Study of drilling of composite material and aluminium stack

Drilling of CFRP/Al or CFRP/Ti, or Al/CFRP/Ti is a challenge to manufacturing engineers. Drilling of CFRP is manageable but the minute drill hits the Al or Ti, those hot and continuous chips destroy the hole. Few studies are carried out in this aspect; CFRP/Al is not reported till date. It is important to select right process parameters when drilling multimaterial stack since each material in the stack requires a different set of machining parameters. Hence, Drilling trials have been carried out in carbon-fibre reinforced plastics (CFRP)/aluminium (Grade 2024) stack without coolant, with plain carbide (K20) drills of various diameters to choose optimum process parameters. The parametric influences on thrust force, torque as well as surface finish were experimentally evaluated. The experimental results show that the quality of holes can be improved by proper selection of cutting parameters. This is substantiated by monitoring thrust force, torque, surface finish, circularity and hole diameter. For the CFRP, the circularity is found to be around 6 μm at low feed rates, when the feed is increased the circularity increases to 25 μm. The wear tests carried out show that, during first 30 holes, thrust force in CFRP undergoes a more important increase (90%) than thrust force of aluminium (6%).     

多齿铣刀侧铣加工多层CFRP铣削力的建模与仿真

由于碳纤维增强树脂基复合材料(CFRP)的层间结合强度较低,进行切削加工时在切削力的作用下容易出现分层和毛刺等质量缺陷。因此,通过对切削力的预测与控制可以有效提高加工质量。采用瞬时刚性力模型对多齿铣刀侧铣多层CFRP材料的加工过程进行铣削力建模与仿真,分析了多齿铣刀特有的几何结构对切削力的影响。试验中保持切削速度恒定,以不同进给速度分别对45°、0°、-45°和90°这4种典型纤维方向的单向CFRP进行侧铣加工,通过测得的切削力数据计算各自的铣削力系数。根据力学矢量叠加原理得到了多向CFRP铣削力系数的简化计算表达式,最后将计算结果代入铣削力模型得到了各时刻的铣削力仿真值。在同样的试验条件下对该多向CFRP进行侧铣加工验证试验,试验结果表明: 该模型能较好地预测铣削力,最大相对误差小于9%,平均相对误差小于5%,可为铣削参数优化和刀具结构优化提供理论基础。     

碳纤维增强复合材料螺旋铣孔切削力及加工质量研究

针对传统钻孔方法加工复合材料时易导致分层、撕裂等缺陷的问题,采用螺旋铣作为新的制孔技术,根据飞机装配现场的实际加工条件,构建以机器人为移动载体、螺旋铣孔终端执行器为加工单元、螺旋铣孔专用刀具为切削工具的加工系统,采用该加工系统对碳纤维增强复合材料(CFRP)螺旋铣孔关键工艺参数进行正交试验,并讨论了刀具主轴转速、每齿进给量和轴向切削深度等工艺参数对切削力的影响规律;通过对加工缺陷的监测,探讨了切削力与CFRP分层、撕裂等缺陷之间的关系;最后对工艺参数进行优化,经试验验证,优化后轴向切削力较优化前降低26%以上,孔入口及出口处均无撕裂、毛刺,加工质量最优。     

The influence of cutting force on surface machining quality

Appropriately controlled cutting forces can contribute not only to the safety and efficiency of machining but also to the quality of machined surfaces. It is even more important when hardened material is cut. The correlation between the cutting force and the surface quality in ball-end milling operations has been investigated by machining P20 steel (HRC 30) work-pieces using solid carbide ball-end cutters. Plane surfaces with different depth of cut were machined using two different cutting strategies. The first strategy cut the test-piece using a cutting force model, whereas the other machined with a feed rate optimization product, which uses the removal rate as an analogue of cutting force to control the feed rate. The test results show that constant surface quality is possible when the cutting forces are controlled through feed rate adjustment. Conversely, a desired surface quality can also be maintained by controlling the cutting force in a predetermined manner.     

Optimization of machining parameters at high speed drilling of carbon fiber reinforced plastic (CFRP) laminates

High speed machining is now acknowledged to be one of the key manufacturing technologies to ensure high productivity and throughput. Drilling of CFRP, though a challenging task, is being performed successfully at low spindle speeds. However high speed drilling in CFRP thin laminates has not been explored much. This paper reports an experimental investigation of a full factorial design performed on thin CFRP laminates using K20 carbide drill by varying the drilling parameters such as spindle speed and feed rate to determine optimum cutting conditions. The hole quality parameters analyzed include hole diameter, circularity, peel-up delamination and push-out delamination. Analysis of variance (ANOVA) was carried out for hole quality parameters and their contribution rates were determined. Genetic Algorithm (GA) methodology was used in the multiple objective optimization (using MATLAB R2010a software) to find the optimum cutting conditions for defect free drilling. Tool life of the K20 carbide drill was predicted at optimized cutting speed and feed.     

