简介概要

3D打印硬质合金研究进展

来源期刊：稀有金属2021年第4期

论文作者：刘云郭子傲王行张锦芳李晓峰

文章页码：484 - 492

关键词：3D打印;硬质合金;成型方式;力学性能;

摘要：硬质合金由难熔金属碳化物（如WC,NbC,TiC等）与过渡族金属元素（Co,Fe,Ni等）组成,因其具有良好的硬度、强度、耐磨性以及耐腐蚀性被广泛用作钻探切削、冲压模具、磨损件等。然而目前在硬质合金的高质量制备存在一些难题,如:生产周期长,易出现晶粒长大;受制于模具的形状,难以制备复杂形状工件。而3D打印技术在的快速发展为复杂硬质合金的制造提供了可能。综述了3D打印硬质合金的研究进展,着重围绕当前硬质合金3D打印的成型方式进行了介绍与对比。3D打印技术通过减少材料的损失和制造步骤的数量,降低生产成本,可以有效实现硬质合金的快速、精确、便捷的生产,未来有望成为生产复杂结构硬质合金的方法之一。但目前3D打印制备的硬质合金零件存在致密度低、组织控制与力学性能不足等问题,需要在未来进行深入研究。

详情信息展示

稀有金属 2021,45(04),484-492 DOI:10.13373/j.cnki.cjrm.XY19090015

3D打印硬质合金研究进展

刘云郭子傲王行张锦芳李晓峰

中北大学材料科学与工程学院

河南黄河旋风股份有限公司

中南大学粉末冶金国家重点实验室

摘要：

硬质合金由难熔金属碳化物（如WC,NbC,TiC等）与过渡族金属元素（Co,Fe,Ni等）组成，因其具有良好的硬度、强度、耐磨性以及耐腐蚀性被广泛用作钻探切削、冲压模具、磨损件等。然而目前在硬质合金的高质量制备存在一些难题，如：生产周期长，易出现晶粒长大；受制于模具的形状，难以制备复杂形状工件。而3D打印技术在的快速发展为复杂硬质合金的制造提供了可能。综述了3D打印硬质合金的研究进展，着重围绕当前硬质合金3D打印的成型方式进行了介绍与对比。3D打印技术通过减少材料的损失和制造步骤的数量，降低生产成本，可以有效实现硬质合金的快速、精确、便捷的生产，未来有望成为生产复杂结构硬质合金的方法之一。但目前3D打印制备的硬质合金零件存在致密度低、组织控制与力学性能不足等问题，需要在未来进行深入研究。

关键词：

3D打印;硬质合金;成型方式;力学性能;

中图分类号： TP391.73;TG135.5

作者简介：刘云（1967-），男，山西应县人，博士，教授，研究方向：铝合金液态成型及过程控制技术、增材制造，E-mail:997627968@qq.com;;*李晓峰，副教授，电话：13633518635,E-mail:lxf@nuc.edu.cn;

收稿日期：2019-09-18

基金：国家自然科学基金项目（51804280）;山西省自然科学基金项目（201701D221086和201801D221146）;山西省科技重大专项项目（20181101009）;山西省教育厅科技创新项目（201802076）;山西省回国留学人员科研资助项目（2017-095）;山西省国际合作项目（201603D421024）;山西省重点研发计划（国际科技合作）项目（201903D421075）资助;

