Effects of machining method and technique on galvanized steel material properties

Abstract

Machining or metal cutting operation is the process of removing unwanted material in the form of chips, from a block of metal, using cutting tool is the most widely used industrial practice for which sustainable manufacturing is an appropriate approach. Aligning with rapidly changing scenario in manufacturing industries, applications of optimization techniques in metal cutting processes is essential for a manufacturing unit to respond effectively to severe competitiveness and increasing demand of quality product in the market. When machining structural parts with high material removal rates, the stiffness of the machine tool can be one of the limited factors due to the machining vibrations or called chatter vibrations. Computer Numerical Controller (CNC) machining is one of the machining method used to get a high accuracy results in product and Wire Electrical Discharge Machining (WEDM) is one of the advanced technique primarily used for hard metals or those that would be very difficult to machine with traditional techniques, such as conventional milling operation. Due to this non-contact operation towards the work piece, it shallower the heat-affected zone (HAZ) which may effect on the material properties. In this paper, galvanized steel has been used to identify the changes in its mechanical properties by using tensile test with both machining methods and techniques to produce the T-bone specimen. Hence the static or tensile properties of the specimen is obtained, a good machining procedure could be determined. Therefore, it will decrease the strength of material and increase the durability and reliability of product.

Keywords

Machining mechanical properties galvanized steel

1. Introduction

Steel are iron-based metals to which other chemical elements have been added. The addition of these elements can create new constituents in the metal that will affect its mechanical properties characteristics in its hardness, tensile and yield strength, ductility and machinability. Due to the many favorable characteristics, steel is well suited and widely used for a broad number of engineering applications. It exhibits a variety of excellent mechanical properties, such as strength, toughness and ductility [1]. In many industrial sectors and especially the aerospace industry, increasingly complex parts have to be manufactured efficiently, while achieving the required surface qualities and reducing the production costs. The process in machining or metal cutting defined as a process of raw material that was cut into a desired final shape and size by a controlled material-removal process. This method either in “traditionally” or “conventionally” machining processes of machine tools. Lathes, milling machines, drill presses, or others, were used with a sharp cutting tool to remove material to get a desired geometry [2]. Number and various advanced machining techniques were used to improvise the cost consumptions include precision CNC machining, electrical discharge machining (EDM), electro-chemical erosion, laser cutting, or water jet cutting to shape the metal workpieces for high quality and precision. Manufacturing and human evolution are strongly related as machining capability has always influenced culture, determined military and economic power [3]. Several studies on effects of machining defects of material mechanical properties using a variety of machining processes have been reported. [4] highlighted that the EDM cutting left surface defects cause slight reduction in fatigue life of an Inconel 718 alloy. [5] stated that the EDM spark dramatically increases the surface roughness and reduces the fatigue strength of the SiC/Al metal matrix composite. [6] defined that Wire-EDM cutting parameters significantly affect the surface quality of AZ61magnesium alloy. Therefore, the purpose of this case study is to investigate the effects of machining by wire-EDM cutting process on galvanized steel effects the HAZ compare to milling cutting machine.

1.1 Working principle of conventional milling machining

In milling operation metal is removed by a rotating multipoint cutter which is fitted on the arbor of the milling machine and fully operated by human as Fig. 1. [7] had determined a technical concept and machining possibilities for milling of micro structures by using a fast tool servo (FTS) for high precision product. The milling machining suits numbers of particular product in certain area. In three-dimensional free-form fabrication, which is required for manufacturing dental restoratives, milling machining is needed but the machinability must be predicted in advance to determine the proper machining conditions, including the workpiece temperature [8].

Figure 1.

Conventional milling machine.

1.2 Working principle of electrical discharge machining

Electrical discharge machining (EDM) is a nonconventional material removal process assisted with computer simulation and NC data and it is widely used to produce complex dimensions of product, punches,molds and dies. The working principle of EDM process is based on the thermoelectric energy. The thermoelectric energy, form spark was created between a work piece and an electrode submerged in a dielectric fluid with conduction of electric current. Several aspects related to surface quality and metal removal rate has been identified by [9] on which are the most important parameters from the point of view of selecting the optimum condition of processes as well as economical aspects in EDM machining. [10] measured machining performance and proposed optimal WEDM machining for SiCp reinforced Al7075 matrix composites (Al7075/SiCp). [11] investigated the effects of the main process parameters on the MRR and surface quality including surface roughness (Ra) and surface crack density (SCD) in machining Ti6Al4V and defined WEDM machining characteristic.

Table 1
Mechanical properties of galvanized steel [12]

Material properties	Value
Density	7.860000E+03 kg/m ${}^{3}$
Young’s modulus	2.000000E+11 N/m ${}^{2}$
Poisson ratio	0.33
Yield stress initial strength hardening strenght limit	3.450000E+08 N/m ${}^{2}$ 528 Mpa 550 Mpa

Figure 2.

Wire EDM machine.

