Mitigation of thermal distortion in wire arc additively manufactured Ti6Al4V part using active interpass cooling

ABSTRACT

In this study, active interpass cooling using compressed CO₂ was innovatively employed in the wire arc additively manufactured Ti6Al4V process with the aim of mitigating part distortion. A comparative analysis between simulation and experimental results was performed to explore the effects of active interpass cooling on the thermal behaviours, geometric features and distortion levels of deposit. The results show that active interpass cooling with CO₂ gas is an effective means of reducing Wire arc additive manufacturing (WAAM)-part distortion by increasing heat dissipation and reducing heat accumulation within the deposition. It can contribute to a maximum reduction of 81% in longitudinal distortion and 69% in transverse distortion for the wall structures produced in this study. Compared to the cooling gas flow rate, cooling time alternation is more effective in mitigating WAAM-part distortion due to more effective heat dissipation per unit time. The findings reveal that using active interpass cooling in WAAM can offer significant cost and build-time savings, as well as providing conditions for the improvement of WAAM-part quality.     

The effect of hot isostatic pressing on thermal conductivity of additively manufactured pure tungsten

The crack-healing behaviors and microstructure evolution of pure tungsten produced by laser powder bed fusion (LPBF) were studied and compared before and after post hot isostatic pressing (post-HIP) treatment. An average thermal conductivity of 133 W·m⁻¹·K⁻¹ at room temperature (RT) was obtained after HIP, which was 16% higher than that of as-built sample (115 W·m⁻¹·K⁻¹). Although the HIP process had little effect on density, it resulted in a large grain size of >300 μm accompanied by a decrease in dislocation density and crack healing, which led to a substantial improvement of thermal conductivity of pure tungsten. The positive correlation between relative density and thermal conductivity of as-built tungsten was reported.     

2219铝合金电弧增材制造组织及力学性能的非均匀性

2219铝合金电弧增材制造技术在航天领域具有广泛的应用前景。采用基于CMT工艺的电弧增材制造技术成形了2219铝合金单壁墙试样,并研究了沉积态、固溶态、固溶+时效态的显微组织与力学性能。结果表明显微气孔、θ(Al₂Cu)及富Fe脆性相在层间密集分布形成薄弱带,是导致水平方向和垂直方向力学性能各向异性的原因。垂直方向拉伸时,微裂纹易于在层间形成,塑性较差、抗拉强度较低。为提高WAAM成形的2219铝合金垂直方向力学性能,需尽可能降低丝材中的Fe、Si杂质含量。     

增材制造高强韧高熵合金中基于切变型相变的微裂纹抑制机理

研究选区激光熔化增材制造FeMnCoCrNi体系高熵合金的微裂纹行为,并采用XRD技术对激光打印后样品表面的残余应力进行分析。结果表明,经激光打印后等原子比FeMnCoCrNi高熵合金显示为稳定的单相面心立方(FCC)结构,出现残余拉应力,并产生微裂纹。相比之下,具有低层错能的非等原子比亚稳FeMnCoCr高熵合金在各种激光能量密度下均出现残余压应力,且无微裂纹形成。在激光熔化后的冷却过程中,亚稳高熵合金中发生的从FCC基体相到密排六方(HCP)相的切变型相变消耗了激光打印过程中的热应力,从而抑制微裂纹的产生。此外,相比于单相稳定高熵合金,亚稳高熵合金在拉伸变形过程中马氏体相变也有助于提高其抗拉强度和延展性。这些结果为增材制造领域设计开发高强、高韧、无裂纹的合金提供有益参考。     

增材制造钛合金凝固晶粒调控研究进展

钛合金由于比强度高、耐腐蚀好、高温性能好等优异的性能而广泛应用于航空航天、船舶、核电等领域。增材制造技术为大型整体关键钛合金构件的短周期、低成本制造提供了变革性途径,但增材制造产生的粗大柱状晶组织导致了构件的各向异性,限制了合金性能的充分发挥。进行晶粒调控以消除各向异性、提高力学性能成为近些年的研究热点。本文简述了增材制造钛合金构件典型晶粒形貌及形成机制,阐述了国内外在增材制造钛合金晶粒调控方面的研究进展,包括工艺参数优化、微合金化改性与新合金成分设计、外加能量场、后续热处理、新型增材制造工艺和多种方法复合等,总结了各类方法的调控机理和调控效果,对增材制造钛合金凝固晶粒调控的未来发展提出了思考与展望。     

