Empirical Modelling of Water-jet Peening of 6063-T6 Aluminium Alloy

Transcription

1 Empirical Modelling Water-jet Peening 6063-T6 Aluminium Alloy N Rajesh, Non-member N Ramesh Babu, Non-member In this article, peening studies carried out on 6063-T6 Al alloy with high pressure water are presented. These studies have been carried out employing a round nozzle by varying the process parameters, such as, water-jet pressure, stand f distance, number passes and traverse speed at three levels using Taguchi s L9 orthogonal array experimentation. The influence these process parameters on residual stress, hardness and surface roughness are investigated. Finally, an empirical model, based on the mean analysis that relates the results with process parameters, is presented. Keywords : Water-jet peening; Residual stress; Hardness; Surface roughness; Empirical model INTRODUCTION Among the different surface treatment processes, water-jet peening is an emerging surface treatment process in which high velocity water droplets continuously impinge over the component surface. These droplets produce high peak loads that can cause localized plastic deformation and thereby creates compressive residual stresses on the surface the component. It fers several benefits, such as, improved fatigue strength and resistance to corrosion, highly flexible process that can simultaneously be used along with other material removal processes possible with high-pressure water-jets. Unlike shot peening, water-jet peening is an environment-friendly process since it does not produce any dust during the surface treatment materials. It is a forcecontrolled treatment that generates compressive residual stresses in the surface layers without modifying the surface topography. Unlike shot peening process, water-jet peening process is governed by the velocity jet, that is, the velocity water droplets impinging on the material surface. In contrast to shot peening, this process is less expensive since shot peening requires substantial investment on the shot material whose cost depends on the size, shape, density, hardness, yield strength and modulus elasticity shots used. This process is capable treating the entire surface uniformly thus requiring simplified quality assurance procedures. Yamuchi, et al 1 made an attempt to investigate experimentally the water-jet peening process. From their investigations, it was observed that the water-jet peening enhanced the fatigue life carbon steel and stainless steel. It is due to an increased crack initiation life and reduced crack propagation life. Though a few attempts were made to perform water-peening experiments in under water 2, most the researchers performed investigations only in air 3 5. The mechanism water-jet peening is essentially due to an impingement droplets water on the surface. These droplets generate a surface pressure distribution producing peak loads that exceed the yield strength the material. These peak loads induce N Rajesh and N Ramesh Babu are with the Manufacturing Engineering Section the Department Mechanical Engineering, Indian Institute Technology Madras, Chennai This paper was received on May 28, Written discussion on the paper will be received until November 30, localized plastic deformation, which is constrained by the surrounding material thus inducing high compressive residual stresses on the surface. In water-jet peening, several parameters, such as, nozzle geometry, water-jet pressure, stand f distance and peening duration influence the process results 1. Different geometries that were employed for water-jet peening include round nozzles, conical nozzles, flat nozzles and Abrasive Water-jet (AWJ) cutting nozzles. Experiments were conducted on 1100-H14 aluminium specimens by employing stand f distances in the range 80 mm mm and water pressures 100 MPa and 140 MPa using conical nozzle spray angle 20. Various parameters, such as, surface roughness, hardness and residual stresses were measured 6. The magnitude compressive residual stresses induced at a sub-surface depth 50µ is about 15% higher than that in the base material hardness and it is due to the negligible amount work hardening on the material surface. This article covers the peening studies that examine residual stresses, hardness and surface roughness induced on 6063-T6 aluminium alloy treated with water-jets. It presents the approach adopted for conducting the experiments with round nozzle spray angle 60. The Taguchi s design experiments was used for conducting the experiments. The target parameters residual stress, hardness and surface roughness on the surface were measured before and after water-jet peening. EXPERIMENTAL PROCEDURE Figure 1 shows the schematic representation water-jet peening. It consists a high pressure intensifier, round nozzle, CNC positioning table and a catcher tank. The intensifier pressurizes the water in the range 60 MPa MPa and the maximum water flow rate depends on the size the nozzle. The pressurized water is delivered to the nozzle through a high-pressure stainless steel hose. The geometry nozzle used for peening experiments is shown in Figure 2. Round nozzle with a conical aperture was used as a tool for water-jet peening. The nozzle, which is fixed to the water-jet head, is kept normal to the workpiece. In this study, water-jet pressure, stand f distance, number passes and traverse rate are considered as input process parameters for water-jet peening. 22 IE(I) Journal-PR

