Ultrasonic Testing for the Measurement of Process Capability in Piston Manufacturing Line

Transcription

1 National Seminar & Exhibition on Non-Destructive Evaluation, NDE 2014, Pune, December 4-6, 2014 (NDE-India 2014) Vol.20 No.6 (June 2015) - The e-journal of Nondestructive Testing - ISSN Ultrasonic Testing for the Measurement of Process Capability in Piston Manufacturing Line Dr. K. N. Subramanya 1, Vijayakumar M N 2, Prashant V 3, Vikram N B 4, M alleshi S Talwar 5 1,2,3,4,5 R V College of Engineering, M ysore road, Bangalore. India 1 sansa96@gmail.com, 2 vijayakumar@rvce.edu.in, 3 prashanthv@rvce.edu.in, 4 vikramnb@rvce.edu.in, x mechmallesh49@gmail.com Abstract: Piston plays vital role in mobility of any vehicle. The manufacturing of piston is highly precise as it includes various sequences of operations such as casting of the aluminum with cast iron rings, outer diameter turning, grooving and chamfering of the piston ring hole. During the process, material causes micro shrinkage, cold shut, porosity and other variation in composition. In order to ensure the quality of the piston manufacturing process, the specimen is subjected to Ultrasonic testing. In this paper the process of manufacturing piston is studied, the Ultrasonic inspection is carried out in extruded area of piston and the flat area of piston, the Ultrasonic back wall effect of the specimen is recorded. The results are verified and validated with master piston. Any deviations in the specimen are not acceptable. Finally, the control charts are plotted and process capability index is determined. Key words: Piston, Ultrasonic Testing, Flaw, Control chart 1.0 Introduction Non- Destructive Testing (NDT) is a way of testing without destroying. This means that the component casting, weld or forging, can continue to be used and that the non destructive testing method has done no harm. NDT ensures that materials can continue to operate to their highest capacity with the assurance that they will not fail within predetermined time limits. NDT can be used to ensure the quality right from raw material stage through fabrication. Few of the popular and frequently used NDT methods are Magnetic Particle Inspection (MPI), Eddy Current test (ED), Ultrasonic Testing (UT), Die Penetrant Testing (DPT), X Radiography Testing (RT) etc. Among all these methods, Ultrasonic testing is popular and most widely used method for detecting the flaws. Ultrasonic Testing This method uses high frequency sound (20Hz to 20 khz) detect imperfections or changes in properties within the materials. It can also be used to measure the thickness of a wide range of metallic and non-metallic materials where access from one side only is available. Different probes are used for different materials. Internal flaw can be identified with clear indication of position of the flaws and number of flaws.

2 2.0 Process S tudy of Manufacturing of Piston A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. In an engine, piston purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod. In some engines, the piston also acts as a valve by covering and uncovering ports in the cylinder wall. Pistons are commonly made of a cast aluminum alloy for excellent and lightweight thermal conductivity. Piston features include the piston head, piston pin bore, piston pin, skirt, ring grooves, ring lands, and piston rings. Piston contain some internal defects those are causing major failure. Hence identification of internal defect is carried out with Non- destructive testing. Piston is manufactured in two stages, firstly through foundry and then followed by machining operations. Quality inspection is carried out in separate unit. In the foundry unit, aluminum alloy is melted up to c to c and poured into cavity made by the die. In this process, composite material can be obtained to withstand high pressure generated, during piston operation. The figure 2.1 below depicts the detailsed process flow of manufacture of piston. Fig. 2.1: The process flow for manufacturing of piston

3 3.0 Problem definition and Objectives of study The initial obser4vation through company records indicated that rejection rate of the piston is 140 out of 1545 components, due to the occurrence of the internal flaws in the casted material. These defective components directly affect the variation in skirt diameter and decreasing process capability to The tests were conducted to find out such flaws and the pistons were rejected based on the order requirement of selection and rejection procedure. The reasons for the occurrence of such flaws were not recognized and the patterns of flaws were not plotted. This provided an opportunity to work in filling this gap. In order to fulfill the requirements and the gap identified, the study tem set an objective to measure and characterize the internal flaws, to identify the causes for such flaws and to statistically control the skirt diameter used process capability measurement. 4.0 Methodology adopted for experimentation and measurement using Ultrasonic tester The following methodology was adopted to thoroughly study the defects through ultrasonic testing method and classify it as crack, cold shut, blow hole, shrinkage, hot tear etc. The rejected pistons were analyzed for their pattern, number of defects and the location of defects using ultrasonic inspection. Non conformity charts were drawn using P chart and the major defects were identified using Pareto analysis. S tep 1: Ultrasonic inspection was carried out using DIGISCAN ultrasonic flaw detector with normal probe of 10mm diameter and frequency of 2MHz. The flaw detector is as shown in Fig.4.1. Initially the instrument was calibrated with master block to correct the deviation in the instrument. Fig. 4.1: Ultrasonic Flaw Detector

4 S tep 2: Velocity and Range value of the instrument were reset as per the material. Of study. The inspection was performed on required gain energy. Defects were identified though the observation of disturbance in initial and final echo displayed on the monitor of UT M achine. A sample flaw identification is depicted in the Fig. 4.2 and is shown below: Fig.: 4.2 Ultrasonic CRT S creen for S ingle Flaw Location S tep 3: Defect were classified as crack, cold shut, blow hole, shrinkage, hot tear etc and report were generated that clearly indicated that a piston of 20 mm thickness has a flaw at 18 mm depth as shown in Fig.4.3 below: Fig.: 4.3 Ultrasonic Flaw Report

