The Influence of Variable Plasma Welding Parameters on Weld Geometry, Dilution Factor, and Microhardness

Abstract

Weld surfacing is the process of applying a layer of metal to the surface of metal objects by simultaneously melting the substrate. As a result of this process, the metal content of the padding weld can be as high as several tens of percents. It is a method used to regenerate machine parts and improve the properties of the surface layer, increasing its resistance to abrasion, corrosion, erosion, and cavitation. It also supports the repair and creation of permanent protective coatings in the engineering, automotive, energy, and aerospace industries. This makes it possible to repair damaged parts instead of completely replacing them, saving time and production costs. Plasma surfacing technology is used for components that require high hardness and corrosion resistance under various environmental conditions. Plasma wire surfacing is not sufficiently presented and described in the current literature, which creates problems in determining the appropriate process parameters. The influence of variable plasma surfacing parameters on steel C45 significantly affects surfacing weld geometry, the dilution factor, and microhardness. Higher currents can increase the dilution factor, integrating more base metal into the weld pool, which may alter the chemical composition and mechanical properties of the weld. Variations in surfacing speed and heat input also affect the microhardness of the surfaced joint, with higher heat inputs potentially leading to softer welds due to slower cooling rates. Optimizing these parameters is essential to achieving desired surfacing weld characteristics and ensuring the structural integrity of C45 steel joints. This paper presents the influence of varying plasma surfacing parameters on the surfacing geometry, the dilution factor, and microhardness. The tests were carried out on a Panasonic TM-1400 GIII automated surfacing machine with CastoMag 45554S solid wire as the filler material. Flat bars of C45 steel were prepared, and then the variable parameters of the surfacing process were developed. Tests were carried out to determine the dilution factor, followed by microhardness measurements. The results showed a significant dependence of the effect of the parameters on the surfacing geometry and the dilution factor.

1. Introduction

Weld surfacing is defined as a method of applying a layer of metal to the surface of metal objects while simultaneously melting a substrate, the content of which in the surfacing can be up to several tens of percents. The advantages of this process are the regeneration of machine parts and the refinement of the surface layer by increasing resistance to abrasion, corrosion, erosion, and cavitation [1]. Surfacing technologies in the global industry are developing in parallel with welding technologies or their modifications. However, cladding technologies require more research effort. There are still no normative studies regarding the quality assurance of welded products, especially in terms of regeneration. In order to increase the durability of machine and equipment parts, and at the same time save costs and, to some extent, protect the environment, surfacing is increasingly used as production and regeneration surfaces in the global industry. The development of research on the functional properties of surface layers and modern welding materials allows the development of production or regeneration technologies, thus contributing to higher efficiency and high economic efficiency [2].

The weld surfacing process has a vital role in modern industries such as mechanical engineering, automotive, power generation, and aerospace. It helps to repair high-performance parts, create durable protective coatings, and improve the performance and durability of components. Using this process allows production to be optimised by repairing damaged parts instead of completely replacing them, leading to time and cost savings [3]. This welding method maximises the functionality of machine part systems, which contributes to significant economic benefits. Plasma surfacing technology is used for components with special properties such as high hardness, abrasion resistance, and corrosion in atmospheric or chemically active environments [4].

Weld surfacing not only makes it possible to restore the dimensions of components altered by wear [5], but at the same time provides the work surface with better wear resistance due to the use of a harder surfacing metal than the base of the component [6]. The higher the hardness of the surfacing metal, the greater the wear resistance of the surface layer and the greater the difficulty of machining. This problem is complicated by the presence of specific irregularities within the surface layer. During machining, this creates impact loads on the cutter, leading to accelerated chipping and puncturing of the cutting edge. It is possible for not only the cutting edge, but the entire replaceable insert to split, i.e., the machining effort then becomes even greater. Another reason limiting the use of surfacing is the tendency to crack. To prevent cracking, preheating of the workpieces before surfacing and heat treatment (tempering) after surfacing are used, which increases the cost of surface work and makes it more difficult to carry out. Therefore, usually the hardness of the surface layer is reduced at the expense of wear resistance [7]. One method of surfacing is wire arc [8] additive manufacturing, (ang. WAAM), which is a popular wire-feed additive manufacturing technology that creates components by depositing material layer by layer [9]. WAAM has emerged as a promising alternative to conventional machining due to its high deposition efficiency, environmental friendliness, and cost competitiveness. The article presents the adaptation of a gantry machine with in situ monitoring and a control system in order to reveal the ability of WAAM technology to produce parts with complex shapes. The machine was retrofitted in several layers called hardware, control, and software layers, respectively. This study demonstrated the feasibility of an additive manufacturing-capable machine as a controlled process [10]. Plasma surfacing with powder additive material requires greater control over temperature and gas parameters to maintain optimal melting conditions, while wire is more resistant to fluctuations in temperature and operating conditions [11]. The use of wire allows for more precise and controlled deposition of thin repair layers [12].

