熔融成形Ti6Al4V/AlSi10Mg合金电子束焊接工艺研究

针对激光粉末床熔融成形的Ti6Al4V合金和AlSi10Mg合金进行电子束对接试验,对比分析不同侧偏束条件下焊接接头的组织和性能。结果表明：钛侧偏束（-0.6 mm）相较于铝侧偏束（+0.6 mm）所获接头的成形更稳定、美观。铝侧偏束时,接头表面出现了大量的气孔及裂纹,接头内部也存在大量的气孔,主要聚集于铝侧熔合线上;钛侧偏束时,接头焊接缺陷的数量明显减少。两种工艺条件下的界面层均形成了一定数量的金属间化合物,铝侧偏束时以TiAl为主,钛侧偏束时形成了TiAl3、Ti Al2、Ti3Al和TiAl等多种金属间化合物。铝侧偏束接头抗拉强度最高为81 MPa,钛侧偏束接头抗拉强度最高可达到128 MPa。两种接头均失效于反应界面层处,呈现出脆性断裂的特征。     

固溶温度对激光粉末床熔化GH3536合金组织演变及力学性能影响

利用激光粉末床熔化（laser powder bed melting, LPBF）制造GH3536镍基高温合金,通过研究不同激光功率和扫描速度对缺陷数量的影响,进行工艺参数优化. 为了缓解沉积态组织的各向异性,消除残余应力,对LPBF制造合金进行固溶处理,探究不同固溶温度对组织及力学性能影响规律. 借助扫描电子显微镜(SEM)和配套的电子背散射仪(EBSD)对试样的显微组织进行观察,并进行力学性能测试. 结果表明,随着固溶温度的升高,沉积态熔池轮廓消失,碳化物溶解,小角度晶界数量减少. 1 100 ℃固溶试样常温拉伸的屈服强度为450 MPa,随着固溶温度的升高,小角度晶界对位错运动的阻碍减弱,屈服强度降低,经过1 220 ℃固溶,试样屈服强度为315 MPa. 1 100 ℃固溶试样的高温抗拉强度为220 MPa,高温拉伸时碳化物沿晶界析出导致晶界脆化,随着固溶温度的增加,沿晶界分布的碳化物数量减少,抗拉强度逐渐增大.     

Graded structures by multi-material mixing in laser powder bed fusion

Multi-material mixing in laser powder bed fusion is demonstrated by fabricating functionally graded structures in which Inconel and stainless steel are metallurgically bonded. Material mixing and elemental diffusion in the transition zone are studied for single scan tracks, hatch scans and multi-layer builds. An adapted recoater enabling powder deposition from multiple sources was utilized to fabricate a more gradual transition zone by alternating material deposition within the build. Homogenization heat treatment enhances elemental diffusion compared to the as-built configuration resulting in a gradual material transition without a distinct interface. Finally, a multi-material heat exchanger is presented as exemplary design implementation.     

Interlaced layer thicknesses within single laser powder bed fusion geometries

The geometrical design freedoms associated with additive manufacturing techniques are currently well exploited and finding commercial application. The capability of layer-based processes to allow modification of composition and microstructure in process to achieve functional grading is currently a growing topic. In this work, a method is demonstrated for varying layer thickness within single components that allows part sections to be interlaced for the purpose of locally manipulating material and structural properties. Demonstrator geometries are explored here which exhibit the interfaces within specimens constituted of both 30 µm and 150 µm. Accordingly, a new design freedom for laser powder bed fusion is created.     

Towards production of landing gear materials with laser powder bed fusion

Titanium 10 V-2Fe-3Al (Ti 10-2-3) coupons were produced via laser powder bed fusion and characterized. Results indicate that coupons produced by the additive manufacturing process have substantially lower high-cycle fatigue strength compared to wrought material (40 vs. 90 ksi, or 276 vs. 621 MPa at runout, respectively). Low-cycle fatigue strength of the printed coupons was also lower than wrought coupons (1000 cycles to failure at 0.75% strain for printed coupons vs 1% strain for wrought), but there was significant overlap in the data between printed and wrought. Failure analysis and optical microscopy revealed that the printed coupons showed microstructural differences when compared to wrought, such as larger grains and smaller dimples in the fracture surfaces. Computed tomography revealed defects in printed coupons. After a 500 h salt-spray, no corrosion was visible in printed Ti 10-2-3 material. At this time, Ti 10-2-3 landing gear components which are not safety critical and do not experience cyclic loading over 10,000 cycles may be considered for production via laser powder bed fusion.     

