Influence of Selective Laser Melting Process Parameters on Microstructure and Properties of a Typical Ni-Based Superalloy

The influence of selective laser melting (SLM) process parameters on the microstructure and mechanical properties of a typical Ni-based superalloy was researched. The optimum parameters of P = 170 W, V = 0.8 m/s were determined, under which the SLMed samples exhibited both the largest relative density of 99.57% and the best mechanical properties, including the microhardness (329.3 ± 3.8 HV), yield strength (726 ± 8.1 MPa), ultimate tensile strength (900 ± 5.9 MPa) and elongation ((31.9 ± 0.24)%). The average grain size ranges of SLMed samples are from 15.2 to 17.4 μm, with a typical mixed grain structure. Owing to the high cooling rate and remelting during SLM process, a large number of low-angle grain boundaries (LAGBs), dislocations and sub-grains were formed, and the fraction of LAGBs reached above 65%. At the same time, the content of low-Σ coincidence site lattice (CSL) boundaries was mostly less than 1%, while there was almost no γ′ phase precipitated in the matrix. The texture of SLMed samples was weak, and there was no obvious preferred growth direction. Combining with the microstructure characterization, both grain refinement strengthening and dislocation strengthening were considered as the main strengthening mechanisms. Moreover, the fracture mechanism of the optimum sample belonged to ductile fracture.     

Si-Assisted Solidification Path and Microstructure Control of 7075 Aluminum Alloy with Improved Mechanical Properties by Selective Laser Melting

The construction and application of traditional high-strength 7075 aluminum alloy (Al7075) through selective laser melting (SLM) are currently restricted by the serious hot cracking phenomenon. To address this critical issue, in this study, Si is employed to assist the SLM printing of high-strength Al7075. The laser energy density during SLM is optimized, and the effects of Si element on solidification path, relative density, microstructure and mechanical properties of Al7075 alloy are studied systematically. With the modified solidification path, laser energy density, and the dense microstructure with refined grain size and semi-continuous precipitates network at grain boundaries, which consists of fine Si, β-Mg₂Si, Q-phase and θ-Al₂Cu, the hot cracking phenomenon and mechanical properties are effectively improved. As a result, the tensile strength of the SLM-processed Si-modified Al7075 can reach 486 ± 3 MPa, with a high relative density of ~ 99.4%, a yield strength of 291 ± 8 MPa, fracture elongation of (6.4 ± 0.4)% and hardness of 162 ± 2 (HV_0.2) at the laser energy density of 112.5 J/mm³. The main strengthening mechanism with Si modification is demonstrated to be the synergetic enhancement of grain refinement, solution strengthening, load transfer, and dislocation strengthening. This work will inspire more new design of high-strength alloys through SLM.     

TiB2/Al-Si-Mg-Er复合材料的组织与性能

采用重熔稀释法制备了Al-7Si-0.5Mg-0.1Er和0.5TiB₂/Al-7Si-0.5Mg-0.1Er合金,研究了TiB₂颗粒增强Al-Si-Mg-Er复合材料的组织性能。结果表明,复合材料铸态组织主要由α-Al基体、共晶Si相和TiB₂颗粒组成。TiB₂粒子的加入使Al-7Si-0.5Mg-0.1Er合金二次枝晶间距减小了7.1 μm。抗拉强度达到217.53 MPa,较Al-7Si-0.5Mg-0.1Er合金提升了12.1 %。TiB₂/Al-Si-Mg-Er复合材料的最优T6热处理工艺为530 ℃×12 h固溶+160 ℃×7 h时效,经该工艺处理后,TiB₂/Al-Si-Mg-Er复合材料抗拉强度达到319.49 MPa,相比热处理前提高了46.9%,相比Al-7Si-0.5Mg-0.1Er合金提高了5.9%;屈服强度达到266.75 MPa,相比热处理前提高了106.4%,相比Al-7Si-0.5Mg-0.1Er合金提高了14.9%。复合材料抗拉强度的提升主要源于TiB₂颗粒加入后产生的晶粒细化、变质和热处理强化。     