2 Table 1 Parameters used for water jet peening Variable Parameters Low Medium High P, MPa SOD, mm NOP TR, mm/min Figure 1 Schematic representation water-jet peening process Figure 2 Round nozzle diagram The material used for the experiments was 6063-T6 aluminium alloy. The characteristics water-jet peening were examined on a square plate 40 mm 40 mm with a thickness 10 mm. For peening studies, the specimens were initially polished using belt grinder having different grades emery paper. After that it was polished in a rotating disc machine using alumina powder. Nine polished specimens were considered for peening experiments. Input parameters water-jet peening, such as, water-jet pressure, stand f distance, number passes and traverse rate were varied to examine their effect on peening results. To analyze the influence various process parameters on the surface treated with water-jets, surface roughness, hardness and residual stresses were measured before and after water-jet peening. The surface roughness was measured using stylus type roughness measuring instrument. The hardness was measured by Vickers hardness testing machine. The residual stress variations were measured on the surface by Rigaku stress analyzer. TAGUCHI S DESIGN OF EXPERIMENTS Design experiments (DOE) is the process planning the experiments considering different parameters influencing the Table 2 Allocation parameters for water jet peening 6063-T6 aluminium alloy Jet Pressure, MPa Stand f Distance, mm Number Passes Traverse Rate, mm/min process and different levels for these parameters so as to acquire the necessary data for building statistical models that can aid in predicting the performance any process. Fractional Factorial Design An Orthogonal Array (OA) is a fractional factorial matrix, which assures a balanced comparison levels any factor or interaction factors. Frequently used orthogonal arrays in design experiments are two levels and three levels OA. In this study, four parameters are chosen. Since, L9 OA considers four parameters at three levels, it is used to select each parameter value for conducting the experiments. The parameter levels are fixed, based on the preliminary experiments. The parameters used following L9 OA are given in Table 1. The allocation parameters is given in Table 2. RESULTS AND DISCUSSION In this section, the influence peening condition on various parameters, namely, residual, stresses, hardness and surface roughness are discussed. Influence Peening Conditions on Residual Stresses In Figure 3, the variation residual stresses on the surface before and after peening is shown. The influence different process parameters on residual stresses is analyzed by means mean Figure 3 Variation residual stresses on 6063-T6 aluminium alloy Vol 86, September

3 P1 : 175 MPa; P2 : 200 MPa; P3 : 225 MPa; S1 : 5 mm; S2 : 7 mm; S3 : 10 mm; N1 : 2; N2 : 3; N3 : 4; T1 : 20 mm/min; T2 : 30 mm/min; and T3 : 40 mm/min Figure 4 Mean response-control factors against residual stresses response analysis which is shown in Figure 4. From these results, a number observations can be drawn. When the specimen is treated using a jet pressure 200 MPa, and stand f distance 5-mm and by traversing the jet thrice over the surface employing traverse rate 40 mm/min, the maximum compressive residual stress 56 MPa is detected on the surface. This corresponds to 64% improvement residual stresses on the surface treated with water-jets. With a traverse rate 30 mm/min, jet pressure 225 MPa and stand f distance 5-mm, these stresses are seen to be only 45 MPa and are compressive in nature. These conditions indicate in 32% improvement residual stresses. In contrast, the specimen treated with a jet pressure 225 MPa, traverse rate 20 mm/min and stand f distance 10-mm, show a reduction in surface residual stresses, that is, 34 MPa (compressive). This corresponds to 7% improvement residual stresses on the surface treated with water-jets in comparison with the stresses on unpeened surface. In order to identify the most significant parameter influencing the residual stresses, hardness and surface roughness, ANOVA analysis was performed. From the ANOVA analysis, results presented in Table 3, one can infer the strongest contribution parameter as stand f distance with nearly 28% contribution in improving the residual stresses on the surface treated with water-jets. A similar trend is reported by earlier researchers 3 6 while peening aluminium samples with water-jets. The reasons for an improvement residual stresses on the surface 6063-T6 aluminium alloy can be attributed to a number observations. At higher stand f distance 10-mm, the jet disintegrates into droplets and then diverges to a large extent with lesser droplet velocities that can produce an effect to lower compressive residual stresses. On the other hand, the specimen Table 3 ANOVA analysis (compressive residual stress) Source Pool Degrees (ρ), % treated with water-jet located at a stand f distance 5-mm is found to induce higher compressive residual stresses due to the droplet region being confined to a smaller zone with significantly higher droplet velocities producing higher residual stresses by water peening. Thus, this