Effects of Shot Peening Process Parameters

Abstract

The shot peening process parameters have a wide selection range, and due to the random nature of the process parameters, the selection represents a challenge. The following study presents a procedure based on 3-D single and multiple shots finite element models to study the effects of process parameters. A selection procedure is proposed to decrease the required experiments needed to set the peening parameters. This study provides a detailed investigation on the effects of process parameters such as shot size, shot velocity, and impact angle. The proposed selection procedure is used to determine the shot peening requirements for AISI 4340 steel a material commonly undergoes shot peening. The selected process parameters are then evaluated using the superposition principle using a simple statistical analysis. Consequently, the parameters are converted to Almen intensity a common measure in the shot peening process. This procedure decreases the selection band to handful of choices rather than the full wide spectrum of each shot peening parameter.

This thesis will provide a detailed insight based on experimental setup and finite element models on how different process parameters affect the peening results and this leads to represent and define the limits of beneficial shot peening through the following objectives. The First objective is to analyze the coverage progression of peening indentation through a novel approach using image processing. The second objective of this research is to develop parametric studies to examine the effects of the process parameters such as shot size, velocity, and impact angle. The third objective is to use a novel interpretation algorithm based on the super position principle in assessing the overall induced residual stresses. The fourth objective is a selection procedure designed to decrease the number of experiments required to select the peening parameters.

1. Introduction

Succeeding the evolution of materials science and the quest for creating lighter and durable metals, many methods of surface treatment arose under the light of this quest to help in enhancing the materials behaviour by proposing new treatments that target to strengthen and decrease the required weight to achieve performance requirements.  The shot peening process is one of the most interesting treatments as this process contains many control variables that lay under a stochastic and probabilistic nature more and more attempts are made every day to control these variables and to represent a controlled environment. Shot peening is simply defined as a mechanical method for pre-stressing materials surface by introducing beneficial compressive surface residual stresses that greatly improves the fatigue strength of metals under carefully controlled variables.  The usefulness of the shot peening process depends mainly on the amount of energy transferred in the workpiece as plastic deformation However, there are limits to shot peening that when are passed, i.e., over peened, would cause deteriorating effects in the material’s surface. Hence, the process effectiveness is established by means of global process control factors that contribute to the overall process success mainly the coverage, intensity and saturation.

The effectiveness of the process is to extend the fatigue life of components that are widely used in a cyclic loading nature. The peening action is straight forward as a shot impacts the surface a dent is created displacing the material by stretching the surface consequently during unloading of the shot the surface layers tend to recover elastic deformation but due to the plastic deformation a layer of compressive residual stress in generated as shown in figure 1.1. Inducing a layer of compressive residual stress reduces the chances of crack initiation be counteracting the tensile stresses that act to initiate the crack therefore lengthen the life of the peened component. Multiple parameters affect shot peening results namely shot diameter, impact velocity, impact rate, impact angle and target material mechanical properties.

Figure 1.1 Typical residual stress distribution beneath the shot impact location

The process is mainly controlled by two parameters peening intensity and surface coverage since these parameters are a representation of all other parameters in the process, therefore the intensity and coverage acts to indicate the effectiveness and to ensure quality of the process on multiple iterations. The intensity of peening measures the amount of energy in the process mainly in the form of kinetic energy generated from the shot momentum (hence it is related to the shot diameter and velocity) transferred from the shot to the target component during the impact. The peening intensity is measured and controlled by the Almen intensity. The surface coverage based on the SAE standard J2277 is defined as the percentage of a surface that has been impacted at least once by the peening media[1], thus the coverage percentage measures the total area dented and indicates the process completion.

The shot peening process was once called a process of black magic due to many uncertainties that are in the process because the process can result in a tougher and fatigue resistant component but can also result in a very high surface roughness with micro crack inclusions because of the impact dents, With the technological breakthrough of finite elements method and increased computational performance it is more practical to study the process control parameters to define a relationship between the process parameters and the target component properties by analytical and numerical methods. This study will provide detailed insight on how different process parameters affect the peening results and this leads to represent and define limits of the beneficial shot peening results

1.      Organization of the thesis

Chapter 2 presents a literature review on the shot peening process and its control methods. Chapter 3 presents the research rationale and objectives. Chapter 4 represents process coverage analysis based on novel image processing method. Chapter 5 presents the finite element analysis, introduces the general model idealization and the results extraction procedure from the single and multiple shot models. Chapter 6 presents experimental setup to shot peen low carbon steel and the experimental verification on the finite element models. Chapter 7 presents a parametric study on the process parameters effects on the induced residual stresses, each parametric study will aim to examine a certain parameter such as shot velocity, shot diameter, and repeated shots in the same domain. The parametric studies will be conducted on AISI4340 high tensile steel Chapter 8 presents a systematic selection procedure on a practical requirement using the results of the parametric studies. Chapter 9 presents the discussions and conclusions of the main findings in this research.