Microstructural Evolution and Anisotropic Weakening Mechanism of ZK60 Magnesium Alloy Processed by Isothermal Repetitive Upsetting Extrusion

The isothermal repetitive upsetting extrusion (RUE) was implemented to process ZK60 magnesium alloy at 380 °C. Then, the relationship between the microstructural characters, including grain refinement and texture evolution, and the mechanical performance of the alloy was investigated. Results showed that after 3 passes of RUE, the average grain size was refined from 115.0 to 26.5 μm, which was mainly caused by the continuous dynamic recrystallization and discontinuous dynamic recrystallization. Meanwhile, the elongation of the alloy increased from 13.8 to 21.6%, and the superplasticity (142%) of the alloy has been achieved in the following high temperature tensile test, which is very beneficial for the further processing of the alloy into components. In particular, the alloy formed a distinctive texture distributed between < 2-1-11 > and < 2-1-14 > , which was greatly related to the Schmid factor of extrusion direction (ED) and transverse direction (TD). This texture changed the initiation ability of basal and prismatic slip in both directions and inhibited the initiation of partial tensile twinning in TD; thus, the anisotropy in both directions was weakened. As expected, the tensile yield strength difference decreased from 25.9 to 3.4 MPa, but it was used as the cost of tensile yield strength in ED.     

An Ultra-High Strength Over 700 MPa in Al-Mn-Mg-Sc-Zr Alloy Fabricated by Selective Laser Melting

Selective laser melting (SLM) provides optimized lightweight structures for aircraft and space applications. However, the strength of the current SLMed aluminum alloys is still lower than that of the traditional high-performance aluminum alloys. This study presents an ultra-high-strength Al-Mn-Mg-Sc-Zr aluminum alloy specifically designed for SLM by increasing the (Mg + Mn) and (Sc + Zr) content simultaneously based on the rapid solidification characteristics of the SLM process. The alloy exhibits good SLM processability with a minimum porosity of 0.23%. After aging at 300 °C, the strength of the alloy was effectively improved, and the anisotropy of mechanical properties was reduced. Additionally, the tensile yield strength and ultimate tensile strength of the alloy reached 621 ± 41 MPa and 712 ± 28 MPa, respectively; these values are superior to those of most SLMed aluminum alloys reported previously. Multiple strengthening mechanisms including solid solution strengthening, precipitation strengthening and grain refinement strengthening contribute to the high strength of the present alloys.     

Effect of Heat Treatment on the Microstructure,Mechanical Properties and Corrosion Resistance of Selective Laser Melted 304L Stainless Steel

The influence of heat treatment holding temperatures from 600 to 1300 °C on the microstructure, mechanical properties and corrosion resistance in selective laser melted (SLMed) 304L stainless steel is investigated in this work. The results reveal that there is no remarkable microstructure change after holding at 600 °C for 2 h, while recrystallization leads to a slight decrease in grain size in the temperature range of 700-900 °C. The heat treatment at temperatures from 1000 to 1300 °C for 2 h obviously affects the morphology of grains in SLMed 304L stainless steel. Combining effects of grain coarsening, delta-ferrite (δ) phases reduction and residual stress release during heat treatment lead to the reduction of yield strength and an increasing elongation. The elongation of the samples heat treated at 800 °C for 2 h is, however, significantly decreased due to the increase in the amount of sigma (σ) phase. A remarkable increase in the amount of δ ferrite results in an increase in yield strength but a decrease in ductility after heat treatment at 1300 °C for 2 h. The corrosion resistance of the samples heat treated at 1300 °C is significantly improved due to the substantial reduction of brittle phase (σ). There is no obvious effect of the presence of δ ferrite on corrosion behavior.     