铝合金电弧增材制造焊道宽度尺寸预测

文中首先采用同一组焊接参数,在四种厚度基板上进行单道20层铝合金薄壁试样成型,发现不同厚度基板下试样稳定区的焊道宽度相同. 基于此结论,利用二次通用旋转组合方法设计试验样本,通过二次回归方程建立工艺参数(焊接电流、焊接速度、送丝速度、层间温度)与成型试样稳定区域焊道宽度尺寸预测模型,经验证发现模型预测效果较好. 结果表明,影响焊道宽度的主要因素有焊接电流、焊接速度和层间温度. 当电流小于95 A时,参数对焊道宽度的影响顺序为:电流大于焊接速度大于层间温度;当电流大于95 A时,顺序变为:电流大于层间温度大于焊接速度. 同时,焊接电流和层间温度间存在交互作用.     

Novel additively manufactured bio-inspired 3D structures for impact energy damping

Additive manufacturing enables the design of complicated structures that would not be feasible to manufacture otherwise. Novel bio-inspired 3D structures were digitally produced employing Voronoi tessellation techniques and manufactured by selective laser sintering from polyamide. Impact tests were performed to analyze the impact absorption capacity of these structures, while FEM modeling offered insights on the fracture mechanisms and the prediction of the material response, which proved to be highly sensitive to strain rate. The results show a considerable impact attenuation in the case of 80% porosity and strut radius of 0.5 mm, encountering small transmitted peak force and smooth deceleration.     

Crack suppression in additively manufactured tungsten by introducing secondary-phase nanoparticles into the matrix

In this study, an effective strategy was developed to suppress cracking by introducing secondary-phase ZrC nanoparticles into a tungsten (W) matrix. Pure W and W-0.5wt%ZrC bulks were additively manufactured via the laser powder bed fusion (LPBF) technique, and their cracking behaviour was compared. It was observed that the crack density of W-ZrC was reduced by 88.7% compared with that of pure W. The grains in W-ZrC were obviously refined compared with the grains in pure W, which significantly increased the cracking resistance. In addition, ZrC diminished the oxygen impurities, further increasing the cracking resistance. This study provides a promising strategy for the additive manufacturing of high-quality W by introducing secondary-phase nanoparticles into the metal matrix.     

A hybrid post-processing method for improving the surface quality of additively manufactured metal parts

A hybrid post-processing method that synergistically combines cavitation peening and electrochemical polishing to achieve superior surface quality of solid and lattice structured additively manufactured (AM) metal parts is analysed. The method enables surface strengthening of AM parts through plastic deformation caused by cavitation while simultaneously improving the surface finish through electrochemical dissolution of surface asperities. Compared to sequential processing, the hybrid process produces higher microhardness and comparable surface roughness in a single step. Results show that coupling of the physical-chemical effects accompanying cavitation and electrochemical reaction can enhance the cavitation intensity and dissolution efficiency in hybrid processing.     

Surface smoothing and repairing of additively manufactured metal products by large-area electron beam irradiation

In general, surface of additively manufactured (AMed) metal products has large roughness due to arrangement of bead shapes, and surface irregularities such as spatter and cavity. Furthermore, surface elemental composition of AMed products may be changed from that of metal powder. In this study, efficient surface smoothing and repairing of AMed metal products by large-area electron beam irradiation were experimentally investigated. Experimental results show that spatter and cavity can be completely removed and surface roughness significantly reduces. Elemental composition of AMed surface can be also changed to that of original metal powder due to the removal of oxidized surface.     

热暴露对欠时效态Al-Cu-Mg-Ag合金拉伸性能的影响

采用拉伸力学性能测试、透射电镜微观组织分析和扫描电镜断口分析等方法,研究热暴露对一种欠时效态Al-Cu-Mg-Ag合金力学性能及微观组织的影响.结果表明:在150 ℃热暴露下,欠时效态Al-Cu-Mg-Ag合金的剩余强度先上升后下降,强度峰值出现在100 h;热暴露1 000 h后,合金的力学性能相对欠时效态合金的无明显下降;在200～300 ℃热暴露时,合金强度随时间的延长而下降,伸长率随着时间的延长而增大;在300 ℃热暴露时,强度明显下降,热暴露10 h后,其抗拉强度只有272.5 MPa,暴露100 h后, 其抗拉强度降至114.5 MPa;欠时效态合金细小分布的Ω相随着热暴露温度的升高,Ω相长大并粗化,θ′相析出,无沉淀析出带(PFZ)变宽;在250 ℃下热暴露时,Ω相明显粗化且数量稀少;合金中的Ω相和θ′相在300 ℃热暴露100 h后,均转变成平衡θ相.     

