Viability of Selective Laser Melting of Commercially Pure Titanium on Stainless Steel Substrate

Abstract

Titanium and Stainless Steel are two metals which are known to be very beneficial in various industries and applications. Many industries optimize steel and stainless steel by bonding them with titanium and its alloys. This is because titanium is half the density of steel, making steel/stainless steel more durable and stronger. However, majority of the bonding has been conducted through conventional welding techniques, such as Gas tungsten Arc Welding (GTAW) and this results in the failure of the welds with cracking. There has been vast research about bonding the two methods using conventional welding techniques, however there isn’t much on using different methods, such as Selective Laser Melting (SLM). With limited research on this method of bonding of the two metals, this project proposal will attempt at determining the viability of fusing commercially pure Titanium on Stainless Steel 304 alloy substrate using Selective Laser Melting.Where an experimental method will be formulated using visual and mechanical tests. This will be conducted by investigating a SLM workpiece provided by Osaka University, Japan. Where the surface morphology and the microstructure will be examined to identify any cracks or unmolten metal present in the fusion. The results will be collected, analysed and reported after these tests are performed.

Introduction

Titanium and Stainless Steel are two metals which are popularly used in a number of industries due to their mechanical and chemical properties. With prior research conducted on similar and dissimilar metals being welded by conventional methods, not much research has been conducted on Selective Laser Melting (SLM) of Commercially Pure Titanium (CP-Ti) on Stainless Steel substrate. Thus, this study will investigate the viability of SLM of CP-Ti on AISI 304 Stainless Steel alloy substrate by observing the surface morphology and microstructure of the materials using various imagining techniques.

Selective Laser Melting (SLM) is an Additive Manufacturing technique which is becoming an increasingly popular method. This is due to its ability to form three-dimensional complicated shapes without a mold (Sato Y., Tsukamoto, M., Shobu, T, 2018). It provides cost savings and weight reduction with respect to the conventional forming methods. Thus, SLM is being used in a variety of industries and applications, some of which are aerospace, automotive engineering, dental technology, medical technology, mechanical engineering components, turbine construction prototyping and rapid prototyping (Concept Laser, n.d.). With such high level of formability and shape complexity adhered through SLM, there are still some issues that have yet to be resolved, such as, dimensional accuracy, surface finishes, processing time and mechanical properties including surface roughness, hardness and crystal orientation (Sato, Y., Tsukamoto, M., Shobu, T, 2016).

Commercially pure titanium and titanium are commercially used in variety of industries, including aerospace, medical, architecture, chemical to name a few. These metals are used in the respected industries due to their mechanical and chemical properties, where they have high corrosion resistance, low density, high strength/ weight ratio, high ductility, low thermal conductivity. However, these metals have little formability into complex shapes/ structures as they are difficult to machine. To address similar issues, prior research and testing has been conducted on Ti-6Al-4V (Ti64) alloy using SLM process. (Liu, Chu, & Ding, 2004) found that the crystal orientation of Ti64 can be controlled by SLM under vacuum conditions and the laser scanning speed. Ti64 has α + β crystal orientations, the β phase had mainly appeared in the SLM process since Ti64 had melted above 1600°C by the laser irradiation and then it solidified.

AISI 304 Stainless Steel alloy is an austenitic stainless steel that can be severely drawn. They are used in industries such as water, dairy and food, architecture and pharmaceutical. This is because AISI 304 SS alloy is corrosion and heat resistant, has good machinability, excellent hot and cold forming process and performance and good weldability. However, it has low mechanical strength. Production lines, through research and development have been optimising AISI 304 grade according to different environmental conditions and specific requirements/ parameters (AZO Materials, 2005).

With the mechanical and chemical properties of CP-Ti and AISI 304 SS alloy, and prior research on different metals being fabricated using SLM, the question raised is “Will Selective Laser Melting Commercially Pure Titanium on AISI 304 Stainless Steel alloy substrate work successfully?”

This paper will be focusing on how well the two dissimilar metals join through a series of visual and mechanical tests and if it will be a successful integration.

