ORIGINAL ARTICLE Experimental study on hole characteristics and surface integrity following abrasive waterjet drilling of Ti6Al4V/CFRP hybrid stacks Maojun Li 1 & Mingjie Huang 1 & Yiwei Chen 1 & Wei Kai 1 & Xujing Yang 1 Received: 5 December 2018 /Accepted: 23 August 2019 # Springer-Verlag London Ltd., part of Springer Nature 2019 Abstract Abrasive waterjet (AWJ) technology has been demonstrated to be a feasible manufacturing process for machining composite and titanium alloys to its specific advantages. However, AWJ cutting of composite/metallic hybrid stacks possesses several chal- lenges. In this work, experimental results were analyzed including hole quality and surface morphology/integrity during abrasive waterjet drilling of Ti6Al4V/CFRP stacks with different operating conditions. The influence of some key parameters involving traverse speed, hydraulic pressure, stand-off distance, and stack layup on hole quality was studied. Experimental array having 16 trials was designed based on full-factorial statistical analysis. Results showed that hole edge rounding was prevalent at the entrance irrespective of processing parameters due to erosion effect of abrasive particles. Ti6Al4V/CFRP layup configuration was preferred to obtain better geometrical consistency in terms of hole diameter. The variation of hole roundness was relatively high (up to 0.57 mm) when drilling hybrid stacks with Ti6Al4V/CFRP configuration. Ridges, fractured fibers, and small voids were generally observed on CFRP surface, while wear tracks and embedded fractured particles were typically found on titanium surface due to various material removal mechanisms. Keywords Abrasive waterjet . Ti6Al4V/CFRP stack . Drilling . Surface integrity 1 Introduction In order to meet the requirements of superior structural strength and corrosion resistance, hybrid structure comprising carbon fiber-reinforced plastic (CFRP) and titanium plate are extensively used in aerospace companies to utilize combined advantages and minimize the weaknesses of individual mate- rial [1]. As bolting and riveting are the major approaches for mechanical fastening CFRP/Ti stacks, the quality of machined holes generally affecting structural strength and fatigue per- formance is crucial for final application of multilayer stacked materials [2]. To avoid the influence of surface defects and improve geometrical accuracy of joint hole, CFRP/Ti stacks are preferred to be pre-loaded and clamped together, then drilled through both layers in single pass [3]. However, both CFRP and titanium alloy are difficult-to-cut materials. Traditional cutting of CFRP laminate normally generated several defects including delamination and fiber burrs on hole exit [4]. Excessive cutting force also leads to debonding of fiber-matrix interface and damage extension during cutting process [5–7]. Regarding machining titanium alloys, the in- herently low thermal conductivity, high hardness, and strength of titanium alloy generally result in relatively high machining temperature and fast tool wear [8]. Therefore, there are several challenges to cut CFRP/Ti stacks due to their poor machin- ability and different physical properties, especially on the lay- er interface [9]. Abrasive waterjet (AWJ) cutting is widely applied as a nonconventional machining method experiencing rapid devel- opment in industrial applications, due to its advantages includ- ing high machining versatility and flexibility, low cutting heat, wide range of process thickness, and small cutting force [10]. During AWJ machining process, abrasive particles are mixed into the high-pressurized water to form the high-speed abra- sive waterjet stream, which impacts the surface of the target material at an extremely fast speed, causing the erosion and removal of workpiece material. In general, AWJ machining process is employed to process several types of materials in- cluding nickel and titanium superalloys, ceramics, and carbon fiber-reinforced composites [11]. Many researchers studied * Xujing Yang yangxujing@hnu.edu.cn 1 State Key Laboratory of Advanced Design and Manufacture for Vehicle Body, Hunan University, Changsha 410082, China The International Journal of Advanced Manufacturing Technology https://doi.org/10.1007/s00170-019-04334-5