2. : Literature Review
   1.      Shot Peening Mechanics

The shot peening mechanics is relatively straight forward, as a shot impacts the surface a dent is created displacing the material by stretching the surface consequently during unloading of the shot the surface layers tend to recover elastic deformation but due to the plastic deformation a layer of compressive residual stress in generated[2].

Crucial to this analysis is to define how the mechanics of peening is related to the mechanics of material deformation, and thus to the generation of stresses.  It is established that the surface of the material upon unloading the shot exhibits an elongation because of the dent deformation, this elongation causes a corresponding compression beneath the impact location thus resulting in a compressive-tensile stress behavior [3].

2.      Stress generation mechanism

Figure. 2.1 graphically describes the deformation process; multiple shots bombard material surface “A” causing an elongation in the surface this elongation is counteracted by a compressive reaction from the material, the surface beneath it denoted as “B” is compressed due to the dent deformation and creates a tensile reaction thus both surfaces A and B create a compressive-tensile stress distribution.

A typical distribution can be identified as shown in figure 2.2,

• σ _max:  Maximum value of the induced compressive residual stress
• σ _surf: The value of the near surface compressive residual stress
• σ _ten: Maximum value of tensile residual stress
• t1: Maximum value of compressive residual stress depth
• t2: Stress shift depth (sign change)

Figure 2.1 Induced residual stress distribution by shot peening

Figure 2.2 Typical residual stress distribution

3.      Classification of Process Parameters

As introduced the shot peening process results depends mainly on the process parameters, thus classifying the process parameters will help in understanding the study of the parameters effects. Dr. S Kyriacou proposed to classify the process parameters as follows [4]

• Shot properties: Material type, diameter, geometry, density and hardness
• Target material mechanical properties: Hardness and yield strength
• Shot flow stream properties: shot mass flow rate, average shot velocity and angle of impact.

General influence of the process parameters

A study by Schiffer et al [5] examined the effect of control parameters on the resulting residual stress distribution using experimental trials. The influence of the hardness of the work piece material, thickness of the work piece, shot diameter, and shot velocity on the residual stress profile presented in figure 2.3. as the hardness of the work piece increase the maximum value of the residual stress increases but with a shorter distribution, a softer material will create a longer distribution but with a lower value of the residual stress. The velocity of the shot determines mainly the maximum value of the residual stress and the slightly affects the distribution. Comparatively the shot diameter greatly affects the depth of the residual stress distribution however on a near constant maximum residual stress value.

Figure 2.3 The effects of peening parameters on residual stress magnitude and distribution.

4.      Process control methods

As introduced the shot peening process relies heavily on the control of the process parameters, however not all parameters are easily qualified and quantified. Almen intensity and coverage percentage used to measure the effectiveness and reproducibility of the shot peening process. In the following section, these two control parameters are discussed in detail.

1.        Almen Intensity

The amount of energy developed in shot peening process is mainly generated form the change in momentum of the shot movement due to impacting the surface of the component, thus measuring the kinetic energy of the shots gives an indication on the total amount of energy transferred to the work piece. J. Almen and Black developed a measurement procedure to measure kinetic energy transferred by a shot to the work piece and called it as peening intensity. This measurement is achieved by determining the arc height of standardized thin metallic strips called Almen strips, with a standard tool called the Almen gage.[6] Figure 5 shows the measurement procedure. First the selection of the strip thickness; there are three standard thickness of strips manufactured from SAE spring steel-1070 hardened to 44-50 Rockwell hardness C, as summarized in Table 2.1 each thickness is used for a certain application. Secondly, the strip is then placed on a holding fixture which is made of a solid block to fix and sustain the test strips during peening. Thirdly the almen gauge is used to measure the arc height of Almen strips. From Figure 2.4 the process starts be fixing the test strip to a fixation block. Consequently, the strip is subjected to shot peening on only one side. The strip tends to bend toward the peened side and creates an arc that is relative to the peening intensity. The arc is measured and recorded in millimeters.

The measured arc heights of the test strips are drawn as a function of the length of exposure time to obtain a saturation curve. To determine the almen intensity the arc height is measured at a saturation time (T) which is the point on the curve in which doubling the exposure time (2T) produces no more than a 10% increase in arc height depicted in figure 2.5 .[7]

Table 2.1 Standard Almen strips dimensions

Type	Width	Length	Thickness	Application
N Strip	18.987 mm	76.098 mm	0.76 mm	Low intensity
A Strip	18.987 mm	76.098 mm	1.28 mm	Average intensity
C Strip	18.987 mm	76.098 mm	2.36 mm	High intensity

