Production of Single Tracks of Ti-6Al-4V by Directed Energy Deposition to Determine the Layer Thickness for Multilayer Deposition

Summary

In this research, a rapid method based on melt pool characterization is developed to estimate the layer thickness of Ti-6Al-4V components produced by directed energy deposition.

Abstract

Directed Energy Deposition (DED), which is an additive manufacturing technique, involves the creation of a molten pool with a laser beam where metal powder is injected as particles. In general, this technique is employed to either fabricate or repair different components. In this technique, the final characteristics are affected by many factors. Indeed, one of the main tasks in building components by DED is the optimization of process parameters (such as laser power, laser speed, focus, etc.) which is usually carried out through an extensive experimental investigation. However, this sort of experiment is extremely lengthy and costly. Thus, in order to accelerate the optimization process, an investigation was conducted to develop a method based on the melt pool characterizations. In fact, in these experiments, single tracks of Ti-6Al-4V were deposited by a DED process with multiple combinations of laser power and laser speed. Surface morphology and dimensions of single tracks were analyzed, and geometrical characteristics of melt pools were evaluated after polishing and etching the cross-sections. Helpful information regarding the selection of optimal process parameters can be achieved by examining the melt pool features. These experiments are being extended to characterize the larger blocks with multiple layers. Indeed, this manuscript describes how it would be possible to quickly determine the layer thickness for the massive deposition, and avoid over or under-deposition according to the calculated energy density of the optimum parameters. Apart from the over or under-deposition, time and materials saving are the other great advantages of this approach in which the deposition of multilayer components can be started without any parameter optimization in terms of layer thickness.

Introduction

Ti-6Al-4V is the most commonly used Ti alloy in the aerospace, aircraft, automotive, and biomedical industries because of its high strength-to-weight ratio, excellent fracture toughness, low specific gravity, excellent corrosion resistance, and heat treatability. However, its further developments in other applications are challenging, owing to its low thermal conductivity and high reactivity features, which result in its poor machinability. Moreover, due to the heat hardening phenomena during the cutting, a specific heat treatment must be undertaken^1,2,3,4.

Nonetheless, additive manufacturing (AM) technologies showed great potential to be used as new manufacturing techniques that can reduce the price and energy consumption, and address some of the current challenges in the fabrication of Ti-6Al-4V alloy.

Additive manufacturing techniques are known as innovative and can fabricate a near net shape component in a layer-by-layer fashion. A layer-by-layer additive manufacturing approach, which slices a Computer Aided Design (CAD) model into thin layers and then builds the component layer by layer, is fundamental for all AM methods. In general, additive manufacturing of metallic materials can be divided into four different processes: powder bed, powder feed (blown powder), wire feed, and other routes^3,5,6.

Directed Energy Deposition (DED) is a class of additive manufacturing and is a blown powder process that fabricates three-dimensional (3D) near net shape solid parts from a CAD file similar to other AM methods. In contrast with other techniques, DED can not only be used as a manufacturing method, but also can be employed as a repairing technique for high value parts. In the DED process, metallic powder or wire material is fed by a carrier gas or motors into the melt pool, which is generated by the laser beam on either the substrate or previously deposited layer. The DED process is a promising advanced manufacturing process that is capable of decreasing the buy-to-fly ratio, and also is capable of repairing high value parts that previously were prohibitively expensive to replace or irreparable⁷.

In order to achieve the desired geometric dimensions and material properties, it is vital to establish appropriate parameters⁸. Several studies have been undertaken to elucidate the relationship between the process parameters and the final properties of the deposited sample. Peyre et al.⁹ built some thin walls with different process parameters, and then characterized them by using 2D and 3D profilometry. They showed that layer thickness and melt pool volume affect the roughness parameters noticeably. Vim et al.¹⁰ proposed a model in order to analyze the relation between the process parameters and geometrical characteristics of a single cladding layer (clad height, clad width, and depth of penetration).

To date, several studies on DED of Ti alloys have been reported, most of which focused on the influence of the combination of parameters on properties of massive samples^11,12,4. Rasheedat et al. studied the effect of scan speed and powder flow rate on the resulting properties of the laser metal deposited Ti-6Al-4V alloy. They found that by increasing the scan speed and powder flow rate the microstructure changed from Widmanstätten to a martensitic microstructure, which results in an increment of surface roughness and the microhardness of deposited specimens⁷. Nevertheless, less attention has been paid to designing the layer thickness setting. Choi et al. have investigated the correlation between the layer thickness and process parameters. They have found that the main sources of error between the present height and the actual height are the powder mass flow rate and layer thickness setting¹³. Their studies did not properly implement layer thickness setting because they involved lengthy and inaccurate processes in the layer thickness setting. Ruan et al. have investigated the effect of laser scanning speed on the deposited layer height at a constant laser power and powder feeding rate¹⁴. They have proposed some empirical models for layer thickness setting which were obtained under specific processing conditions, and thus the layer thickness setting may not be precise due to the utilization of specific process parameters¹⁵. In contrast to previous works, the layer thickness setting process proposed in this manuscript is a rapid method which can be performed without wasting time and materials.