study clearly indicates the suitable choice stand f distance for effective peening aluminium alloy to induce higher compressive residual stresses on its surface. The data generated from L9 OA experimentation were employed to develop an empirical model which can be represented as σ RS = p d 0.75 (d 7.5) n 0.35 v (1) where p is the pressure; d, the stand f distance; n, the number passes; v, the traverse rate; and σ RS is the residual stresses. Influence Peening Conditions on Hardness In Figure 5, the variation hardness on the surface before and after peening is shown. The influence different process parameters on hardness is analyzed by means mean response analysis (Figure 6). When the specimen is treated with a jet pressure 175 MPa, and a stand f distance 5-mm employing a traverse rate 20 mm/min, the surface hardness 31.8 VHN (Vickers Hardness Number) is induced. This corresponds to 15% improvement hardness on the surface treated with water-jets. With a traverse rate 20 mm/min, jet pressure 200 MPa and stand f distance 7mm, the surface hardness is seen to be only 30.7 VHN. These conditions result in 9% improvement in its surface hardness after peening. But, when the specimen is treated with a jet pressure 225 MPa, traverse rate 20 mm/min and stand f distance 7 mm, the hardness induce on the surface is seen to be 29.5 VHN. From the ANOVA analysis results given in Table 4, it can be seen that the jet pressure is seen to be the strongest contributing parameter with nearly 42% contribution in improving the hardness surface treated with water-jets. This particular observation is in agreement with the observation made by earlier Figure 5 Variation hardness on 6063-T6 aluminium alloy with water peening p d n e v P1 : 175 MPa; P2 : 200 MPa; P3 : 225 MPa; S1 : 5 mm; S2 : 7 mm; S3 : 10 mm; Error N1 : 2; N2 : 3; N3 : 4; T1 : 20 mm/min; T2 : 30 mm/min; and T3 : 40 mm/min Total Figure 6 Mean response-control factors against hardness 24 IE(I) Journal-PR

4 Table 4 ANOVA analysis (hardness) Source Pool Degrees (ρ), % p d n e v Error Total Figure 7 Variation surface roughness on aluminium alloy 6063-T6 with water peening researchers 6 7 when aluminium specimens were treated with water-jets. The reasons for an increase in hardness on the surface 6063-T6 aluminium alloy can be attributed to a number observations. At higher stand f distance 10-mm, the induced compressive residual stresses is less. This can lead to a decrease in the depth sub-surface layers and that can produce an effect resulting in higher hardness on the surface the material. On the other hand, the specimen located at a stand f distance 5-mm can induce lower hardness on the surface but can increase the hardness in the sub-surface layers than the hardness on the original surface. Hence, this study clearly indicates the suitable choice pressure for effectively peening the surface aluminium alloy to produce higher hardness on the water peened surfaces. By using the data generated from L9 OA experimentation, an empirical model was developed which can be represented as VHN = (p 200) ( p 200) d n v (2) where p is the pressure; d, the stand f distance; n, the number passes; v, the traverse rate; and VHN is the Vickers Hardness Number. Influence Peening Conditions on Surface Roughness In Figure 7, the variation roughness on the surface before and after peening is shown. The influence different process parameters on surface roughness is analyzed by means mean response analysis which is shown in Figure 8. When the specimen is treated with a jet pressure 175 MPa, and a stand f distance 7 mm employing a traverse rate 20 mm/min, the roughness on the surface is modified to 1.10 µm from 0.90 µm. This corresponds to 64% increase in the roughness the surface treated with water-jets. With a traverse rate 40 mm/min, jet pressure 200 MPa, number passes 3 and stand f distance 5-mm, the roughness on the water peened surface is found to be only 0.82 µm. These conditions result in 60% increase in water peened surface roughness. But, when the specimen is treated with a jet pressure 225 MPa, traverse rate 20 mm/min and stand f distance 10 mm, the roughness over the surface is found to be 0.63 µm. This has resulted in 58% increase in roughness water peened surface when compared to the roughness surface before peening. From the ANOVA analysis results given in Table 5, it can be seen that the number passes is seen to be the strongest contributing P1 : 175 MPa; P2 : 200 MPa; P3 : 225 MPa; S1 : 5 mm; S2 : 7 mm; S3 : 10 mm; N1 : 2; N2 : 3; N3 : 4; T1 : 20 mm/min; T2 : 30 mm/min; and T3 : 40 mm/min Figure 8 Mean response-control factors against surface roughness parameter with nearly 38% contribution in changing the roughness induced on the surface treated with water-jets. The reasons for an increase in roughness on the surface 6063-T6 aluminium alloy can be attributed to a number observations. At higher stand f distance 10-mm and higher number passes, the jet diverges with lesser droplet velocities at the periphery resulted in less variation surface roughness. On the other hand, the specimen located at a stand f distance 5-mm has modified the surface roughness to the least due to the droplet region being confined to a smaller zone with significant droplet velocities but not causing any erosion to the surface. Hence, this study clearly indicates the suitable choice stand f distance and number passes for peening the surface aluminium alloy effectively in maintaining the surface roughness to the same level as was noticed on the surface unpeened specimen. An empirical model was developed with the data collected from L9 OA experimentation which can be represented as Ra = p d n (n 3) v (3) where p is the pressure; s, the stand f distance; n, the number passes; t, the traverse rate; and Ra is the surface roughness. These empirical models can be employed to predict the variation in surface residual stresses, hardness and surface roughness on 6063-T6 aluminium alloy treated with water-jets employing the Table 5 ANOVA analysis (surface roughness) Source Pool Degrees (ρ), % p d n v e Error Total Vol 86, September