Research Progress of 3D Printing Cemented Carbide

Liu Yun Guo Ziao Wang Hang Zhang Jinfang Li Xiaofeng

School of Materials Science and Engineering,North University of China

Henan Huanghe Whirlwind Limited Company

State Key Laboratory For Powder Metallurgy,Central South University

Abstract：

Cemented carbide was composed of refractory metal carbides(such as WC,NbC,TiC,etc.) and transition metal elements(Co,Fe,Ni,etc.).Because of its good hardness,wear resistance and corrosion resistance,it was widely used for drilling and cutting,stamping dies and wear parts,etc.However,there were some difficulties in the high-quality preparation of cemented carbide at present,such as,long production cycle and easy grain growth;restricted by the shape of the mold,it was difficult to prepare workpieces with complex shapes.The rapid development of 3 D printing technology provided the possibility for the manufacture of complex cemented carbide.This paper reviewed the research progress of 3 D printing cemented carbide,focusing on the folding way of cemented carbide 3 D printing for introduction and comparison.3 D printing technology could effectively realize the fast,accurate and convenient production of cemented carbide by reducing the loss of materials,the number of manufacturing steps and reducing production costs.It was expected to become one of the methods for producing complex structure cemented carbide in the future.However,the current cemented carbide parts prepared by 3 D printing had problems such as low density,insufficient structure control and mechanical properties,which required in-depth research in the future.At present,the methods of additive manufacturing of cemented carbide were mainly pided into direct forming and indirect forming.The direct forming methods were mainly selective laser melting(SLM),laser engineered net shaping(LENS) and selective electron beam melting(SEBM);the indirect forming methods were mainly binder jetting 3 D printing(BJ3 DP),3 D gel-printing,and selective laser sintering(SLS).The direct forming method used high-energy laser beam or electron beam as the heating source to selectively melt the powder and deposit it layer by layer.The formed sample had better compactness,and the mechanical properties,such as hardness was better than that of those prepared by indirect forming.Although this method had high forming accuracy and relatively good performance,the micropores and cracks produced by this method during the rapid solidification process had become problems that researchers need to solve urgently in the future.In comparison,after rebinding and high-temperature sintering,the blank formed by indirect forming had higher compactness and low porosity was close to that of traditional preparation methods to produce cemented carbide,but the post-processing process increased the cost.Shrinkage reduced accuracy.In addition,due to the larger layer thickness,the larger surface roughness of the green body and the larger shrinkage rate after sintering were also the current disadvantages of the indirect forming method.Regardless of the forming method,the control of structure and defects and their effects on performance had not yet found a specific corresponding relationship,so it was necessary to study a set of forming mechanisms to effectively control residual stress and pores.For the current mainstream SLM,fully dense cemented carbide produced with a density greater than 99% had not yet been produced,and it was necessary to perform heat treatment and infiltration after forming to increase its density.The green body prepared by BJ3 DP could reach fully dense cemented carbide samples after sintering,but the performance was not enough for industrial production.At present,the research on the preparation of cemented carbide by SEBM was still in its infancy,and there were few related studies.The high energy and fast-moving speed generated by the electron beam could inhibit the growth of grains to a certain extent,and the substrate preheating temperature could reduce high residual stress effectually,and it could be used as one of the feasible methods in the future.Defects were always the bottleneck of additive manufacturing applications.Most of the holes and cracks were caused by the residual stress with the rapid movement of the high-energy laser beam,which were mainly distributed in the initial deposition layer,because the substrates selected by the researchers were mostly 304 L or 316 L stainless steel.The large thermal expansion coefficient and undercooling of the two materials led to cracks,which could reduce the residual stress by increasing the preheating temperature of the substrate to inhibit the cracks.The powder morphology had a great influence on the structure morphology after forming.Compared with irregular powder,spherical powder had good fluidity,which could effectively inhibit the agglomeration of WC and segregation of Co,and the crystal grain dispersion was more uniform.In the method of preparing cemented carbide by additive manufacturing,the heat distribution was not uniform during the laser/electron beam melting process,the rapid solidification process and the rapid movement of the laser,so that the layered structure and uneven size could be observed in the microstructure,agglomeration of WC grains and Co.Not only that,according to the ternary phase diagram of WC-Co,the high residual stress generated during the high-temperature melting process of Co evaporation,carbon loss,and local powder melting by the laser beam promoted the formation of η phase and reduced the hardness the mechanical properties of the alloy.In contrast,during the preparation process of BJ3 DP and 3 DJP technology,the sample was heated evenly,the grain size of the sample was more uniform,and the brittle phase was greatly reduced.As a result,the samples prepared by BJ3 DP and 3 DJP technology were more similar to that prepared by traditional powder metallurgy processes.

Keyword：

3D printing; cemented carbide; folding way; machnical property;

Received： 2019-09-18

硬质合金是以难熔金属碳化物（如WC,Nb C,Ti C等）为基体，Fe,Co,Ni等过渡族金属为粘结相，通常采用粉末冶金工艺制备而成 ^[1,2,3] 。由于其具有良好的硬度、耐磨性及耐腐蚀等，硬质合金在金属切削加工、矿山采掘、石油钻井及国防军工等方面获得了广泛应用，被誉为“工业的牙齿” ^[4,5,6,7,8] 。