CFRP复合材料/钛合金叠层螺旋铣孔工艺

利用螺旋铣变偏心距加工的特点,提出了在刀具回程过程中进行二次精加工的工艺策略。通过正交试验研究了回程各工艺参数对CFRP/Ti-6Al-4V叠层孔加工质量和加工精度的影响规律,并依此优化工艺参数。试验结果显示:采用优化参数的回程精加工工艺提高了叠层孔的制孔精度,避免了复合材料孔壁加工损伤,复合材料孔粗糙度均值从Ra3.52降低到Ra1.31,入口撕裂明显改善,钛合金孔出口无毛刺。     

EXPERIMENTAL INVESTIGATIONS OF DRILLING ON CFRP COMPOSITES

The paper outlines the various problems associated with the drilling of CFRP composites. The technique of dimensional analysis is used to investigate the complex correlation between thrust force, cutting speed, feed, hole diameter, point geometry and material thickness during the drilling of holes in CFRP composites, A new non-dimensional number (t/D), thickness of layered composites to drill diameter, is found to influence the thrust force. Four drill point geometries specifically found effective in drilling of FRP were tried and among them the eight facet drill point geometry was found to give the best results.     

Study of delamination in drilling carbon fiber reinforced plastics (CFRP) using design experiments

In this paper is presented a new comprehensive approach to select cutting parameters for damage-free drilling in carbon fiber reinforced epoxy composite material. The approach is based on a combination of Taguchi’s techniques and on the analysis of variance (ANOVA). A plan of experiments, based on the techniques of Taguchi, was performed drilling with cutting parameters prefixed in an autoclave carbon fiber reinforced plastic (CFRP) laminate. The ANOVA is employed to investigate the cutting characteristics of CFRP’s using high speed steel (HSS) and Cemented Carbide (K10) drills. The objective was to establish a correlation between cutting velocity and feed rate with the delamination in a CFRP laminate. The correlation was obtained by multiple linear regression. Finally, confirmation tests were performed to make a comparison between the results foreseen from the mentioned correlation.     

Influence of machining parameters and new nano-coated tool on drilling performance of CFRP/Aluminium sandwich

Drilling and fastening of hybrid materials in one-shot operation reduces cycle time of assembly of aerospace structures. One of the most common problems encountered in automatic drilling and riveting of multimaterial is that the continuous chips curl up on the body of the tool. Drilling of carbon fiber reinforced plastic (CFRP) is manageable, but when the minute drill hits the aluminium (Al) or titanium (Ti), the hot and continuous chips produced during machining considerably damage the CFRP hole. This study aims to solve this problem by employing nano-coated drills on multimaterial made of CFRP and aluminium alloy. The influence of cutting parameters on the quality of the holes, chip formation and tool wear were also analyzed. Two types of tungsten carbide drills were used for the present study, one with nano-coating and the other, without nano coating. The experimental results indicated that the shape and the size of the chips are strongly influenced by feed rate. The thrust force generated during drilling of the composite plate with coated drills was 10–15% lesser when compared to that generated during drilling with uncoated drills; similarly, the thrust force in the aluminium alloy was 50% lesser with coated drills when compared to thrust force generated without coated drills. Thus, the use of nano-coated drills significantly reduced the surface roughness and thrust force when compared with uncoated tools.     

Experimental investigation on drilling of natural flax fiber-reinforced composites

Natural fiber composites (NFCs) have strong potential to replace glass fiber-reinforced plastics. An instrumental operation in machining composite structures is hole making which facilitates assembly of parts. Understanding the effects of drilling process parameters on feature properties of NFCs has great benefits. In this regard, to make a good quality and accurate hole in composite structures, appropriate selection of drill bit and cutting parameters is important. This paper investigates delamination behavior and hole quality of flax/epoxy composite laminates in response to feed, spindle speed, and three different types of drill bit. As indicated by analysis of variance results, drill bit type and feed have greater influences on the thrust force. It appeared that delamination factor and surface roughness were significantly influenced by drill bit, but not by feed and spindle speed. The choice of drill bit has great impact on the delamination factor (67.27%) and surface roughness (74.44%), respectively.     