Microstructure and residual stress improvement in wire and arc additively manufactured parts through high-pressure rolling

Parts manufactured by Wire and Arc Additive Manufacture (WAAM) can have significant residual stress and distortion, as well as large grain sizes. To overcome these problems, each layer on a linearly deposited steel WAAM part was rolled with either a ‘profiled’ roller, which had a similar shape to the deposited layer, or a ‘slotted’ roller, in which a groove prevented lateral deformation. Both rollers reduced the distortion and surface roughness, but the slotted roller proved more effective – eliminating the distortion. The residual stresses in the rolled WAAM parts were measured and were lower than those in the unrolled control specimen – particularly adjacent to the baseplate. Rolling also induced additional grain refinement when the rolled material was reheated during the subsequent deposition pass. The application of rolling may be a key technology for enabling implementation of WAAM on large-scale structures.     

Effect of Nd addition on microstructure and tensile properties of laser additive manufactured TC11 titanium alloy

Single-layer and multilayer laser additive manufacturing (LAM) for TC11 alloy with different Nd additions was conducted and the effect of Nd addition on microstructure and properties was studied. With the addition of Nd, the aspect ratio of melting pools of single-layer specimens increases and the columnar-to-equiaxed transition occurs. The original β grain size and α plate width of TC11?1.0Nd are significantly reduced compared with those of pure TC11 specimens. It is proposed that the evenly distributed fine Nd₂O₃ precipitates of about 1.51 μm are formed preferentially during rapid solidification of melting pool, and they serve as heterogeneous nucleation particles to refine the microstructure in the subsequent solidification and solid-state phase transformation. Due to the multiple effects of Nd on the microstructure, the ultimate tensile strength of TC11?1.0Nd increases, while the yield strength, ductility and microhardness decrease compared with those of pure TC11.     

The superior thermal stability and tensile properties of hot rolled W-HfC alloys

Herein, a W-0.5wt%HfC (WHC05) alloy is fabricated by conventional sintering and multi-step hot-rolling. The high-temperature stability and tensile properties at different temperatures, ranging from room temperature to 600 °C, are studied to demonstrate the influence of HfC addition. The results reveal that the WHC05 alloy has a higher recrystallization temperature (1400 °C–1500 °C) than the previously reported as-rolled pure W (1200 °C) and as-rolled W-0.5wt%ZrC (WZC05 ~ 1300 °C) alloy. Moreover, after recovery and recrystallization (annealing at 1600 °C), the WHC05 alloy maintained a high ultimate tensile strength of ~300 MPa and exhibited a desirable increase in total elongation (>35%), which is ~1.6 times higher than the recrystallized WZC05 at 500 °C. The superior thermal stability and excellent high-temperature mechanical properties can be ascribed to the unique microstructure and uniform dispersion of nano-sized HfC particles in W matrix. The influence of annealing temperature on grain structure, grain orientation and distribution of nano-sized HfC particles has been studied to unveil the possible mechanism of enhanced thermal stability and superior mechanical properties.     

The use of off-line part production to predict the tensile properties of parts produced by Selective Laser Sintering

Rapid Manufacturing, and specifically Selective Laser Sintering, has the potential to become one of the most useful manufacturing techniques of the future, largely as a result of the extremely high levels of geometric complexity which can be produced. As the use of these technologies becomes more widespread, so the amount, and variety, of materials available for use in these processes also increases. However, the Selective Laser Sintering process can be both expensive and time-consuming when testing new materials. A method of off-line casting has been proposed here, and the tensile properties of parts produced using this method are compared with the properties achieved in Selective Laser Sintered parts produced in the same materials. For the materials tested it has been shown that the casting method provided an acceptable correlation with the properties of the Selective Laser Sintered parts, rendering this a suitable method of assessing the properties of a Selective Laser Sintering material without the requirement to produce a full build.     

