Forming quality analysis of TC4 titanium alloy based on SLM orthogonal process

The TC4 titanium alloy bulk and irregular porous structures were prepared using Selective Laser Melting (SLM). This study systematically investigated the effects of laser power, scanning speed, scanning spacing, and laser energy density on the forming quality of TC4 titanium alloy specimens, through orthogonal experimental design. The research results indicate that laser power is the primary factor affecting the density of TC4 titanium alloy bulk during the SLM process. With the increase of laser energy density, the density of the specimens shows an initial increase followed by a decrease, while Vickers hardness and surface roughness exhibit a decreasing trend followed by an increasing trend. Moreover, the surface of the specimens transforms from a rough surface with a significant presence of pores, voids, or unmelted powder particles to a relatively smooth and flat surface, accompanied by a reduction in the size and quantity of pore defects.


Introduction
Titanium alloys are widely used in aerospace, chemical, machinery manufacturing, and medical fields because of their high strength, low density, high corrosion resistance, and good biocompatibility [1] .People are increasingly demanding precise and complex structural components.However, the traditional manufacturing process has been unable to meet these requirements, and new manufacturing technologies are needed to improve the preparation quality and performance of titanium alloy products.Based on its technical characteristics, additive manufacturing (AM) has shown unique advantages in the preparation of precision, complex titanium alloy components, and therefore has gradually become the mainstream of the titanium alloy manufacturing process [2] .Laser selective zone melting molding is one of the widely used 3D printing technologies today.It utilizes a laser beam to sinter powders and is capable of rapidly producing complex products [3] .Therefore, SLM technology has become one of the most important means of preparing high-quality titanium alloy products, which can accurately control the material distribution in each part and reduce the structural defects of the parts [4] .In the industrial field, this technology is widely used in the prototyping and development of molds and mechanical parts, as well as in the rapid manufacturing of small-lot parts, and in the medical field, it is also used to produce various custom implants [5] .
However, the quality of SLM molding of titanium alloys varies.Therefore, it is necessary to optimize the forming process parameters and corresponding heat treatment to achieve the goal of improving the quality of prototypes.Many national and international scholars have studied this issue, mainly including delamination thickness, scanning strategy, protective atmosphere, preheating temperature, and other process parameters [6] .Wu et al. [7] obtained the optimal process parameters through orthogonal tests and metallographic observations, and the scanning spacing had the greatest influence on the compressive strength and elastic modulus of the formed samples.Liu et al. [8] found that the densification reaches 99% and above when the energy density is in the range of 35-100 J/mm 3 .Xu et al. [9] studied the effect of different heat treatments on the fatigue properties of titanium alloy porous structure and found that the fatigue strength showed an increasing trend with the increase of heat treatment temperature.Sun et al. [10] studied the effect of energy density on the organizational defects of TC4 titanium alloy based on SLM and found that when the laser body energy density increases, the surface roughness of the specimen decreases.
In this experiment, TC4 titanium alloy block specimens were prepared based on SLM.The effects of laser power, scanning speed, scanning spacing, and laser energy density on the forming quality of the alloy were investigated by orthogonal tests.These findings provide an important reference for the high-quality forming of TC4 titanium alloy based on SLM forming.

Experimental materials and equipment
In this experiment, Ti6Al4V (ELI) commercial powder was used as the raw material for SLM forming, and Scanning electron microscopy of TC4 powder is shown in Figure 1.As can be seen from Figure 1, Ti6Al4V (ELI) commercial powder has no adhesion and less powder between satellite balls, high sphericity, and no irregular particles.The particle size of the alloy powder ranges from 10-60 μm.The chemical composition of the titanium alloy powder is shown in Table 1.
Table 1.Chemical composition of TC4 titanium alloy powder.The appearance of the SLM forming equipment and forming specimens are shown in Figure 2.For this test, the EOS M290 laser selective melting and forming equipment made in Germany was selected, which adopts the ytterbium fiber laser with a maximum power of 400 W. The maximum size of the studio is 250 mm (X) × 250 mm (Y) × 325 mm (Z), 99.999% high purity argon gas, SLM forming block samples in Figure 2.There are more factors affecting the SLM molding specimen.In this paper, we select the laying powder layer thickness of 0.03 mm, and the laser spot scanning diameter of 0.05 mm, and design a 3factor, 4-level orthogonal test using different laser powers P, scanning speeds v, and scanning spacings h.The specific Orthogonal test design and test results are shown in Table 2.

Performance Characterization
The concept of energy density is introduced, which is defined as the energy given by the laser to a unit volume of powder material, see Equation (9).Energy density combines the effects of laser power (P), the thickness of the powder layered layer (t) scanning speed (v), and scanning spacing (h) and is an important factor affecting the quality of the characterization molding is not numbered.For example: Using water as the solution, the mass of the specimen in air was first measured using an electronic balance (BSA124S), and then the mass of the specimen in water was measured.The densification of the formed specimen was measured based on Archimedes' principle as the ratio of the density of the formed specimen to the theoretical density of TC4 titanium alloy, which satisfies Formula (2) [11] .
Where the theoretical density of TC4 titanium alloy: , the density of water: , m 1 for the sample in the air mass, m 2 for the sample completely into the water after the mass.Each specimen was measured independently in five groups of data, and the average value was taken.The densities of the samples are closely related to their mechanical properties and biocompatibility, as well as to the molding quality of the parts.