Plasma surfacing by using a plasma jet and a conductive filler wire is carried out using a direct current of simple polarity. The arc burns between the tungsten cathode and the filler wire, applied from the side at right angles to the plasma torch axis. A low intensity pilot arc (15–25 A) also burns continuously between the cathode and the plasma torch nozzle. The arc ensures reliable ignition and stable combustion of the main arc. The parent metal heats up under the heat of the plasma jet and the heat transferred by the secondary metal droplets. The effective heat output of this heat source depends on the arc current and the distance between the wire and the parent metal. By keeping the current constant and varying the current and thus the melting rate of the secondary wire, the power consumed to heat the parent metal can be varied over a relatively wide range. Thus, with plasma jet surfacing, it is possible to control the thermal and diffusion processes at the fusion edge, which determine the depth of penetration of the parent metal and the content of the parent metal in the metal, as well as the length, composition, and properties of the fusion zone [13]. Plasma jet deposition is widely used on ships for the deposition of corrosion-resistant and anti-friction alloys. The deposition of various shafts, equipment rods and other components is carried out using copper alloys with solid supplementary wires or powder wires. Argon is used as a plasma-forming gas and as a shield [14,15].

Steel C45, also known as AISI 1045, is a medium carbon steel that is commonly chosen for plasma welding due to its specific properties that make it suitable for this kind of process. C45 steel has a carbon content of approximately 0.45%, which provides a good balance between hardness and ductility. This makes it easier to weld compared to high-carbon steels, which can be more prone to cracking. C45 can still be welded effectively with the proper techniques and precautions. Preheating and post-weld heat treatment are often employed to prevent issues such as cracking and to improve the overall quality of the weld. Compared to alloy steels or other high-performance materials, C45 steel is relatively cost-effective, making it a practical choice for many industrial applications [16].

The study [17] focused on the regularities of interfacial interactions, intermetallic phase formation (IMP) features, and defects during the surfacing of steels on titanium using four methods: P-MAG, CMT, indirect arc plasma surfacing with a conductive wire, and PAW. A general trend in the occurrence of IMPs in the surfacing of steels on titanium was found with all the methods considered. It was found that the plasma spraying technique using an indirect arc with a conductive wire was less critical with regard to IMP formation. The study showed the possibility of forming, in addition to TiFe2 and TiFe phases, a Ti2Fe phase at low heat input. The indirect arc plasma surfacing technique with a conductive wire minimises the thermal effect on the base metal. When applied at the transition boundary of a titanium-lined steel layer, the phase composition and layer structure, in some cases, approach that of the transition zone of the original ‘titanium–steel’ bimetallic sheet produced by rolling.

In the article [18], the method of multilayer plasma surfacing in an alloyed nitrogen shielding atmosphere was improved. The paper presents a comparative study of the residual stresses developed when surfacing with 3Kh2V8, 4Kh4V10Yu, and R18Yu steel wire. It is shown that the R18Yu-based wire product provides the lowest residual stresses. In addition, an improved method of multilayer plasma surfacing of heat-resistant steels in a nitrogen atmosphere is presented as preventing cracking and increasing surface metal hardness without subsequent heat treatment.

The article [19] is based on the results of fabricating multilayer semi-finished products and laminates, shaping the structure and properties of homogeneous and heterogeneous materials using plasma surfacing with straight and reverse polarity currents. It is shown that reverse-polarity plasma surfacing provides good layer fusion with minimal heat input to the base material and filler. This reduces the thickness of the fusion zone and the mixing of metal in the layers. In addition, cathode cleaning reduces the likelihood of interlayer defects.

The paper [20] presents the results of a study of the structure, phase formation, and properties of chromium–nickel alloys in plasma surfacing of high-alloy steel wire. In order to investigate the possibility of structure modification during argon arc deposition, an additional ultrasonic action was applied to the deposited material using a waveguide connected to the bottom surface of the sample. A comparison of the surface alloy structures obtained by laser deposition showed that arc welding of the alloy combined with ultrasonic action created an additional effect of increasing phase dispersion, which led to an increase in the strength of the alloy at high temperatures.

In the study [21], a medium entropy CoCrFeMnNiW alloy coating on Q235 was produced using plasma spraying technology. The wear resistance of the prepared single-layer coating and the double-layer coating was investigated using a friction and abrasion tester. The microstructure and performance of the CoCrFeMnNiW coating were investigated using optical microscopy, a nano-hardness test, SEM, and a hardness tester. The results showed that the microstructure of the coating was composed of a melt zone, equiaxial dendrites near the fusion zone, coarse co-luminal and near-surface crystals in a specific direction between the melt zone, and near-surface fine equiaxial grains.