Microstructure and mechanical properties of HIP-ZTC4 during thermal exposure and tensile deformation

Casting titanium alloy TC4(ZTC4) after hot isostatic pressing(HIP) is one of the preferred materials in the field of aerospace manufacturing. In this paper, HIPZTC4 after a long-term thermal exposure was investigated.In order to evaluate the variation of mechanical properties with service time, the tensile properties of this alloy after exposure at 400 °C for 100, 500, and 1,000 h were investigated. Microstructure of samples was observed by the means of optical microscopy(OM), scanning electron microscopy(SEM), and transmission electron microscopy(TEM). Tensile test was carried out under different test temperatures and fracture appearance was studied. The results show that the strength increases with exposure time due to the harder a2(Ti3Al) phase precipitated in the a phase and superficial oxygen layer formed, which results in the fact that the resistance of crack propagation could be increased and cracks first initiate between surface oxidation and the matrix. Besides, the tensile temperature also has a significant effect on the mechanical properties of HIPZTC4. The yield strength and tensile strength decrease with the increase of tensile temperature, while the total elongation increases.     

Relating tensile, bending, and shear test data of asphalt binders to pavement performance

Eight different asphalt binders representing a wide range of applications for pavement construction were tested in uniaxial tension, bending, and shear stresses. Theoretical analyses were performed in this study to convert the data from the three engineering tests to stiffness moduli for predicting pavement performance. At low temperatures, high asphalt stiffness may induce pavement thermal cracking; thus, the allowable maximum stiffness was set at 1,000 MPa. At high temperatures, low asphalt stiffness may lead to pavement rutting (ruts in the road); master curves were constructed to rank the potential for rutting in the asphalts. All three viscoelastic functions were shown to be interchangeable within the linear viscoelastic region. When subjected to large deformation in the direct tension test, asphalt binders behaved nonlinear viscoelastic in which the data under bending, shear and tension modes were not comparable. The asphalts were, however, found to exhibit linear viscoelasticity up to the failure point in the steady-state strain region.     

热拉伸变形对AZ21B镁合金板材力学性能与组织的影响

沿着与板材轧制方向成不同角度的方向截取试样,研究不同拉伸温度下AZ21B镁合金板材的力学性能和组织。结果表明:与轧制方向成相同角度的AZ21B镁合金板材试样,其综合力学性能因温度的变化而不同,其抗拉强度随温度的升高而下降,伸长率随温度的升高而增大;同时由于轧制会使镁合金板材产生很强板织构,造成板材的力学性能各向异性,当温度在室温(25℃)、150℃、200℃、250℃时,与板材轧制方向成0°试样的抗拉强度最大,当温度在300℃、350℃时,与板材轧制方向成90°试样的抗拉强度最大;在室温至250℃拉伸变形时,出现少量的孪晶,而在250℃以上拉伸变形时发生完全动态回复和再结晶。室温下拉伸试样的断口表现为明显韧脆性断裂。     

Testing the tensile properties of ceramic-matrix composites

This article reports the development of a mechanical testing methodology (including fixtures, extensometry, temperature control, and calibration) and procedure (including control mode options, and analysis) for ceramic-matrix composites (CMCs). Six different CMCs were tensile tested at room temperature (RT) and 1,000°C. An appreciable reduction in fracture resistance was measured for most of the systems at the high temperature, but there was little effect of an order-of-magnitude change in strain rate at either temperature. The reduced properties at 1,000°C could be essentially duplicated at RT for several of the composites by prior exposure in air at 1,000°C. The same exposure in vacuum was much less damaging; thus, oxygen would appear to be the culprit. Fractographic evidence indicated that fiber pullout in any system was a function of test temperature rather than fracture strength. Moreover, the incidence and magnitude of energy-release events during testing varied among the systems and revealed no unique connection with the state of damage induced by the prior exposures. These observations do not fit readily into any simple theoretical model. However, they do serve to identify potential limitations for this class of composite in terms of sensitivity to thermal transients in a non-inert environment.     

Constitutive prediction on the variability of tensile properties due to microporosity variation

The dependence of tensile properties on microporosity variation was investigated through a constitutive prediction based on previous experimental results of A356 aluminum alloy; the microporosity on a fractured surface was measured by means of quantitative fractography. The constitutive model, which takes into account the strain rate sensitivity, accurately predicts the tensile strength and elongation of the A356 alloy. In contrast, the results of a simple constitutive prediction that ignores the strain rate sensitivity diverge considerably from the overall trend of the experimental results. The significance of strain rate sensitivity on the constitutive prediction is more remarkably increased as the strain hardening exponent decreases or as the microporosity decreases.