The main focus of this work is to develop a rapid method for the determination of layer thickness based on the characteristics of the single tracks of the Ti-6Al-4V alloy at optimum DED process parameters. Thereafter, the optimal process parameters are employed to determine a layer thickness and fabricate high-density Ti-6Al-4V blocks without wasting time and materials.

Protocol

1. Powder Characterization

1. Put 3 g of starting Ti-6Al-4V powders on a double-sided sticky carbon tape, which is located on an aluminum pin stub, and insert inside the specimen chamber of a Field-Emission Scanning Electron Microscope (FESEM) to analyze the morphology of the powder¹⁶.
2. Measure the apparent density of the powder by filling a 30 cm³ container, and measure the weight of powder according to the ASTM-B212 standard.
3. Perform chemical analysis of starting powder (5 - 10 g), used powder (5 - 10 g), and deposited block (20 g) by means of elemental (e.g., Leco) and inductively coupled plasma (ICP) analyzer¹⁷.

2. Directed Energy Deposition of Single Tracks

1. Powder Loading
   1. Wear personal protective equipment, including an FFP3 respiratory mask compliant to the requirements of EN 149, powder-free disposable nitrile gloves, and protective plastic glasses.
   2. Open the hopper of the powder feeding system and use an extractor fan (e.g., ATEX) for removing the residual powders.
   3. Remove the hopper by following the appropriate instructions given by the constructor and clean every component by using ethanol-soaked paper towels.
      NOTE: This step is fundamental to avoid contamination by different kinds of metal powders.
   4. Reassemble the powder feeding hopper by following the instructions given by the constructor. Leave aside only the top cap of the hopper in order to perform the loading of the powder.
   5. Load the hopper with Ti-6Al-4V powders having granulometry in the range of 50 - 150 µm. Depending on the size of the available hopper, fill it completely.
   6. Close the top cap of the hopper very tightly in order to avoid any gas leakage.
2. Preparation of the Sample
   1. Pick a Ti-6Al-4V sheet with 50 mm x 50 mm dimensions and 4 mm thickness.
   2. Clean the surface of the titanium sheet with ethanol-soaked paper towels. Measure the weight of the sheet with a centesimal balance.
   3. Place the sheet on the working area according to the position of the marker. The working area is where the deposition will take place, so it is determined according to the programmed path of the robot.
3. Preparation of the Robot and Preparation of the Deposition Equipment
   1. Mount the nozzle on the laser head so that the angle between the nozzle axis and the laser axis is 35°.
   2. Move the robot to the starting working point to perform the calibration.
   3. Check the distance between the nozzle and the working plane and, if needed, manually correct the nozzle position until the measured distance is 5 mm.
      NOTE: Since the working area lays on a horizontal plane, this distance is the vertical distance between the metal sheet and the tip of the nozzle.
   4. Check the centering of the nozzle outlet with the laser: first, switch on the laser guide by clicking the "Laser Guide ON" command on the software for the control of the laser. Then, put a thin rod, measuring 0.8 mm in diameter and 200 mm in length, inside the nozzle. Verify that the tip of the rod and the spot of the laser guide are coincident. If not, manually adjust the position of the nozzle, respecting the distances and angles previously indicated.
      NOTE: In this case, the outer diameter of the nozzle is 1 mm; if a nozzle with a smaller diameter is employed, use a rod whose diameter is smaller than that of the nozzle.
   5. Verify the calibration data written into the robot control software: click the button "Apply" on the software, and wait for the compilation of the code.
      NOTE: The software will check for errors in the code; if no errors are detected, the code is stored on the robot controller. If errors are detected, the code won't be compiled, and further revision will be needed.
   6. Enable the laser source module by clicking the "Laser enable" command on the laser control software.
      NOTE: The employed laser source is a continuous fiber laser emitting in the infrared region (1064 nm) with 5 kW maximum power.
   7. Enable the robot's motors by manually pushing the button "Robot Motors on" on the control cabinet of the robot, and check that the related safety LED is lit up: if so, it means that the motors are enabled.
4. Start the Deposition Process
   1. Select the proper file on the existing program list and load the working path into the main robot routine.
   2. Check the laser and robot parameters: set the laser power to the specific laser power (325, 650, 980, 1500 W) and the robot speed to a given speed (30, 40, 50, 60 mm/s).
      NOTE: These parameters are written into the control software of the robot, according to the specific language of the machine.
   3. Confirm the new parameters by pushing the button "Apply" on the software, and wait for the compilation of the code. The software will check for errors in the code; if no errors are detected, the code is stored on the robot controller and it is ready to be launched. Launch the robot routine by pushing the "START" button on the robot control software.
5. Pick up the sample by using the special nipper, and clean the sample surface with an ethanol-soaked paper towel for removing any residual powder.