Test results and discussion
The design scheme and test results of orthogonal tests of SLM forming TC4 titanium alloy are shown in Table 2.In the SLM forming process, a high-energy laser beam scans the metal powder and forms a melt pool.The laser energy density affects the melting process, temperature gradient, solidification process, and interfacial surface tension, which may lead to the emergence of defects such as adhesion of powder particles on the surface of the formed specimen, reduction of densification, and excessive residual stress.Therefore, the appropriate laser energy density is a key factor in obtaining formed specimens with excellent forming quality and outstanding mechanical properties.3. From Table 3, it can be seen that the effects of various parameters on density are: P> v> h.The effect of various parameters on the density are shown in Figure 3.According to the results of orthogonal experiments, the influence of each process parameter on densification is summarized as follows.When the laser power is too low, the energy density is insufficient, the specimen cannot completely melt the metal powder, and the powder cannot be bonded, resulting in poor liquid phase fluidity in the metal layer.The tissue defects increase, and the densification decreases with the increase in energy density.The densities of the specimens decreased significantly with increasing scanning speed and spacing.At scanning speeds between 1000 mm/s and 1600 mm/s, the densities of the specimens decreased consistently.The high scanning speed limits the liquid phase unfolding, resulting in fewer solid phase particles.When the scanning pitch was increased, the excessive increase in scanning pitch resulted in increased difficulty in lapping between scanning trajectories on the scanning surface, and the formation of holes due to incomplete melting of powder.

Effect of laser energy density on hardness and surface roughness
The Vickers hardness of SLM-formed TC4 titanium alloy samples at different effect of energy density on hardness is shown in Figure 4.As from Figure 4, when the energy density increases, the Vickers hardness of the specimen firstly decreases and then increases.When the laser energy density is low, the powder can't be completely melted, which results in the discontinuity of the molten pool, and a large number of unmelted powder particles and gases are retained in the shaped specimen, so the hardness of the specimen decreases.The hardness of the specimen decreases.After the energy density reaches a certain threshold, the melt pool is formed and stabilized, and the hardness of the specimen increases.
According to Figure 5, the effect of energy density on surface roughness of SLM-formed TC4 titanium alloy is as follows: with the increase of energy density, the surface roughness decreases and then increases.The minimum value is 4.95 μm, which corresponds to the energy density of 71.429 J/mm 3 .At low energy density, there are more unmelted powder and holes.In the appropriate energy density range, the powder melting state is good, the surface is flat and the forming quality is high.At too high energy density, the melt pool is poorly bonded and more semi-molten powder and pores are formed, resulting in increased surface roughness.Therefore, proper energy density improves the forming quality and reduces impurities and pores.Figure 5.Effect of energy density on surface roughness.Part of the pore defects of the sample under different energy densities are shown in Figure 6.There are two common organizational defects during the SLM forming of TC4 titanium alloys: porosity and pore defects (Figure 6(a)).Porosity is caused by undischarged gases and powder vapors and has a small diameter.Pore defects are caused by poor process parameters and are mainly unfused defects.The unfused defects are area-type defects, and their formation is mainly due to insufficient energy density caused by too low laser power or too fast scanning speed, which triggers the poor fusion phenomenon.A moderate increase in energy density changes the surface from a rough surface with a large number of pores, pores, or unmelted powder particles (Figure 6 (a)) to a flatter, smoother surface (Figure 6(c)).When the energy density is too low, the articulation of the scanning trajectory on the specimen surface fails to melt the TC4 titanium alloy powder sufficiently to form a bond, which results in the appearance of holes.When the energy density reaches 71.429 J/mm 3 , the densification also grows to 99.913%.At this time, the spacing of the holes on the specimen surface increases, and the holes become smaller and the number of holes decreases significantly.This indicates that the process parameters have a significant effect on the SLM forming surface, and an appropriate increase in energy density has a favorable effect on SLM forming.

Conclusions
(1) The experimental results show that the influence of process parameters on the density of specimens is P> v > h.Laser power is one of the most important factors affecting the density of molded parts.Under the optimal process parameters of P=225 W, v=1000 mm/s, h=0.01 mm, the highest density of 99.964% was achieved.
(2) The influence of laser energy density on the density, hardness, surface roughness, and tissue defects of the formed parts is consistent, low laser power and high scanning speed caused by insufficient melting and spheroidization effect, which not only causes a large number of tissue defects but also makes the surface roughness higher, i.e., appropriate energy density is conducive to obtaining a better quality of formed specimens.When the input laser energy density is 71.429J/mm 3 , the SLMformed specimen has the least tissue defects on the side surface, the surface roughness is 4.95 μm, the densification reaches 99.913%, and the Vickers hardness is 378 HV.

Figure 3 .
Figure 3.The effect of various parameters on the density.The range analysis results of density are shown in Table3.From Table3, it can be seen that the effects of various parameters on density are: P> v> h.The effect of various parameters on the density are shown in Figure3.According to the results of orthogonal experiments, the influence of each process parameter on densification is summarized as follows.When the laser power is too low, the energy density is insufficient, the specimen cannot completely melt the metal powder, and the powder cannot be bonded, resulting in poor liquid phase fluidity in the metal layer.The tissue defects increase, and the densification decreases with the increase in energy density.The densities of the specimens decreased significantly with increasing scanning speed and spacing.At scanning speeds between 1000 mm/s and 1600 mm/s, the densities of the specimens decreased consistently.The high scanning speed limits the liquid phase unfolding, resulting in fewer solid phase particles.When the scanning pitch was increased, the excessive increase in scanning pitch resulted in increased difficulty in lapping between scanning trajectories on the scanning surface, and the formation of holes due to incomplete melting of powder.

Figure 4 .
Figure 4. Effect of energy density on hardness.Figure 5. Effect of energy density on surface roughness.

Figure 6 .
Figure 6.Part of the pore defects of the sample under different energy densities.

Table 2 .
Orthogonal test design and test results.