Strong and ductile Al–Zn–Mg–Zr alloy obtained by equal angular pressing and subsequent aging

An ultrafine-grained Al–Zn–Mg–Zr alloy with superior mechanical performance was obtained by high passes of equal angular pressing (ECAP) and subsequent aging. After 8 ECAP passes and aging, the yield strength (YS) and ultimate tensile strength (UTS) of the solid-solutioned alloy are significantly improved from (98±10) and (226±7) MPa to (405±9) and (427±9) MPa, respectively. A large elongation is also maintained ((17.4±2.5)%). The microstructure features including grain refinement, morphology of precipitates, and dislocation density, were revealed with multiscale characterizations, including transmission electron microscopy, electron backscattered diffraction, and X-ray diffraction. After 8 passes of ECAP, the original coarse elongated grains are refined to a unique bimodal grain structure consisting of ultrafine equiaxed and lath-like grains. Additionally, the effects of ECAP and subsequent aging on the strengthening contribution of a variety of strengthening mechanisms, such as dislocation strengthening and precipitation strengthening, were discussed in detail.     

等通道转角挤压变形Mg-1.5Mn-0.3Ce镁合金的组织、织构与力学性能

采用等通道转角挤压(ECAP)工艺以Bc路径在623K温度下对Mg-1.5Mn-0.3Ce镁合金进行变形,观察显微组织与织构,测试了力学性能。显微组织分析表明,镁合金经ECAP变形晶粒尺寸明显得到细化,经6道次ECAP变形后晶粒尺寸由原轧制态的约26.1μm细化至约1.2μm,且细小的第二相粒子Mg12Ce弥散分布于晶内及晶界处;同时经ECAP变形后,原始轧制织构随变形道次的增加不断减小,并开始转变为ECAP织构,织构强度不断增强;力学性能结果表明,由于晶粒细化作用大于织构软化作用,前3道次ECAP变形镁合金强度随道次的增加不断提高,与Hall?Petch关系相符,在第3道次时其抗拉强度和屈服强度达到最大值,分别为272.2和263.7MPa;在4道次之后形成较强的非基面织构,镁合金强度下降,与Hall?Petch呈相悖关系。断口分析表明,轧制态与ECAP变形镁合金的断裂方式都是沿晶断裂,由于6道次变形镁合金晶粒细化,存在更多的韧窝并获得16.8%最大室温伸长率。     

稀土元素Er对Ti-6Al-4V-0.5Si合金组织与性能的影响

利用粉末冶金工艺制备了Ti-6Al-4V-0.5Si-xEr(wt%)合金,随后采用OM、XRD、TEM和拉伸试验机等分析手段研究了Er元素含量对固溶时效态(950 ℃×30 min(WQ)+480 ℃×4 h(AQ))试验合金显微组织和性能的影响。结果表明:试验合金经固溶时效处理后均为等轴和片状的双态组织。烧结过程中产生的Er₂O₃氧化物颗粒可以作为形核中心促进α相和β相的析出,起到细化晶粒的作用。随着Er元素含量的增加,晶粒尺寸由10~20 μm细化至5~10 μm。当Er元素含量为1.2%时,试验合金的抗拉强度达到峰值,为930.5 MPa,此时伸长率为9.24%,比未添加Er元素时Ti-6Al-4V-0.5Si合金分别提高了22.3%和10.0%。试验合金的拉伸断口形貌显示有韧窝出现,仅有少量的解理台阶,韧窝的存在可以分散材料断裂时产生的应力,使材料断裂前承受更大的变形。     

Effect of Nanoscale Cu-Riched Clusters on Strength and Impact Toughness in a Tempered Cu-Bearing HSLA Steel