Experimental study on mechanical drilling of carbon/epoxy composite-Ti6Al4V stacks

This paper concerns the experimental studies on the drilling process of multilayer carbon/epoxy composite-Ti6Al4V stacks and their individual material layers using tungsten carbide drills. The significance of the current work aims to reveal the impact of tool wear on the drilling process for CFRP/Ti6Al4V, CFRP and Ti6Al4V. Four groups of machining tests including drilling individual CFRP, individual Ti6Al4V, multilayer CFRP/Ti6Al4V stacks with and without a cooling treatment were conducted. Drilling forces, cutting temperatures and hole quality attributes were experimentally investigated in terms of the drill wear extents. The mechanism controlling the tool wear effect on the drilling machinability of CFRP/Ti6Al4V was revealed, providing several implications for the industrial manufacturers. The results highlight the significant impact of the titanium chip ejection on the composite surface quality and the necessity of a rigorous tool wear control to guarantee the damage-free drilling of CFRP/Ti6Al4V stacks.     

Prediction of cutting force based on machining parameters on AL7075-T6 aluminum alloy by response surface methodology in end milling

Cutting forces modeling is the basic to understand the cutting process, which should be kept in minimum to reduce tool deflection, vibration, tool wear and optimize the process parameters in order to obtain a high quality product within minimum machining time. In this paper a statistical model has been developed to predict cutting force in terms of geometrical parameters such as rake angle, nose radius of cutting tool and machining parameters such as cutting speed, cutting feed and axial depth of cut. Response surface methodology experimental design was employed for conducting experiments. The work piece material is Aluminum (Al 7075-T6) and the tool used is high speed steel end mill cutter with different tool geometry. The cutting forces are measured using three axis milling tool dynamometer. The second order mathematical model in terms of machining parameters is developed for predicting cutting forces. The adequacy of the model is checked by employing ANOVA. The direct effect of the process parameter with cutting forces are analyzed, which helps to select process parameter in order to keep cutting forces minimum, which ensures the stability of end milling process. The study observed that feed rate has the highest statistical and physical influence on cutting force.     

基于径向基函数神经网络的CFRP切削力预测

碳纤维增强树脂基复合材料(CFRP)加工中基体相极易因切削力过大而破坏,并迅速扩展至加工表面以下而形成损伤。为了准确预测其切削力并加以控制,基于实验切削力数据建立了人工神经网络切削力模型,预测了不同纤维角度、切削深度和刀具角度下加工CFRP的切削力变化规律,并完成了不同刀具角度及切削参数下典型纤维角度CFRP单向板的直角切削实验,对预测模型进行验证,其预测精度可达85%以上。结合成屑过程在线显微观测结果可知:纤维角度是影响CFRP切削力的主要因素, 0°~135°范围内,切屑形成方式为切断型和开裂后弯断型;切削力随纤维角度增大呈先减小后增大的趋势, 135°时最大,随切削深度增加,切削力总体呈增大趋势。      

Experimental investigation on the ball burnishing of carbon fiber reinforced polymer

The polymer-based materials are generally used in all industrial applications. Even if polymer bars can be machined easily, they need surface finishing treatment after the machining process. The ductile properties make coarse roughness on the surface that causes the elastic structure of polymer materials. The carbon fiber reinforced composite materials differ from polymer-based materials by high strength and stiffness. Its structure exhibits similar performances such as metallic materials. The effect of ball burnishing on the surface quality of rod workpiece was investigated in this study. To enhance the surface quality of high strength carbon fiber reinforced polymer rod workpiece, burnishing process was performed in three different parameters (force, feed rate, number of passes) and under four different mediums (dry, wet, boron oil, and mineral oil) conditions. The results of the experiments were analyzed, and optimum burnishing parameters were determined and discussed in detail. The best surface roughness value of the CFRP material used in the experiments was obtained as burnishing force: 250 N, feed rate: 0.05 mm/rev, four passes and wet medium. According to the results of variance analysis, it was found that the CFRP workpiece is the important leading factor for surface roughness with a contribution ratio of 62.47%.     

Circular cutting strategy for drilling of carbon fiber-reinforced plastics (CFRPs)

The drilling process of carbon fiber-reinforced plastics (CFRPs) is the most commonly employed machining operation due to the necessity of joining these materials. However, these materials are prone to delaminate during the process, and the presence of this defect is the most cause of rejection for CFRP products, especially those produced for the aeronautic industry. Therefore, this article aims to study a drilling strategy (named circular drilling strategy) by using dedicated tools with different diameters, in order to reduce the extension of delaminations. Holes with different diameters (6, 8, and 10 mm) were obtained both with the conventional and with the proposed drilling strategy under distinct cutting conditions that mainly differ in the feed rates (62, 125, and 250 mm/min) and cutting velocities (50, 75, and 100 m/min). The effect of the cutting parameters and tool diameter on the cutting forces and delamination factor was studied for both the conventional and circular drilling process. The results proved that the proposed technique produces better hole quality and lower thrust forces than the conventional one under the same cutting conditions.     