传统硬质合金成型方法主要以粉末冶金为主，将粉末混合均匀后进行压制（或注塑、挤压），最后用高温烧结等热处理方法保证其致密度和力学性能。同时该制备手段也存在一些问题，如生产周期长，易出现晶粒长大；受制于模具的形状，难以制备复杂形状的硬质合金工件；机械加工复杂零件时，容易发生开裂等现象；使用模具制备硬质合金零件时，需组合多套模具实现，增加了成本。

增材制造（3D打印）技术是以三维模型为基础，利用“离散-堆积”原理，通过逐层堆积的方式成型实体零件的制造方法。因其具有快速成型复杂零件的优点，一定程度上为实现制备硬质合金零件提供了解决方案。经过近几年的发展，3D打印技术在制备精密零件发挥了越来越重要的作用。目前3D打印硬质合金成型包含了喷胶粘粉技术（binder jetting 3D printing,BJ3DP），选择性激光烧结技术（selective laser sintering,SLS），选区激光熔化（selective laser melting,SLM）等。本文将针对不同快速成型方式制备硬质合金的进展和存在的问题进行综述。

1 喷胶粘粉成型3D打印（BJ3DP）硬质合金

BJ3DP通过计算机执行STL文件以粘合剂粘合连接粉末层并逐层沉积的方法构成生坯 ^[9,10,11,12] 。成型后在200℃下使得粘合剂进一步固化，之后将生坯按照粉末冶金加工工艺进行脱脂和烧结等。相关文献报道了关于使用BJ3DP工艺制造Ni合金、Ti合金、钢、陶瓷、WC-Co硬质合金等 ^[13,17,18] 。对于硬质合金而言，粘结剂饱和度、粉末层厚、粉末的装载量以及粘结相（如Ni,Cu,Co）的选择与加入方式的不同，对生坯及烧结后零件的微观组织和力学性能都有很大的影响。

Enneti等 ^[18,19] 研究了不同粘结剂饱和度和粉末层厚对WC-12Co生坯强度的影响，研究表明随着粘结剂饱和度的增加，生坯强度随之增加；粉末层厚增加至一定厚度，由于层厚较高，易出现孔洞，降低生坯强度。经过试验所得最佳参数粉末层厚度为70μm,45%的粘结剂饱和度。为使试样烧结后组织致密化，在烧结前进行渗碳处理，经过1485℃烧结后，力学性能良好，硬度为HV₃₀1256，断裂强度为（17±1)MPa·m^1/2，与常规硬质合金性能相近。图1为1485℃下烧结的WC-12Co合金的微观结构，合金组织致密，晶粒细小，孔洞与裂纹几乎消失，同时出现WC晶粒局部团聚。

Cramer等 ^[20,21] 通过BJ3DP工艺制备WC预制棒，再无压渗透Co，最后烧结得到相对密度大于97%的WC-Co复合材料，真空烧结过程中WC发生分解生成η相（如图2所示）。自由Co的含量在28%时，硬度为（14.1±0.4)GPa，致密度最低；自由Co含量在35%，硬度为（9.61±0.3)GPa，零件的致密度达到最高97%。相比于其他成型方法，BJ3DP成型的硬质合金收缩率降低了15%。

目前BJ3DP制备硬质合金大多数采用渗碳或熔渗的后处理方式增加致密度，进而提高性能。然而，烧结后尺寸变形大，表面质量差，对成型复杂结构零件依然是考验。

2 选择性激光烧结（SLS）成型硬质合金

选择性激光烧结（SLS）是快速成型（RP）技术之一，分别为间接激光烧结（EMLS）和直接激光烧结（DMLS) ^[22] 。它可以直接以金属粉末制造任意形状的三维（3D）零件，适用于硬质合金进行小批量制造。

Wang等 ^[23] 在SLS成型WC-Co过程中，模拟激光束面扫时通过边界和最深层的熔化颗粒计算出烧结区尺寸，通过实验对模拟结果进行验证。从图3可以看出在较高的激光功率下和较低的扫描速度时，实际测量的层厚和宽度和模拟结果有所不同；随着扫描速度的提升，数据基本一致，这表明扫描速度微小的差异所带来的颗粒之间热传递的影响可以忽略。仿真模型在将使以后的实验研究中尽可能减少成本和时间，并通过选择适当的激光加工参数进行优化。

图1 1485℃压力下烧结30 min的样品腐蚀后的SEM照片

Fig.1 SEM images of etched microstructures of samples pres-sure sintered at 1485°C for 30 min ^[19]