What distinguishes this research from the existing literature is its comprehensive analysis of the specific impacts of variable plasma surfacing parameters on C45 steel, particularly focusing on weld geometry, the dilution factor, and microhardness. Unlike previous studies that may have addressed these factors individually or in different contexts, this research provides a holistic examination, integrating these aspects to offer a clearer understanding of their interdependencies. The concrete contribution of this study lies in its detailed investigation of how specific changes in plasma surfacing parameters can be optimized to achieve precise weld characteristics, thereby enhancing weld quality and performance. This work also provides practical guidelines for industry professionals aiming to improve plasma surfacing outcomes for C45 steel, bridging the gap between theoretical research and practical application. In this study, the aim was to investigate the effects of varying plasma surfacing parameters such as arc intensity, arc voltage, deposition speed, feed rate and torch distance from the work surface on the surfacing geometry, the dilution coefficient, and microhardness. The parent material was C45 unalloyed steel, the filler material was 45554S stainless steel wire, and the surfacing was carried out under protective argon gas shielding.

2. Materials and Methods

The first step in the research was to prepare the specimens and develop a set of plasma resurfacing parameters variable on the basis of the initial samples. The samples were then incubated in resin so that metallographic tests and microhardness measurements could be carried out. The metallographic tests are related to the determination of the dilution coefficient, which is important for surfacing technology [22].

The dilution coefficient in welding and surfacing technology refers to the proportion of base material mixed with the filler material in the weld pool. It significantly impacts the chemical composition, mechanical properties, corrosion resistance, microstructure, and wear resistance of the weld or surfaced layer. Controlling the dilution coefficient is crucial for achieving desired material properties and cost efficiency in surfacing applications [22].

The tests were carried out on flat bars made of unalloyed C45 steel with dimensions of 300 × 100 × 6 mm. The surface of the samples was cleaned and degreased before surfacing. The CastoMag 45554S stainless (Gliwice, Polandsteel wire with a diameter of 1.2 mm was used in the process. The wire was used for welding and surfacing difficult-to-weld steels in protective gas Argon N5.0. It is characterised by a high corrosion resistance, resistance to sudden temperature changes, cracking resistance, and the absence of spatter. The chemical composition is shown in

Table 1. Chemical composition of Castolin Eutetic CastoMag 45554S wire [23].

Element	C	Si	Mn	Cr	Ni	Mo	Nb
Percentage content	0.035	0.800	1.600	19.500	11.500	2.800	0.700

based on the Castolin Eutetic manufacturer’s (Gliwice, Poland) process card [23].

The surfacing tests were performed on an automated workstation equipped with a Panasonic TM-1400 GIII robot (Gliwice, Poland) (Figure 1), a Castolin Eutectic GAP 2501 DC (Gliwice, Poland) source, and a Castolin Eutectic E52 torch (Gliwice, Poland).

The main variable parameters were the intensity of the main arc, the plasma arc voltage, the surfacing speed, the feeding speed of the additional material, and the distance of the torch from the working surface.

Table 2. Variable surfacing parameters.

Arc Intensity [A]	Arc Voltage [V]	Plasma Surfacing Speed [m/min]	Feed Speed of Additional Material [m/min]	Torch Distance from Work Surface [m/min]
120	28	0.25	2.5	12
140	32	0.50	3.2	16
160	35	0.75	3.9	20

shows the variables identified through preliminary testing. The constant parameters, which remain consistent throughout the process, are shown in

Table 3. Constant plasma surfacing parameters.

Parameter	Value
Internal arc current	50 A
Generating gas output Plasma Varigon H9	1 dm3/min
Shielding gas output Argon N5.0	1.5 dm3/min

.

The starting points were obtained after a number of tests to achieve the most stable surfacing possible. All the variable parameters were manipulated so that the surfacing weld stitch was as beneficial as possible. Eighteen samples of five replicates of each whole surfacing stitch were conducted considering the trivalent nature of each variable (

Table 4. Plasma surfacing parameters for individual tests.