A study on milling of glass fiber reinforced plastics manufactured by hand-lay up using statistical analysis (ANOVA)

Milling is the most practical machining (corrective) operation for removing excess material to produce a well defined and high quality surface. However, milling composite materials presents a number of problems such as surface delamination associated with the characteristics of the material and the cutting parameters used. In order to minimize these problem is presented a study with the objective of evaluating the cutting parameters (cutting velocity and feed rate) related to machining force in the workpiece, delamination factor, surface roughness and international dimensional precision in two GFRP composite materials (Viapal VUP 9731 and ATLAC 382-05). A plan of experiments, based on an orthogonal array, was established considering milling with prefixed cutting parameters. Finally an analysis of variance (ANOVA) was preformed to investigate the cutting characteristics of GFRP composite materials using a cemented carbide (K10) end mill.     

Thrust force model for ultrasonic-assisted micro drilling of DD6 superalloy

As a typical refractory material, the DD6 nickel-based single-crystal superalloy has important applications in the aviation industry. Ultrasonic-assisted drilling is an advanced machining method that significantly improves machining of refractory materials. The drilling thrust force influences the hole surface quality, burr height, and bit wear. Therefore, it is necessary to predict the thrust force during ultrasonic-assisted drilling. However, there are few reports on the modeling of the thrust force in the ultrasonic-assisted drilling of micro-holes. A thrust force prediction model for ultrasonic-assisted micro-drilling is proposed in this study. Based on the basic cutting principle, the dynamic cutting speed, dynamic cutting thickness, and acoustic softening effect caused by ultrasonic vibrations are factored into this model. Through model calibration, the specific friction force and specific normal force coefficients were determined. The model was verified through ultrasonic-assisted drilling experiments conducted at different feed rates, spindle speeds, frequencies, and amplitudes. The maximum and minimum errors of the average thrust force were 10.5% and 2.3%, respectively. This model accurately predicts the thrust force based on the parameters used for ultrasonic-assisted micro-hole drilling and can assist in the analysis and modeling of DD6 superalloy processing.The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-021-00381-y     

芳纶纤维增强树脂复合材料液氮冷却钻孔试验

芳纶纤维增强树脂（AFRP）复合材料是一种公认的难加工材料,加工中极易出现毛刺、烧蚀等缺陷,目前缺乏对其有效的加工工艺方法。为提高其加工质量,研究了液氮作为冷却介质的AFRP复合材料钻孔试验。在相同切削参数下进行了干式切削和超低温加工对比试验,测量了切削过程中的轴向切削力和孔临近区域的温度,并计算了孔的进出口毛刺面积和分层因子,分析了AFRP复合材料缺陷的成因,探讨了不同加工条件下缺陷的变化规律。结果表明:与干式切削相比,采用液氮超低温冷却加工的切削力升高了约15.2%,切削温度降低了约141.6℃,毛刺面积减少了约24.7%,因切削热产生的烧蚀现象得到抑制,明显改善了AFRP复合材料的加工质量。      

进给速度对不同纤维方向CFRP铣削表面形貌的影响

采用金刚石涂层硬质合金铣刀对0°、45°、90°、135°4种纤维方向的碳纤维增强复合材料(CFRP)进行了顺铣加工试验。通过对铣削力和加工表面形貌的对比,分析了纤维方向和每转进给量对加工表面质量的影响。结果表明:主切削力随着每转进给量的增大而增大,0°方向纤维受到的主切削力最大,90°方向纤维受到的主切削力最小;0°方向纤维表面破碎树脂与纤维的残留随着每转进给量的增大而增多,135°方向纤维表面树脂粘附逐渐减小;90°方向纤维表面会有大小不同的微坑,同时在样件上、下表面易产生分层缺陷,45°方向纤维表面多呈现沟槽或波浪形形貌。     

Prediction and optimisation models for turning operations

Selection of process parameters has very significant impact on product quality, production costs and production times. The quality and cost are much related to tool life, surface roughness and cutting forces which they are functions of process parameters (cutting speed, feed rate, depth of cut and tool nose radius). In this paper, empirical models for tool life, surface roughness and cutting force are developed for turning operations. The process parameters (cutting speed, feed rate, depth of cut and tool nose radius) are used as inputs to the developed machineability models. Two data mining techniques are used; response surface methodology and neural networks. The data of 28 experiments have been used to generate, compare and evaluate the proposed models of tool life, cutting force and surface roughness for the selected tool/material combination. The resulting models are utilized to formulate an optimisation model and solved to find optimal process parameters, when the objective is minimising production cost per workpiece, taking into account the related boundaries and limitation of this multi-pass turning operations. Numerical examples are given to demonstrate the suggested optimisation models.