图2 熔渗Co后金属陶瓷材料烧结1,60 min和WC粉末的X射线衍射（XRD）图谱

Fig.2 XRD patterns of cermet processed for 1,60 min and WC powder ^[20]

Subrata等 ^[24] 通过SLS技术成功地制备了WC-Co硬质合金。研究发现WC-Co硬质合金的密度主要取决于粉末的组成、层厚度、阴影距离。通过数学方法计算分析，减少粉末阴影距离和增加粉末中Co含量百分比可降低试样的孔隙率。Subrata ^[25] 还采用SLS制备了WC-Co硬质合金坯体，然后通过熔渗铜获取到WC-Co-Cu硬质合金，从表1可得SLS制备的合金烧结前致密度只有40%～60%，熔渗铜液后其致密度达到了90%以上，合金具有较好的摩擦磨损性能。

顾冬冬教授 ^[26,27] 采用机械球磨的方法混合WC,Co,Cu 3种粉末后选择性激光烧结制备WC-Co∶Cu质量比为20∶80的WC-Co-Cu基复合材料。参数为700 W的激光功率，0.06 m·s^-1的扫描速度，0.15 mm的扫描线间距和0.3 mm的粉末层厚度。试样的平均显微硬度可达HV_0.1268。之后还采用直接激光烧结技术制备相对密度为94.3%的WC-Co/Cu纳米复合材料，具有3.47 GPa的动态纳米硬度，通过扫描电镜（SEM）表征发现复合材料中WC增强颗粒得到保留，并且晶粒没有明显的生长。

图3 模拟和测量数据在烧结区与扫描速度的对比

Fig.3 Sintering zone dimension versus scan speed:compari-son between simulations and measurements with differ-ent Nd:YAG laser power ^[23]

(a)8.18 W;(b)17.9 W;(c)28.1 W

表1 激光烧结后样品熔渗前和熔渗后的密度下载原图

Table 1 Density of laser sintered samples with and with-out infiltration ^[25]

3 选区激光熔化（SLM）打印硬质合金

选区激光熔化技术（SLM）使用高能激光束选择性地熔化和凝固粉末层，逐层打印零件。因其具有灵活性和高性能、复杂结构复合材料零件近净成型的潜力，镍合金 ^[28,29] 、钛合金 ^[30,31] 以及不锈钢 ^[32,33,34] 等材料的选区激光熔化目前逐步受到了广泛的关注，部分零件已经得到了实际应用。

Khmyrov等 ^[35] 采用球磨法制备WC-Co纳米级复合粉末，通过选区激光熔化（SLM）获得WC-Co硬质合金试样，发现碳化钨颗粒分布在Co基体中，硬质合金未出现明显裂纹。然而Co含量过高，合金的强度与硬度均降低。此外，提高激光扫描速度能够降低成型过程中球化效应和细化合金的微观结构，减少SLM的热循环。Li等 ^[36] 用球形WC粉末和Ni Al Co Cr Cu Fe高熵合金粉末混合，通过SLM制备出平均密度为14.69 g·cm³WC-20Co硬质合金。如图4所示，从表面至芯部显微硬度逐渐增加，这是由于选区激光熔化时高激光功率引起的基板金属元素扩散稀释和元素蒸发造成，导致晶粒分布不均匀。

Eckart等 ^[37] 对球形WC,Co粉末混合后采用不同的SLM成型工艺制备了WC-17Co硬质合金，高激光能量密度有助于在激光束和粉末材料相互作用的区域形成连贯的熔池，得到高致密度的样品，但是制备的硬质合金脆性增加。南京航空航天大学顾冬冬教授 ^[38] 采用机械球磨混合W,Ni，石墨粉末后SLM制备了WC-Ni硬质合金，其致密度达到了96%，维氏硬度超过了HV 1800；图5所示为SLM成型后的WC-Ni硬质合金显微组织，扫描速度偏大，合金内部出现较多空洞和裂纹，且局部晶粒发生长大。

图4 沿Z方向不同高度的维氏硬度和压痕断裂韧性

Fig.4 Profile of Vickers hardness and indentation fracture toughness at different height level along Z direction ^[36]

图5 激光扫描速度为1.2,1.0和0.8 m·s-1SLM加工试样的低倍微观组织（SEM)

Fig.5 Low-magnification SEM images of SLM-processed samples at laser scan speeds of 1.2 m·s^-1(a),1.0 m·s^-1(b)and 0.8 m·s^-1(c) ^[38]