Literature Review

Research shows that unalloyed Commercially Pure Titanium and Stainless-Steel metals are difficult to bond by conventional welding techniques, such as Gas Metal Arc Welding (GMAW), Gas Tungsten Arc Welding (GTAW), Shielded Metal Arc Welding (SMAW) and Flux-Cored Arc Welding (FCAW) to name a few. Operations with these methods are known to result in a very brittle bond in the titanium or stainless-steel parts and eventually causes one or the other to fracture. As Additive Manufacturing is a fast-growing process in the metallurgical sector, this is a given opportunity to study the bonding and identify the surface morphology and microstructure of unalloyed CP-Ti on AISI 304 stainless steel alloy substrate by Selective Laser Melting to determine whether this method is viable.

Surface Morphology and Microstructure

Surface morphology is the study of the shape and distribution of materials at a surface using imaging techniques such as Optical Microscopy and/or Scanning Electron Microscopy (SEM), (ASMAC, 2013). The change in surface morphology of materials are significantly important for each layer of the SLM process, and it determines the final product properties and is affected by the process parameters such as the hatch distance and scan speeds, where cracks and un-melted materials are identified. (Yadroitsev & Smurov, 2011) studied the surface morphology of SS grade 904L powder on SS grade 304L substrate under SLM and obtained results at different hatch distances.

The process parameters for this experiment are as follows; laser power was 50 W, scanning speed was 0.14m/s and hatch distances varied from 60 – 280 µm with step of 20 µm. SS grade 904L had a layer thickness of 50 µm on the SS grade 304L substrate. As seen in figure 1 above, there is a sequence of tracks with identical geometric characteristics at hatch distance of 120 µm and 240 µm, which indicates that when hatch distance is decreased the thermo-physical conditions of the fusion changes (Yadroitsev & Smurov, 2011).

Microstructure is “The arrangement of phases and defect within a material” (The Inference Group., 2014). Microstructure can be observed using a range of microscopy techniques.

As shown above, (Gammon, et al., 2004) studied the microstructures of unalloyed titanium sheets being annealed and cold worked. Figure 1a, shows grains of alpha of CP-Ti sheet (99.0%), where the sample was as-rolled to 1 mm thick at 760°C and air cooled. Figure 1b shows recrystallized alpha grains, particles of TiH (black) and particles of beta and was annealed 2 hours at 700°C and air cooled. Figure c shows recrystallized grains of primary alpha and transformed beta. And figure 4 shows serrated alpha plates, particles of TiH and retained beta between alpha plates.

Gas Tungsten Arc Welding

Gas Tungsten Arc Welding (GTAW) of CP-Ti and AISI 304 Stainless Steel alloy has been carried out to understand why they do not join. Kumar (n.d) found that joining the two dissimilar metals using GTAW results in a failure, where the reason for failure was stated as “…formation of TiFe and TiFe₂ intermetallic compound at the weld bead”.

Figures 1 and 2 above shows element composition at given temperatures. And it is known that FeTi and Fe₂Ti phases are both hard and very brittle in nature, thus the failure of the weld. The failure of the experiment is due to the differing thermal properties of CP-Ti and AISI 304 SS alloy. Firstly, the melting temperature of CP-Ti is high than of AISI 304 SS alloy, which will lead to uneven distribution of the material within the fusion zone. Secondly, AISI 304 SS alloy has higher thermal conductivity that CP-Ti, where the heat from the fusion zone will dissipate faster than CP-Ti, thus increasing thermal shearing stress within the weld seam. And lastly, the thermal expansion of AISI 304 is said to be twice of CP-Ti, indicating the creation of shearing force, thus causing fracture of the weld piece.

With this experiment, it can be seen that GTAW is not a desirable process for joining CP-Ti and ASIS 304 SS alloy, therefore Selective Laser Melting will be considered and used for this research project to determine if a favourable thermal environment is created to join the two metals.

Selective Laser Melting

SLM involves a metal powder bed which is melted by a high-energy fibre laser. A thin layer of powder is deposited over a substrate layer, and the laser beam melts and fuses the powder particles. All of this occurs in a closed chamber filled with inert gas, ideally Argon or Nitrogen, to minimize the oxygen contamination during the process. There are a few important parameters which affect the mechanical properties of components. These are: the laser power, laser scan speed, hatch distance, hatch overlaps and hatch style, to name a few (Konda Gokuldoss, P., Kolla, S., & Eckert, J. (2017).

Since CP-Ti is being used for this research project, it is a difficult metal and cannot form into complicated structures using the conventional welding methods, thus SLM is a desirable process to shape the metal.