5 Table 6 Comparison predicted and experimental values residual stresses, hardness and surface roughness on water peened aluminium alloy P, MPa d, mm n v, mm/min Residual Stresses, MPa Hardness, VHN Surface Roughness, µm Predicted Experimental Predicted Experimental Predicted Experimental pressures in the range 175 MPa-225 MPa, stand f distance in the range 5 mm-10 mm and traverse rate in the range 20 mm/min-40 mm/min. The same relationships can also be employed to select the process parameters for producing the desired residual stresses, hardness and surface roughness on the surface to be treated with high pressure water-jets. In Table 6, the predicted values along with experimental results are compared for the purpose demonstrating the applicability empirical models for predicting process results with the process parameters known a priori. From these results, one can notice a slight deviation in the results predicted with the model compared to the results obtained from the experiments. This can be attributed to the nature experimentation planned using L9 OA experimental method. CONCLUSION Attempts have been made to investigate the influence high pressure water-jets on 6063-T6 aluminium alloy resulted in empirical models that can predict the residual stresses, hardness and surface roughness at different parameters, such as, jet pressure in the range 175 MPa-225 MPa, traverse rate in the range 20 mm/min-40 mm/min and stand f distance 5 mm-10 mm. The ANOVA analysis carried out with the experiments using L9 OA experimentation clearly indicates the stand f distance as the most significant factor in improving the residual stresses on the surface treated with water-jets. Water-jet pressure is the most significant factor in improving the hardness and number passes is the most significant factor in increasing the surface roughness. Lower stand f distance 5-mm is found to be better over higher stand f distances due to the fact that the droplets produced at low stand f distance possess the capabilities for effective peening surfaces. However, the influence other parameters in different ranges needs to be investigated in future. ACKNOWLEDGMENT The authors express their sincere thanks to Science and Engineering Research Council Department Science and Technology, Government India for the financial support. REFERENCES 1. Y Yamuchi, H Soyama, Y Adachi, K Sato, T Shindo, R Oba and R Oshima. Suitable Region High-speed Submerged Water-jets for Cutting and Peening. JSME International Journal (Series B), vol 38, no 1, 1995, p M Mochizuki, K Enomoto, S Sakata, H Saito and K Ichie. A Study on Residual Stress Improvement by Water-jet Peening. Proceedings the Fifth International Conference on Shot Peening, Oxford University, Christ Church: England, 1993, p H K Toenshf, F Kroos and E Brinksmeier. Mechanical Surface Bonding by Water Peening. International Conference on Surface Engineering, Bremen, Germany, 1993, p H K Toenshf and F Kroos. Increasing Fatigue Strength by Water Peening. International Conference on Residual Stresses, Baltimore, Maryland, USA, 1994, p H K Toenshf and F Kroos. High Pressure Water Peening a Mechanical Surface Strengthening Process. Annals the CIRP, vol 46, 1997, p R Daniewicz and. S D Cummings. Characterization a Water Peening Process. Transactions the ASME, Journal Engineering Materials and Technology, vol 121, 1999, p M Ramulu, S Kunaporn, D Arola, M Hashish and J Hopkins. Water-jet Machining and Peening Metals. Transaction the ASME, Journal Pressure Vessels and Technology, vol 122, no 1, 2000, p H K Toenshf, F Kroos and M Hartmann. Water Peening an Advanced Application Water-jet Technology. Proceedings the Eighth American Water- jet Conference, Houston, Texas, USA, 1995, p K Hirano, K Enomoto, M Mochizuki, E Hayashi and S Shimizu. Improvement Residual Stress on Material Surface by Water-jet Peening. Transactions the Fourteenth International Conference on Structural Mechanics in Reactor Technology (SmiRT 14), Lyon, France, 1997, p S Kunaporn, M Ramulu, M Hashish and J Hopkins. Ultra High- pressure Water-jet Peening- Part I: Surface Texture. Proceedings the Eleventh American Water-jet Conference, vol 1, 2001, p B M Colosimo, M Monno and Q Semeraro. Process Parameters Control in Water-jet Peening. International Journal Materials and Product Technology, vol 15, no 1, S Kunaporn, M Ramulu, M Hashish and J Hopkins. Ultra High- pressure Water-jet Peening- Part II: High Cycle Fatigue Performance. Proceedings the Eleventh American Water-jet Conference, vol 1, p IE(I) Journal-PR