Figure 2.4 Almen intensity measurement procedure

Figure 2.5 Almen saturation curve

2.        Coverage Control

One of the process main parameters is the coverage percentage, normally users and peeners identify the coverage percentage needed before beginning peening, thus coverage in the shot peening process works as an indication variable that would indicate the percentage of peened surface area and thus the completion of the process. Coverage may be defined as the percentage of area that is dented in a given exposure unit time, [8]and according to the SAE specification SAE J2277, 2009[9]: “Coverage is defined as the percentage of a surface that has been indented at least once by the peening media. However, it is very difficult to obtain accurate measurements of coverage above 98%. Consequently, full coverage is therefore defined as being at least 98% denting of the surface to be peened.”. The mechanical improvement that results from shot peening however are not just based on the dents that arise from complete or incomplete Coverage requirements, hence, improvement comes primarily from surface residual compressive stress and, but to a lesser extent, surface work-hardening. It is becoming established that the maximum compressive residual stress levels and optimum work hardening generally occurs with significantly less than 100% coverage. Therefore, coverage control is an essential measure in the process.[10]

Achieving Full Coverage 

As more applications require achieving full coverage, it is imperative to understand how this would be achieved. Coverage to be measured as it had been defined would be to calculate the percentage of area left undented to the full area. Thus, if we would describe the process of indentation to cover all surface area given, one would easily understand that at the beginning of the process coverage rate is at the highest because of the higher probability that all shot impacts the surface with lower probability of overlapping.  However, as exposure time increases the surface area left uncovered has decreased by indentation and this result in higher probability of over lapping. Hence, the coverage rate is decreased. The coverage rate known to shot peeners to be an exponential rate as described in figure 2.6 where 100% coverage would be an asymptote to the curve that practically cannot be achieved [11].

Hence, Coverage development can be defined as;

A=-ln⁡100-C1100

Equation 2.1

Where “ln” stands for ‘natural logarithm’ and C1 is the coverage percentage measured after one pass. More on surface coverage will be discussed further in chapter 4 using a novel image processing algorithm based on MATLAB code to examine the coverage development.

Figure 2.6 Coverage development relative to exposure time

5.      Finite element models of shot peening process

Succeeding the advancement in the commercial finite element code and the computational processing abilities the analysis of the process received great consideration this gave endless possibilities to profoundly investigate the shot peening process.  

1.        Existing Models

The shot peening process is based on high speed dent formation on the surface of target material, this deformation is caused by the energy released from a shot impacting an elastic-plastic material. Al-Obaid [12] pioneered the first finite element simulation that was based on the analysis of OBAID computer program, the simulation was carried out by using 20 node solid brick elements and a mesh consisting of nine layers of elements through the depth of a workpiece as shown in figure 2.7.

Figure 2.7 Initial attempt to model the shot peening process

An axisymmetric model was introduced by Mori et al. [13] using a deformable shot and work piece Figure 2.8. a 2D dynamic analysis of a single rigid spherical shot impacting an elastic plastic component was developed by A. Levers et al [14] to examine the residual stress profile. A parametric study was developed by Schiffner et al [5] based on an axisymmetric model to simulate the residual stress distribution. Pedro et al, [15] examined  the influence of the work piece material mechanical properties on the residual stress distribution.

Figure 2.8 2D Axisymmetric finite element model

Using symmetry cell Meguid et al. [16] developed a reduced size model as shown in Figure 2.9 and due to the small size of this model it could simulate multiple impacts. Majzoobi et al [17]. using a similar model examined the effect of varying the impact velocity on the residual stress distribution.

Figure 2.9 Periodic symmetry cell model

A 3D multiple impact model was developed by Meguid et al.[2] shown in figure 2.10 to examine the residual stress distribution resulting from single and double impacts taking in consideration of the impact distance between the shots on the induced stresses profile. Guagliano et al, [18] developed a multiple impacts model as shown in figure 2.11 to examine the influence of repeated impact on the residual stress distribution

Figure 2.10 Separation distance in FEA of shot peening

Figure 2.11 3D axisymmetric multiple impact model

A full 3D model with silent boundary condition based on infinite elements was developed by Schwarzer et al.[19] as shown in Figure 2.13. This model can represent the process with predefined impact locations taking advantage of silent boundary conditions.

Figure 2.13 3D finite element model with infinite element on boundary conditions

A finite element model developed by Miao et al [20], presented a novel approach to simulate the process as a dynamic multibody simulation, moreover the model handled the impact locations by a random location generator code. Figure 2.14 shows the model in different methods of studying normal and oblique impacts.

Figure 2.14 A 3D random FEM in normal and oblique impacts

6.      Literature review summary

The peening action is achieved as a shot impacts the surface a dent is created displacing the material by stretching the surface consequently during unloading of the shot the surface layers tend to recover elastic deformation but due to the plastic deformation a layer of compressive residual stress the induced residual compressive stress counter acts the tensile strength to hamper crack initiation thus, extending and increasing the fatigue life and strength of a part. Almen intensity and coverage percentage are the main control parameters and are used to ensure the quality and reproducibility of the process. Determining the influence of process parameters using on experiments are expensive, impractical and time consuming. A selection procedure for the process parameters for a general use is lacking.

Most existing finite element methods studies the process assuming the impact location and number of shot do not  influence the results Although the finite element model developed by Miao et al [20] shown in figure 2.14, presented a new method of analyzing the process the induced residual stress is not collectively evaluated along the surface area.