Figure 4 shows the setup for the SLM system, where the system has a fiber laser, which can be used at the required power and wavelength and a galvanic mirror setup to focus on the powder surface. The chamber will have a set pressure of either argon or nitrogen in order to prevent the metal powder from oxidizing. The metal powder will be supplied from the powder feeder (powder cylinder), where a roller on top of the powder bed will smoothen the powder and the fiber laser will then irradiate and melt the powder bed which will create a melt pool for metallic structures to form.

SLM is the most versatile additive manufacturing process as it can process a wide range of materials including Al-based alloys, Ti-based alloys, Fe-based alloys, Ni-based alloys, Co-based alloys, Cu-based alloys and their composites (Gokildoss, Kolla, & Eckert, 3028).It also has the ability to tune mechanical properties of alloys during the process by varying the process parameter, depending on the requirement.  Process parameters include hatch style, base plate heating, contour variation, internal heat treatment, etc.  SLM is relatively low cost, has increased functionality and production of near-net-shaped components, ready to use. However, SLM does have a few disadvantages to it such as, having high power usage, and high initial costs. It is a slow process and the optimization of the process parameters is time consuming. Furthermore, cracking will occur to brittle and high temperature materials which cannot accommodate high internal stress during the fabrication process.

Commercially Pure Titanium is characterized into four distinct grades which are 1,2,3 and 4 respectively. The grades are identified through the resistance to corrosion, formability and strength requirements of specific applications of CP-Ti. Grade 1 has the high corrosion resistance, formability and low strength titanium’s, whereas grade 4 has the highest strength and moderate formability. Even with the grading system, consumers utilize the excellent corrosion resistance, formability and weldability of the CP-Ti for various applications (Fort Wayne Metals, 2018).

Surface contamination of CP-Ti, where there is a fresh titanium oxide surface, is quite reactive towards organic and inorganic contaminants. This contamination contributes to the lack of integration in titanium parts. As (Sittig, Textor, Spencer, Wieland, & Vallotton, 1999) have found from their study of titanium as implant materials, the surface roughness determines the shear strength of the implant- bone interface-important for long term fixation. This has a major influence on the properties and evolution of the implant-tissue interface.

To achieve the desirable surface, industries have begun using a variety of surface treatment processes such as cleanliness, passivation and specific topography.

AISI 304 Stainless Steel is the standard “18/8” stainless steel. The grading of this alloy refers to its durability, quality and temperature resistance., where the numbers (18/8) are the amount of chromium and nickel present in the composition. 18 refers to 18% chromium and 8 refers to 8% nickel in this grade. Grade 304 has at least 50% iron and 0.8% carbon, where the chromium binds oxygen to the surface in order to protect iron from oxidizing. Nickel improves the corrosion resistance of the metal (mightynest, 2014).

AISI 304 SS has an austenitic structure which allows it to be severely drawn, resulting as a dominant grade which is used in applications such as sink and saucepans (AZO Materials, 2005). The alloy is corrosion and heat resistant both, which gives it a desirable trait of being resistant to oxidation. It has good machinability and fusion welding performance is excellent with and without fillers. This results in SS 304 being used in a variety of applications such as food, cutlery and flatware, dairy and pharmaceutical production equipment, to name a few.

Experimental Procedure

An experimental procedure addressing all aspects of the research question could not be found in the literature review, however, aspects of the methods used in earlier studies such as SLM process of the individual metals is possible to use. It must also be mentioned that this research project is in collaboration with Hugh Lowther (2018), a student at Auckland University of Technology, where he has studied this topic for his final year project and developed and carried out an experimental procedure. It has been reported that his experimental procedure has worked well for his final year project, thus the experimental procedure for this research project will be based on his method.

Test Sample Details

As research has shown, experimental procedures have been conducted using the conventional GTAW method using similar metals, this project will use selective laser melting to join the two dissimilar metals. The test samples for this project are supplied from Osaka University and the samples are of CP-Ti printed on AISI 304 Stainless Steel alloy substrate (see figure 7)

The sample consists of a 3mm thick AISI 304 stainless steel alloy substrate, which is the baseplate. There are nine 10mm x 10mm x 0.1mm commercially pure Titanium plates laser melted onto the baseplate. The CP-Ti powder was made by a pre-mixed atomization process and was spherical in shape, where the particle diameter varied from 4-88 µm, with a median particle diameter of 35 µm (Sato et al., 2016).