The role of scan strategies in fatigue performance for laser powder bed fusion

The integrity of additively manufactured components is limited by the number, size, type and location of defects encapsulated in the build. Our ability to manufacture fatigue resistant components by the powder bed fusion process is still nascent as a result. The location of defects within a build volume is known to be of significance but efforts are yet to achieve superior manufacturing strategies resulting in tolerable fatigue performance. In this work the role of laser scan strategies is investigated in determining fatigue performance of printed components. Fractography and X-ray computed tomography data are presented to support this.     

Oxide dispersion strengthened 304 L stainless steel produced by ink jetting and laser powder bed fusion

This paper discusses the fundamentals of a novel hybrid method to synthesize oxide dispersion strengthened (ODS) 304 L stainless steel (SS) alloy using a modified laser powder bed fusion (LPBF) machine. Previously, ODS metal matrix composites have been produced by LPBF via ball-milling, which is expensive to scale. Here, we selectively dope yttria nanoparticles into a SS matrix by jetting a precursor chemistry onto the SS substrate prior to laser conversion and consolidation. The new alloy shows good room temperature mechanical properties. Microstructures are studied using electron microscopy, energy dispersive spectroscopy and electron backscatter diffraction.     

STL-free design and manufacturing paradigm for high-precision powder bed fusion

High-precision powder bed fusion (PBF), together with highly complex geometries necessitate a much more scalable representation of the geometry and an efficient computational pipeline. This paper presents a new digital design and manufacturing paradigm to solve the scalability and efficiency challenges by using the concept of STL-free workflow. It seamlessly integrates implicit solid modelling for design and direct slicing for manufacturing without any intermediate steps related to STL meshes. The presented paradigm has been validated by two case studies involving complex geometries filled with multiscale triply periodic minimal surfaces (TPMS), which are fabricated by PBF with laser beam size 25 µm.     

On the use of metastable interface equilibrium assumptions on prediction of solidification micro-segregation in laser powder bed fusion

ABSTRACT

Many models have been developed to explore solidification segregation and dendrite structure in additively manufactured parts. However, these models tend to be computationally expensive and consider only a limited number of alloying elements, compromising their practical application. In this work, a methodology to extend the Scheil model, based on interface metastable equilibrium assumptions, is established to predict the spatial compositional maps due to micro-segregation for a laser-powder bed fusion (L-PBF) build of alloy 718. The compositional maps are contrasted against experimental data measured in a unit dendrite cell by transmission electron microcopy. The validity of Scheil's implicit assumptions under the rapid solidification conditions in L-PBF is further discussed. The extended Scheil model is shown to be computationally efficient and readily applicable to multi-component systems.     

Laser ultrasonic testing for near-surface defects inspection of 316L stainless steel fabricated by laser powder bed fusion

The laser powder bed fusion (L-PBF) method of additive manufacturing (AM) is increasingly used in various industrial manufacturing fields due to its high material utilization and design freedom of parts. However, the parts produced by L-PBF usually contain such defects as crack and porosity because of the technological characteristics of L-PBF, which affect the quality of the product. Laser ultrasonic testing (LUT) is a potential technology for on-line testing of the L-PBF process. It is a non-contact and non-destructive approach based on signals from abundant waveforms with a wide frequency-band. In this study, a method of LUT for on-line inspection of L-PBF process was proposed, and a system of LUT was established approaching the actual environment of on-line detection to evaluate the method applicability for defects detection of L-PBF parts. The detection results of near-surface defects in L-PBF 316L stainless steel parts show that the crack-type defects with a sub-millimeter level within 0.5 mm depth can be identified, and accordingly, the positions and dimensions information can be acquired. The results were verified by X-ray computed tomography, which indicates that the present method exhibits great potential for on-line inspection of AM processes.     

Laser calorimetry for assessment of melting behaviour in multi-walled carbon nanotube decorated aluminium by laser powder bed fusion

Material development for powder bed fusion is critical to enhance the utility of the process. Establishing process parameters for new materials limits the rate at which process performance can be appraised. Adapted laser calorimetry is a useful technique for rapid material screening. Here, multi-body feedstocks which have the potential to improve laser coupling and mechanical properties of structures (Al + multi-walled carbon nanotubes) are investigated to demonstrate the flexibility of laser calorimetry for understanding consolidation and heat transfer phenomena in powder bed fusion. It is shown that the modifications to powder surface condition/chemistry improve consolidation behaviour in this case.