Alexey等 ^[39] 用WC-12Co纳米粉末（20 nm）和微米粉末（2.5μm）通过选区激光熔化制备硬质合金试样，显微硬度测试结果显示纳米粉末制成的试样显微硬度高于微米粉末试样。Chen等 ^[40] 通过喷雾干燥得到规则球形A粉末和冷压成型后压碎成不规则形状的B粉末，如图6所示。之后通过选区激光熔化制备出WC-20Co的试样A，其致密度达96%、维氏硬度10 GPa，试样B致密度只有86%。如图6(d）所示试样B的显微组织有较多的裂纹、孔洞。与不规则形状的粉末相比，使用球形WC-Co粉末制备的硬质合金具有更高的相对密度。

图6 WC-Co颗粒的形态及SLM制成的硬质合金的SEM照片

Fig.6 SEM images of WC-Co granules and cemented car-bides ^[40]

(a)Granule A,made by spray drying;(b)Granule B,madeby cold pressing followed by crushing;(c)Cemented carbidesmade from(a)Granule A;(d)Cemented carbides made from(b)Granule B after SLM processing

4 其他3D打印硬质合金成型方法

4.1 3D冷打印技术制备硬质合金

3D冷打印技术（3D gel printing）是基于增材制造理念，以金属/陶瓷和凝胶体系制备出的稳定悬浮浆料为原料，通过选择逐层堆积和固化悬浮浆料，制造三维实体的技术 ^[41] 。从成型条件和原理可以得出3D冷打印技术具有常温/低温打印、低成本、高自由度等特点，同时免去了高能激光束/电子束成型时产生的一系列问题。3D冷打印技术的优点适用于硬质合金复杂结构设计和短流程的要求。

Zhang等 ^[41] 制成WC-20Co浆料后打印烧结成型。如图7所示，粘结剂装载量为56%（体积分数）的浆料通过3D冷打印技术制得的WC-20Co硬质合金，相对密度为99.93%，绝对密度达13.55 g·cm³、洛氏硬度为HRA 87.7、断裂强度为2612.8 MPa，具备良好的力学性能。

图7 不同装载量喷胶粘粉成型的WC-20Co浆料打印烧结后样品的SEM图像

Fig.7 SEM images of sintered samples printed by 3DGP using WC-20Co slurries with different solid loading ^[41] (a)56%;(b)53%;(c)50%;(d)47%

4.2 选择性电子束熔化（selective electron beammelting,SEBM)

SEBM可用于制备硬质合金零件，可一步成形，现在主要应用于金属合金，其高温的特性适用于难熔金属和对氧气、氮气亲和力高的金属，在制备硬质合金领域有广阔前景，扫描速率过高易导致高孔隙率，需要后期处理提高致密性。SEBM原理虽然类似于SLM，铺粉后将粉末床熔化，逐层堆叠，但是其热源是高能光束，且SEBM所需的环境是真空状态，需要基板预热温度比SLM高许多，扫描速率对成形效果具有重要影响。

为探究不同扫描速率对试样孔隙率的影响，Konyashina等 ^[42] 对扫描速率分别为4500,2250和800 mm·s^-1成形后的WC-13Co孔隙率进行研究。如图8所示，结果发现高扫描速率下成形后孔隙率较高，热等静压（HIP）处理后孔隙未能完全消除；中等扫描速率下成形后孔隙率明显降低，HIP处理后孔隙基本消除；低扫描速率成形后核心区域没有孔隙，外围1 mm有少许孔隙，XRD显示只存在WC和Co，不存在η相。利用SEBM成形复杂结构的硬质合金零件，表面粗糙度仍然很高，需要后期的加工。

4.3 定向能量沉积技术制备硬质合金

激光在基板快速移动的同时在沉积区域产生熔池，材料以粉末或丝状直接送入熔池中，融化后快速凝固，逐层沉积完成零件打印，这种成型方法被称为定向能量沉积技术（directed energy deposition,DED）。DED主要是以激光近净成型制造（laser engineered net shaping,LENS）和金属直接沉积（direct metal deposition,DMD）技术为主。Xiong等 ^[43] 利用LENS技术制备的WC-10Co薄壁硬质合金，致密度为97%，平均维氏硬度值约为HV 1326。作者分析顶部受到的重熔和加热次数少于中间部分，晶粒长大不明显；底部随着试样高度的升高散热速率不断降低，WC晶粒不规则均匀长大。