In reference to figure 5, the sample was created using the SLM process, laser power of 200 W was used with a focus diameter of 100 µm. The laser power density was calculated by dividing the laser power by the laser spot area:

I= PS

Where:

P = Laser Power (W)

S = Spot Area (cm²)

The CP-Ti plates have been produced in a grid pattern, as seen in figure 8 above, each plate corresponds to a combination of hatching distance and laser scanning speed. For example, plate A, was produced with a laser scanning speed of 30 mm/s and a hatching distance of 120 µm, whereas the plate I was produced using laser scanning speed of 10 mm/s and hatching distance of 90 µm.

Surface SEM Scan

Scanning Electron Microscopy of a samples surface is done to observe points of interest on the surface to identify texture, cracking, different microstructures and porosity (Swapp, n.d.). Scans will be done on the left-hand side border of each sample to identify the points of interest. Multiple images will be taken at different magnifications on a micro scale to observe the grain structures.

Surface Roughness

The surface roughness of the samples will be measured using a Taylor Hobson Precision FORMTALYSURF 50 machine, which is a stylus type surface roughness tester (STSR). It will be carried out to quantify the extent of waviness, which is the height difference between the crests and valleys on the CP-Ti surfaces. It will be examined with the microhardness data and microstructural analysis to determine which combinations of laser scanning speed and hatching distance produce the optimum CP-Ti plates for the given laser power density.

The surface roughness will be measured before the samples are cut so multiple passes can be carried out on different areas of the CP-Ti plates. The surface roughness will be characterised by a mean roughness value which is the average distance of the displacement of the stylus from the centre line.

Surface Roughness will be defined as (Ra):

Ra=a+b+c+…n

Where:

a,b,c = absolute values in µm

n = number of readings

Figure 10: Sample schematic provided by Hugh L.

Cutting

A brass fine-wire Electric Discharge Machine (EDM) will be used for the initial cuts of the sample. This process uses electrical discharge while fully submerged in water to erode the material away. To ensure the CP-Ti – Steel interface microstructure will not be affected, care should be taken to prevent any work hardening, localised heating and any form of damage.

Mounting, Grinding, Polishing and Chemical Etching

The samples will be mounted on polyfast resin cylinders using the Struers LaboPress-3 machine. These mounts will enable a secure hold of the samples where the desired surface will be exposed for testing and analysis.

A Metaserv Rotary Grinder will be used to prepare the samples for microscopic analysis, where fine grit sanding papers will be used to grind the samples to achieve a flat surface. Water will be used as a lubricating coolant to ensure no work hardening or thermochemical effects on the surface.

The samples will be polished using water as a lubricant and coolant to keep the samples temperature low and a solution of 3% of Hydrogen Peroxide will be used.

Chemical etchant will be applied to the samples surface for a certain amount of time to expose the materials microstructure. Kroll’s Titanium etch with a composition of 92.82% water (H₂O), 6.11% Nitric acid (HNO₃), and 1.07% hydrofluoric acid (HF) will be used to etch the Titanium surfaces, and a solution of 47.5% water (H₂0), 47.5% hydrochloric acid (HCl), and 5% Nitric acid (HNO₃) will be used for the stainless-steel surface.

Optical and Scanning Electron Microscopy (SEM)

Optical microscopy will be done to examine the samples cross-sections and observe any defects and cracking. Different microstructures will be observed as well as the relative thickness of the microstructural layers.

Hardness Testing

Hardness tests enable the evaluation of a materials properties, such as the strength, ductility and wear resistance. It is affected by any heat treatment or hardening processes that the materials have gone through. The hardness testing will be done using a micro Vickers hardness machine. The Vickers hardness value will be calculated using this formula:

HV= 2 Fsin⁡136°2d2

Where:

F = load applied (g)

d = average diagonal length of indentation

Energy Dispersive X-ray Spectroscopy (EDS)

EDS is a chemical microanalysis technique which is used in combination with SEM to detect x-rays emitted form a sample during the process where an electron beam is used to characterize the element composition of the sample (MEE, 2014). Regions of the samples microstructure which contain brittle intermetallic phases will be identified and to model the chemical distribution of the samples cross-section will be done by spot EDS with regular interval, ensuring all different microstructures are analysed.

Results will be collected and delivered in the required formats to determine the viability of this research project proposal.

Conclusion

This investigation seeks to fill the knowledge gap presented in this research proposal, to determine what combinations of the scanning speeds and hatching distances (if any) produces any ideal, close to desired products, which contains smooth surface finishes, no cracking and no brittle intermetallic phases.

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