3. Analyze the Single Tracks

1. Analyze the single tracks from the top by a stereo microscope at 5X magnification.
   NOTE: In this stage, some images are taken by means of a stereo microscope and analyzed visually.
2. Cut the single tracks from the middle of the deposited tracks perpendicular to the deposition direction by using a precise cutting tool.
3. Mount the cross section of single tracks in epoxy resin. Choose a mounting cup and place the clean and dry specimen in it. Measure the correct amounts of resin carefully by weight (10 g/sample) and then mix with a liquid hardener (6 g/sample). Pour the resin-mixture over the specimen and leave the filled mounting cups to cure for 30 min at room temperature. Thereafter, grind the mounted specimens with 500, 800, and 1,200 grit size SiC paper, and then polish by using diamond paste down to the finer sizes of diamond particles (1 µm).
4. Analyze the polished surfaces in terms of shape and porosity by means of an optical microscope. Acquire the images of melt pools with the optical microscope at 10X magnification, and then analyze them using Image-J software.
5. Measure the height of melt pools by measuring the distance between the top and bottom of the melt pool. Afterwards, plot the measured height of melt pools as a function of specific energy density, which is calculated according to its corresponding equation. Indeed, the energy density can be calculated by the following equation:
   
6. Fit a 2^nd order polynomial on the experimental results to obtain the equation corresponding to the layer thickness as a function of specific energy density.
7. Consider a specific energy density, and calculate the layer thickness according to their relationship, which was observed in the previous step.
8. To verify the method, fabricate a multilayer block by considering the calculated layer thickness, and then characterize the section of blocks in terms of porosity and final height.
9. Analyze the microstructure of the fabricated block after a polishing step in the same procedure as section 3.3. In fact, after final polishing, etch the samples for 30 s with Kroll's Reagent, which contains 92 mL distilled water, 6 mL nitric acid, and 2 mL hydrofluoric acid.

Results

For the experimental studies, irregular Ti-6Al-4V powder with an average size of 50 - 150 µm and apparent density of 1.85 g/cm³ was employed as depositing material (Figure 1). The chemical analysis of the powder confirmed that the oxygen and nitrogen contents of the powder did not change before and after the deposition process, while in both cases the oxygen content was higher than the standard oxygen content of Ti-6Al-4V powder for additive manufa...

Discussion

In this work, the focus was on the slicing thickness setting in the DED process of Ti-6Al-4V, according to the geometry of melt pool characteristics. For this purpose, a two-step protocol was defined and utilized. The first part of the protocol was an optimization of process parameters for single scan deposition and, during this step, the optimum parameters were achieved and the melt pool geometries were measured. In the second part of the protocol, the specific energy density of specimens at the optimum parameters was c...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Ti-6Al-4V powder	Xi’Tianrui new material		As starting material
ISOMET precision cutter	Bohler		To cut the samples
Polishing machine	Presi		To polish the samples
EpoFix resin	Presi		To mount the samples
Diamond paste	Presi		For polishing
Optical Microscope	Leica		Microstructural observation
Field emission scanning electron microscope	Merlin-Zeiss		Microstructural observation
Stereo microscope	Leica
LEC1- CS444 ANALYSER	IncoTest		Chemical analysis
LEC3 - ELTRA OHN2000 ANALYSER	IncoTest		Chemical analysis
LEC2 - LECO TC436AR ANALYSER	IncoTest		Chemical analysis
ICP	IncoTest		Chemical analysis
IRB 4600	ABB		Antropomorphic robot
GTV PF	GTV		Powder feeding system
YW 52	Precitec		Laser head
Nozzles	IRIS		Nozzle for feeding powders
YLS 3000	IPG Photonics		Laser source