The effect of Cu-riched clusters on strength and impact toughness in a tempered Cu-bearing high-strength low-alloy (HSLA) steel is investigated. With increasing the tempering temperature, it is found that the yield strength increases firstly, achieving the maximum value (~ 1053 MPa) at the tempering temperature of 450 °C, and then decreases significantly with the rise of tempering temperature. The tempering temperature-dependent yield strength is closely related to the precipitation of Cu-riched clusters. When tempering at 450 °C, the peak strength will be reached as the nanoscale Cu-riched clusters with small size and high number density will cause a strong precipitation strengthening (~ 492 MPa) due to the dislocation shearing mechanism. However, the Cu-riched clusters will coarsen with further increasing tempering temperature, resulting in obvious decrement of yield strength owing to the dislocation bypassing mechanism. Compared with the yield strength, the variation in impact energy displays an inverse tendency and the impact energy is only 7 J for the sample tempered at 450 °C. The fracture mode can be well explained by the competition between the cleavage fracture strength (σ_F) and “yield strength” (σ_Y). Although transgranular cleavage fracture can be found in samples tempered at 450 and 550 °C, the crack propagation along the lath boundaries is prevented in the sample tempered at 550 °C. The reason is that the number density of Cu-riched clusters at lath boundaries decreases and the segregation of Mo element at the lath boundaries is induced, which will increase the bonding energy.     

Microstructure and tensile property of the ECAPed pure magnesium

Microstructure and texture development of the equal channel angular pressed (ECAP) pure Mg with different rotation routes of A, Bc and C were investigated and related to the tensile mechanical property. Results showed that grain size was greatly decreased after 1 pass but varied with processing routes after the following ECAP. Due to a relatively high processing temperature, the refinement degree was limited via these three routes processing. Different preferred orientations of the basal plane were formed with three routes processing and there existed a rotation angle in (0 0 0 2) pole figure from normal direction to transverse direction. Negative slope of the yield stress versus d^−1/2 was obtained with routes Bc processing, which was attributed to the texture modification. Texture strengthening greatly increased the 0.2% yield strength, but decreased the ductility.     

Selective Laser Melting of Al-7Si-0.5 Mg-0.5Cu: Effect of Heat Treatment on Microstructure Evolution,Mechanical Properties and Wear Resistance

Al-7Si-0.5 Mg-0.5Cu alloy specimens have been fabricated by selective laser melting (SLM). In this study, the effects of solution treatment, quenching, and artificial aging on the microstructural evolution, as well as mechanical and wear properties, have been investigated. The as-prepared samples show a heterogeneous cellular microstructure with two different cell sizes composed of α-Al and Si phases. After solution-treated and quenched (SQ) heat treatment, the cellular microstructure disappears, and coarse and lumpy Si phase precipitates and a rectangular Cu-rich phase were observed. Subsequent aging after solution-treated and quenched (SQA) heat treatment causes the formation of nanosized Cu-rich precipitates. The as-prepared SLMs sample has good mechanical properties and wear resistance (compressive yield strength: 215 ± 6 MPa and wear rate 2 × 10^-13 m³/m). The SQ samples with lumpy Si particles have the lowest strength of 167 ± 13 MPa and the highest wear rate of 6.18 × 10^-13 m³/m. The formation of nanosized Cu-rich precipitates in the SQA samples leads to the highest compressive yield strength of 233 ± 6 MPa and a good wear rate of 5.06 × 10^-13 m³/m.     

不同热变形工艺大塑性变形加工超细晶1570C铝合金的组织、强度和超塑性

将Al-5Mg-0.18Mn-0.2Sc-0.08Zr-0.01Fe-0.01Si(质量分数,%)合金铸锭进行多向等温锻造(应变12)或等径角挤压(应变10,325℃),再进行热轧(325℃)和冷轧(20℃),对比研究合金变形后的组织和力学行为。结果表明,对(亚)晶粒尺寸dUFG=2μm的超细晶组织合金进行多向等温锻造后,其室温延展性提高,超塑性伸长率可达2800%。通过热轧进一步细化晶粒,后续再通过冷轧形成高位错密度的严重变形组织,合金的屈服/极限抗拉强度从多向等温锻造后的235/360 MPa分别提高到热轧和冷轧后的315/460 MPa和400/515 MPa。同时,热轧使超塑性伸长率提高到4000%,而冷轧后的伸长率仍然足够高(高达1500%)。与多向等温锻造相比,经等径角挤压后合金的晶粒细化程度更高(dUFG=1μm),强度和超塑性性能均得到提高。然而,热轧后此效果有所减弱,加工后的板材具有同等的性能。与多向等温锻造后再冷轧的合金相比,经等径角挤压后再冷轧的合金强度更高,超塑性性能略好。     