5 S tep 4: During testing, number of defects were listed from sample data. Table 4.1 shows the number flaws present in each piston. Table 4.1: Ultrasonic Flaw Report S l. No S ample number Number of defects S l. No S ample number Number of defects 1 P1 2 9 P9 3 2 P P P P P P P P P P P P P8 2 S tep 5: Using the data of defects for the sample pistons, M initab statistical software was used to perform a test using Non conformities control chart i.e., P chart and the analysis is depicted in the Fig.: 4.4. P chart was created to evaluate the p rocess and identify whether the defects are in control or outof control. The specimen is accepted or rejected based on the points lying inside or outside control limits respectively. Fig: 4.4 Depiction of P chart using Minitab S tep 6: Pareto chart was plotted for analysis of experimental data to highlight the most important among a set of factors. M ajor issues were quickly identified and resolved. Minitab software with

6 quality tool was used to analyze the experimental data for Pareto analysis and the same is shown in Fig. 4.5 below: Fig. 4.5 Pareto chart for vital few trivial many defects S tep 7: Table 4.2 shows the skirt diameter value measured during piston manufacturing operation. Skirt diameter is contributing to maximum number of defects. Hence, the inspected defect free piston outer diameter is measured through gauge and these value are listed below: A Sample size of 3 is considered with skirt diameter: 50±5 for study: Table 4.2 Measurement of skirt diameter of a piston O uter head O uter body O uter bell diameter (D1) diameter (D2) diameter (D3) O uter head diameter (D1) O uter body diameter (D2) O uter bell diameter (D3)

7 S tep 8: Control chart was plotted with help of M initab software. Skirt diameters values were used to determine X bar and R charts and the same is shown in figures 4.6 below: Xbar-R Chart of skirt diamater Summary Report Is the process mean stable? Evalu ate th e % of ou t-of-control subgroups. 0% > 5% Comments The process mean is stable. No subgroups are out of control on the Xbar ch art. Yes 0.0% No 8 Xbar and R Charts Investigate any out-of-control su bgroups. UCL=7.630 Mean 4 _ X= LCL= UCL=9.62 Range 5 _ R= LCL= Control li mits use StDev(within) Subgroups: 19 Mean: StDev( within): StDev(overal): Fig 4.6 X bar and R Chart of Skirt diameter S tep 9: Process capability studies were done to check the capability of producing virtually all conforming product. Statistical process controls used to monitor the process and conventional acceptance efforts reduced. This helped in cost savings in eliminating non-value added inspections and eliminated scrap, rework and increases customer satisfaction. The following figure 4.7 indicates the process capability report of skirt diameter of selected samples which includes determination of the process capability (Cp = 1.42) and process performance (Cpk = 0.83). Process Capability Report skirt daimeter Process Data LSL -9 Target * USL 10 Sample Mean Sample N 19 StDev(Overall) StDev(Within) LSL USL Overall Within Overall Capability Pp 1.35 PPL 1.91 PPU 0.79 Ppk 0.79 Cpm * Potential (Within) Capability Cp 1.42 CPL 2.00 CPU 0.83 Cpk Fig 4.7- Process capability indices obtained from Minitab

8 5.0 S ummary of Results: The manufacturing process of piston from scrap to end product was studied. The initial defects in the casting were explored with ultrasonic testing equipment. Then, the root causes of the defects in piston were identified. Finally, the process capability study was conducted and it was found that out of all casting defects, crack and shrinkage were the major defects occurring in skirt area of piston.a comparison of before and after study indicated that the process capability (Cp) increased from 0.89 to 1.42) and process performance(cpk) increased from 0.56 to This has enhanced the confidence among the people in the testing department on the benefits of using NDT testing and also ability of the test to enhance the quality parameters, which is helpful for better salability and satisfaction of internal customers. 6.0 References [1.] Antonio Toro, Juan A. Chavez, M iguel J. Garcia Hernandez, Andres Bulked, Peters Tomek, Gabor Tóth, Anna Gironés, Jordi Salazar (2013); Ultrasonic inspection system for powder metallurgy parts; M easurement 46 (2013), pp: [2.] Bernard H. Hertlein (2013); Stress wave testing of concrete; Construction and Building Materials: 38 (2013), pp: [3.] Fitting DW, Adler L.(1981), Ultrasonic spectral analysis for nondestructive evaluation. New York: Plenum Press. [4.] Vanlanduit Steve, Guillaume Patrick (2003), On-line monitoring of fatigue cracks using ultrasonic surface waves; NDT&E International Journal. [5.] Renaldas Raiˇsutis, RymantasKazys, Egidijus ˇZukauskas (2010); Application of ultrasonic guided waves for non-destructive testing of defective CFRP rods; NDT&E International 43, pp: Acknowledgement: This study is a part of funded project sponsored by Vision Group of Science and Technology (VGST), Government of Karnataka under Karnataka Fund for Infrastructure Strengthening in Science and Technology Level II ( ) for Establishing Non Destructive Testing Laboratory for Post Graduate Courses and Research. We sincerely thank VGST for their financial support for purchase of Ultrasonic Testing Equipment.