The machine capability depends on its cutting operation procedure and the quality effects on the workpiece. In this case study, two methods of machining which are by conventional milling and WEDM machining has been used and the effects on Galvanized Steel characteristic has been investigated. Galvanized Steel was extremely used as a fenced in road traffic area due to its mechanical properties. Table 1 shows the mechanical properties for galvanized steel [12].

The machining accuracy, surface finished and time required to complete a job is extremely predictable, are all required factors of machining procedure to comply and control the quality of the material and product.

2. Methodology

In this case study, tensile test has been carried out to investigate the effects on material characterization of galvanized steel mechanical properties due to the machining process by EDM wire cutting and milling machine. The experimental procedure follows:

2.1 Specimens preparation

In order to test this galvanized steel, an “ASTM Designation: E 8M-04” was referred to determine the best method to be applied. (Standard Test Methods for Tension Testing of Metallic Materials) which is ASTM A 653 [13]. Figure 3 shows the required dimension of sample according to the standard. The test specimen was prepared for the experiments and from the original size of road barrier sample (1.5 m long), it is cut into shorter length approximately 300 mm by using the power saw cutter machine.

Figure 3.

Specimen for tensile test.

2.2 Machine tool

The specimen preparation was carried out by machining with:

1.
WEDM machine. Model: ACUTEX SP-500 Technology Series (Fig. 4).

Figure 4.
WEDM ACUTEX SP-500 technology series machining process.

2.
Conventional milling machine. Model: MASTIKA YSM16SS.SF (Fig. 5).

Table 2
Specimen parameter set up

Crosshead speed 5 mm/min

Sample rate 2 pts/s

Temperature 73 F

Humidity 50%

Specimen gauge length 57 mm

Extensometer gauge length 25 mm

Width 10 mm

Thickness specimen 1 2.67 mm

Thickness specimen 2 2.67 mm

Figure 5.
Conventional milling MASTIKA YSM16SS.SF machining process.

Instron 3367 model has been used for the experiment of tensile test. Instron 3367 can test wide variety of mechanical properties like Compression, Shear or Torsion, Tensile, Flexure or Bending and it capable of generating 30 kN (6744 lbs) of force on the specimen. This testing system provided simplicity, performance, and affordability for quality control and product testing. This testing system was full software control, Instron Series IX/S Automated Material Tester Version 8.25.00. The specimen was clamped at a specific grip separation as Fig. 6a and b. An extensometer was clamped at the center section of the specimen according with parameter set up as shown in Table 2.

Figure 6.
(a) Specimen was clamped at a specific grip separation. (b) Tensile test instrument-instron 3367.

3. Results and discussion

Crosshead speed	5 mm/min
Sample rate	2 pts/s
Temperature	73 F
Humidity	50%
Specimen gauge length	57 mm
Extensometer gauge length	25 mm
Width	10 mm
Thickness specimen 1	2.67 mm
Thickness specimen 2	2.67 mm

Testing procedures were used to develop better information on known materials or to test new materials usage and maintaining the quality of the materials used. Base on simple understanding, strength of the materials refers on the ability of a structure to resist loads without failure. This failure may occur by rupture because of the excessive stress or it may take place owing to excessive deformation. The results discussion will be partially considered into:

ii. i.
Results verification by tensile test using Galvanised Steel with Instron Tensile machine, specimen machined by EDM Wire Cutting and Milling machining.
ii.
Material Characterization by HAZ factors on machining operation.

3.1 Data results of tensile test

Table 3
Tensile test data

Machining	Max. load (kN)	Stress at max. load (Mpa)	Strain at max. load	Young modulus (Mpa)	Stress at upper yield (Mpa)
Wire EDM	14.789	443.102	0.1880	224083.141	376.538
Conventional milling	16.172	470.500	0.0021	219000.567	238.000

During the test, a phenomenon called “Necking” occur at the specimen as the gripper slowly moving upward with constant rate of 5 mm/min. After a while the specimen began to elongate significantly. The experiment stopped when the specimen broke apart into two pieces. The result regarding the material properties like Young Modulus, Yield Stress, Ultimate tensile strength, stress and strain were produced and displayed in the monitor soon after the experiment ended. Tensile properties include the resistance of materials to pull or stretch forces. The amount of force required to break a material and the amount it extends before breaking are important properties. For most materials, the initial resistance to force, or modulus, and the point of permanent deformation, are obtained from plots of force against elongation. Through the analysis of force elongation or stress-strain curves, it conveys much about the material tested, and predicting its behavior determined by experiment to obtain the slope of a stress-strain curve. It was defined as the rate of change of stress with strain and the formula is express as below:

$\displaystyle E=\frac{\text{tensile stress}}{\text{tensile strain}}=\frac{% \sigma}{\varepsilon}=\frac{\frac{F}{A_{o}}}{\frac{\Delta L}{L_{o}}}=\frac{FL_{% o}}{A_{o}\Delta L}$

where, $E$ : Modulus Young (modulus of elasticity), $F$ : Force applied to the object, $A_{o}$ : Original cross section area through which the force was applied, $\Delta L$ : Total length of the object changes, $L_{o}$ : Original length of the object

Under the tensile loading, the steel specimens under goes four stages of deformation as Fig. 7.