Temperature-Dependent Compressive Deformation Behavior of Commercially Pure Iron Processed by ECAP

To explore the temperature dependence of deformation behavior of BCC structural materials and the relevant effect of pre-annealing, commercially pure iron(CP Fe) produced by equal-channel angular pressing(ECAP) is selected as the experimental material. The influences of deformation temperature T and pre-annealing on deformation behavior,surface deformation characteristics and substructures of ECAP Fe were systematically studied. The results show that ECAP Fe undergoes a remarkable strain softening stage after a rapid strain hardening during uniaxial compression, and the softening degree and the yield strength rYSfirst decrease and then increase with raising temperature. Pre-annealing at400 °C effectively weakens the strain softening degree and increases rYS. To understand the influence of deformation temperature on deformation behavior, as well as the relevant pre-annealing effect, deformation and damage characteristics and dislocation structures are studied in detail. In a word, the strain softening of ECAP Fe is associated not only with internal structural instability, but also with temperature, and pre-annealing at 400 °C improves high-temperature mechanical properties of ECAP Fe.     

Microstructure and Mechanical Properties in Friction Stir Welded Thick Al-Zn-Mg-Cu Alloy Plate

In this work, 20-mm-thick aluminum-alloy plates were joined via friction stir welding. The temperature gradient was reduced by reducing the surface welding heat input to achieve uniformity of the mechanical properties across the thick plate joints. The welding temperature was measured using thermocouples. The microstructures were observed via electron backscatter diffraction and transmission electron microscopy. The tensile properties of the samples sliced along the thickness direction of the joint were evaluated. The results show that the highest welding peak temperature is 430 °C on the advancing side on the top surface of the joint. The grain size gradually decreased along the thickness direction, and grain refinement was due to the combination of continuous, discontinuous, and geometric dynamic recrystallization. The tensile properties of the sliced samples were found to be uniform, and the ultimate tensile strength reached 62% of that of the base metal. The main strengthening mechanism of the Al-Zn-Mg-Cu alloy joints consists of precipitation strengthening. In addition, the η` →  η phase transition and grain coarsening in the heat-affected zone were found to be responsible for the fracture of the joints.     

二次挤压对挤压和ECAP变形后ZK60镁合金显微组织和力学性能的影响

测试四种状态下ZK60合金的显微组织和力学性能,四种状态分别为：挤压;挤压＋4道次ECAP;挤压＋4道次ECAP＋二次挤压;挤压＋4道次ECAP＋退火＋二次挤压。在室温下成功地进行ZK60的二次挤压,得到超细晶组织。结果表明：ECAP和二次挤压可以显著细化晶粒。挤压＋4道次ECAP＋二次挤压后的ZK60合金的屈服强度为342MPa,但是其伸长率只有0.8%。在二次挤压之前进行退火,ZK60合金的伸长率可以提高到4.5%,而屈服强度基本不变,抗拉强度达到 388 MPa。     

Enhanced Mechanical Properties of Pure Zirconium via Friction Stir Processing

Friction stir processing (FSP), as a new kind of severe plastic deformation technique, can refine and homogenize the microstructure of metallic material. In this study, the effect of FSP on the microstructure and mechanical properties of pure Zr was investigated using electron backscatter diffraction analysis, microhardness and room-temperature tensile testing. The fine-grained (FG) structure with an average grain size of?~?5.3 μm was obtained in the processed zone, where the average microhardness was?~?198 HV, 1.6 times higher than that of base metal. Furthermore, tensile property of FG pure Zr exhibited obvious anisotropy owing to strong texture. Both grain size and texture had a significant effect on strength and ductility of FG pure Zr. High yield strength (248 MPa) and ultimate tensile strength (483 MPa), as well as good uniform elongation (10%) were achieved, which demonstrated that FSP was an effective method to fabricate bulk FG pure Zr with high strength and good ductility.     

Preparation of Biodegradable Mg/β-TCP Biofunctional Gradient Materials by Friction Stir Processing and Pulse Reverse Current Electrodeposition

Mg alloys, as a new generation of biodegradable bone implant materials, are facing two tremendous challenges of enhancing strength and reducing degradation rate in physiological environment to meet clinical needs. In this study, tricalcium phosphate(β-TCP) particles were dispersed in Mg–2 Zn–0.46 Y–0.5 Nd alloy by friction stir processing(FSP) to produce Mg-based functional gradient materials(Mg/β-TCP FGM). On the surface of Mg/β-TCP FGM, the hydroxyapatite(HA) coating was prepared by electrodeposition. The effects of FSP and electrochemical parameter on the microstructure, microhardness, bonding strength and corrosion performance of the Mg/β-TCP FGM were investigated. After four passes of FSP, a uniform and fine-grained structure was formed in Mg/β-TCP and the microhardness increased from 47.9 to 76.3 HV. Compared to the samples without β-TCP, the bonding strength of the Mg/β-TCP FGM increased from 23.1 ± 0.462 to 26.3 ± 0.526 MPa and the addition of degradable β-TCP contributed to the in situ growth of HA coating. The thickness of HA coating could be dominated by controlling the parameters of electrodeposition. According to the results of immersion tests and electrochemical tests in simulated body fluid, it indicated that the degradation rate of the Mg/β-TCP FGM could be adjusted.     

Strengthening of Ultrafine Lamellar-Structured Martensite Steel via Tempering-Induced Nanoprecipitation

An ultrafine lamellar-structured martensite steel fabricated by heavy warm rolling (HWR) has shown an excellent combination of strength and ductility. By appending tempering at 400 °C to HWR, we show that the comprehensive mechanical property of a lamellar-structured low-carbon martensite steel can be further improved to reach a yield strength of ~ 1.8 GPa, an ultimate tensile strength of ~ 2.0 GPa and a total elongation of ~ 9.3%. This is achieved by tempering the HWR steel from 300 to 750 °C, and the optimum tempering temperature is thus obtained. We find that the tempered ultrafine lamellar martensite contains high-density nanoprecipitates dispersed within the aligned martensite laths with reduced crystallographic variations. The ultrahigh strength of the steel is rationalized as mainly the result of grain boundary strengthening and precipitation strengthening, which contribute to yield stress by 610 MPa and 440 MPa, respectively. The good ductility is believed to be closely related to the capacity of the tempered grains to accommodate dense dislocations upon plastic deformation. The present thermomechanical processing provides a feasible routine for producing steels with ultrahigh-strength and good-ductility.     

纯镁切应力变形后的微观组织演变、力学性能、腐蚀行为

纯镁为密排六方结构,具有较少的独立滑移系导致其塑性较差。研究了纯镁变形后的微观组织演变、力学性能、腐蚀行为。结果表明,纯镁经过等径角挤压（ECAP）变形后晶粒明显细化以及基面织构发生了弱化,导致纯镁的塑性得到了显著地提高。等径角挤压变形后纯镁强度降低主要是因为基面织构弱化影响大于晶粒细化。此外,等径角挤压变形后纯镁自腐蚀电位和腐蚀电流密度明显增加,纯镁的抗腐蚀性能显著提高。纯镁的腐蚀机理可能从局部腐蚀向均匀腐蚀转变,从而减少了样品在标准模拟体液浸泡中的腐蚀脱落,确保了试样的完整性。