In the first stage of deformation, the linear elastic region and elastic are constant and the slope of the line in this region where stress is proportional to strain were called as modulus of elasticity or Young’s modulus. The modulus of elasticity (E) defines as the properties of a material as it undergoes stress, deforms, and then returns to its original shape after the stress is removed. At the second stage of deformation is Yield Point and at some point, the stress-strain curve deviates from the straight-line relationship and in the situation Hooke’s Law no longer applies as the strain increases faster than the stress. From this point, in tensile test, some permanent deformation occurs in the specimen and the material is said to react plastically to any further increase in load or stress. The material will no longer return to its original, it is under unstressed condition when the load is being removed.

In third stage of deformation was plastic region where at this region the steel specimen is no longer to retain its original shape even after the load is removed and the deformation becomes permanent. In a certain time of this region, ultimate tensile strength is reached. The strength of material in its ability to withstand external forces without breaking and lastly at the fourth stage of deformation was breaking strength and at this point, the deformation is too much and the atomic structure of the steel specimen begin to fail. In certain moment, crack is formed and propagated. This is when the specimen broke into two pieces. Yield strength or yield point of a material was defined in engineering and materials science as the stress at which a material begins to deform plastically [14]. Base from the data of tensile test in Table 3, it shows that the Stress at upper Yield value of material gives the value of 376.538 Mpa by WEDM machining compare to Conventional Milling machining where the Stress at upper Yield value was 238.000 Mpa. The theory of Galvanized Steel Yield properties is 345 Mpa, therefore it shows that machining by WEDM maintain the characteristic of Galvanized Steel compare to Conventional Milling Machine.

3.2 Data of HAZ calculation

Table 4
HAZ calculation

Machining	Voltage (V)	Current (A) 3-Phase	Feedrate (mm/min)	Effiency(%)	Heat Input (KJ/mm)
Wire EDM	220	34.378	300	0.60	0.908
Conventional milling	600	57.735	552	0.85	3.200

Figure 7.

Stages of metal deformation.

Base from on this finding results, Table 4 shows that the value of Heat Input for WEDM machining shallower than the conventional milling machining subsequently effects the Mechanical Properties of Galvanized Steel where the Stress value is lower for WEDM Machining compare to conventional milling machining. Therefore, the true elastic limit for the material by EDM Wire cut is 224.083 Gpa compare to milling machining, the E value is 219 Gpa. Young’s modulus value represents the tensile modulus, is a measure of the stiffness of an elastic material and is a quantity used to characterize materials. It is defined as the ratio of the uniaxial stress over the uniaxial strain in the range of stress in which Hooke’s Law holds. In WEDM machining, the process of tooling by using electricity to cut the materials, there is an area that is known as the Heat Affected Zone and means that wherever the electrical wire passes to make the cut, heat is a generated by product of the electrical cut. The area where the cut is made is known as the heat affected zone [15]. Heat-affected zone (HAZ) is the area of base material, either a metal or a thermoplastic, which has had its microstructure and properties altered by welding or heat intensive cutting operations. The extent and magnitude of property change depends primarily on the base material, the weld filler metal, and the amount and concentration of heat input either by the welding process or heat intensive cutting operations. Due to the heat generated in flame-cutting, by milling operation the steel along the cut edge is changed in a couple of different ways. First and foremost, the very surface of the edge will be a thin layer of brittle, resolidified steel, altered in chemistry. The thermal diffusivity of the base material plays a large role if the diffusivity is high, the material cooling rate is high and the HAZ is relatively small. Alternatively, a low diffusivity leads to slower cooling and a larger HAZ. By taking the results into consideration and through this study, few factors could be determined in future of manufacturing technology and materials long lasting life. Machining process consist of man, machine, material, method, cost and environment. Therefore, to achieve high quality product, a must in high precision and to ensure the product in precise dimensions, high skill of machinist need to run the machine, but not all skilled machinist capable to understand the product requirement base on material effects.

4. Conclusion

Durability contribute to the component life and strength of material and reliability of the product assurance towards its ability of performance. A good machining procedure could decrease the wastage and sufficiently increase the quality of the product itself. WEDM machining is capable of producing intricate parts with high dimensional accuracy and excellent surface finish. Methods with techniques in machining and products output identify the saliencies of human progress. Manufacturing evolution provided more efficient techniques to extend the humans capabilities and cost reduction in various areas. From the Neolithic Revolution to the Digital Revolution, passing through Renaissance and the Industrial Revolution, men invented new tools/systems to achieve the best quality in products making. The Industry 4.0 era supplied manufacturing systems with new technologies aiming to digitalize the smart and modern factory in simulation, machining, inspections, maintenance and quality assurance. The manufacturing environment is changing rapidly especially due to the rapid development of digital technologies. This rapid development of digital technologies and their implementation from the manufacturing-industrial enterprises create a new era, known as “the fourth industrial revolution” or “Industry 4.0” [16]. The implementation of such trends and advanced digital technology may be critical for an industry to remain competitive, but it is also connected with a change in the skill set required from engineers and blue-collar workers [17].

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