	Plasma Surfacing Parameters
Sample No.	Arc Intensity [A]	Arc Voltage [V]	Plasma Surfacing Speed [m/min]	Feed Speed of Additional Material [m/min]	Torch Distance from Work Surface [m/min]
1	160	28	0.50	3.9	12
2	160	32	0.25	2.5	16
3	160	35	0.75	3.2	20
4	120	28	0.50	2.5	16
5	120	32	0.25	3.2	20
6	120	35	0.75	3.9	12
7	140	28	0.25	3.9	20
8	140	32	0.75	2.5	12
9	140	35	0.50	3.2	16
10	160	28	0.75	3.2	16
11	160	32	0.50	3.9	20
12	160	35	0.25	2.5	12
13	120	28	0.25	3.2	12
14	120	32	0.75	3.9	16
15	120	35	0.50	2.5	20
16	140	28	0.75	2.5	20
17	140	32	0.50	3.2	12
18	140	35	0.25	3.9	16

); in order to quantify the influence of the parameter variables, an orthogonal randomised experimental plan as described in the article [24] was used.

Samples for metallographic examination were prepared by cutting the specimens at a predetermined distance from the stitch and then into smaller fragments (Figure 2 and Figure 3) using a Struers Labotom cutter5. Non-continuous surfacing samples were eliminated, and further tests were carried out to refine the final parameters.

The prepared samples were encapsulated in epoxy resin so that their dilution factor could be examined. This is determined from the formula as follows [22]:

$$D={\displaystyle \frac{B}{A+B}}100\% [-]$$

(1)

where

• D is the dilution factor [−];
• A is the cross-sectional area of the surfacing [mm²];
• B is the cross-sectional area of the surfacing mixed with the substrate material [mm²].

3. Results

The results section details the findings from metallographic tests, surfacing parameters analysis, dilution factor assessments, and hardness tests. These analyses provide crucial insights into the microstructural characteristics, optimal surfacing conditions, material mixing ratios, and mechanical properties such as hardness. Together, these results contribute to a comprehensive evaluation of the surfacing process and its impact on the final properties of the welded or surfaced material [25].

Figure 4 shows selected samples during metallographic measurements. It is noticeable that the shape of the stresses varies considerably.

Table 5. Summary of parameters of selected samples.

Sample No.	Arc Intensity [A]	Arc Voltage [V]	Plasma Surfacing Speed [m/min]	Feed Speed of Additional Material [m/min]	Torch Distance from Work Surface [m/min]	Dilution Factor D [−]
1	160	28	0.50	3.9	12	0.430
11	160	32	0.50	3.9	20	0.373
18	140	35	0.25	3.9	16	0.303
15	120	35	0.50	2.5	20	0.288
16	140	28	0.75	2.5	20	0.265
6	120	35	0.75	39	12	0.093

presents a summary of the parameters of the samples in Figure 4 in order from largest to smallest dilution factor value. The metallographic tests were carried out on a Zeiss Smart Zoom 5 microscope (Warsaw, Poland).

The results show that the same wire feed rate of 3.9 m/min occurred for samples 1, 11, and 18, and that there is also overlap in the first two samples between the arc current of 160 A and the hardfacing speed of 0.50 m/min. The results of the dilution factor for all the samples are shown in

Table 6. Results of measurement of the dilution factor of all samples.

Sample No.	D [−]
1	0.430
2	0.547
3	0.343
4	0.335
5	0.286
6	0.093
7	0.346
8	0.284
9	0.276
10	0.319
11	0.373
12	0.623
13	0.310
14	0.053
15	0.288
16	0.265
17	0.325
18	0.303

.

The highest dilution factor among all the surfacing tests carried out was achieved by test number 12 (Figure 5) and the lowest by test number 14 (Figure 6). The detailed parameters of these two trials are shown in

Table 7. Summary of sample parameters 12 and 14.

Sample No.	Arc Intensity [A]	Arc Voltage [V]	Plasma Surfacing Speed [m/min]	Feed Speed of Additional Material [m/min]	Torch Distance from Work Surface [m/min]	Dilution Factor D [−]
12	160	35	0.25	2.5	12	0.623
14	120	32	0.75	3.9	16	0.053

.

By comparing the parameters and dilution factor results for test pieces 12 and 14, it can be seen that the arc intensity of the plasma surfacing process has a strong influence on the shape of the padding weld. A higher arc intensity can result in a faster melting of the deposit, which can be advantageous in the case of hard-welded materials such as C45 steel. In addition, as test No. 14 shows, insufficient melting of the material can occur if the surfacing speed is too low, leading to poor coating quality and poor adhesion to the substrate. Excessively high surfacing and supplementary material feed rates can result in uneven material deposition, excessive spatter, and loss of process control, leading to an imperfect coating and reduced process efficiency. Therefore, it is important too maintain appropriate process parameters to ensure the quality and efficiency of plasma surfacing. Hardness measurements were performed on a FM-800 micro hardness tester in accordance with PN-EN ISO 6507-1:2018 [26] and PN-EN ISO-9015-2:2016 [27]. The results are shown in

Table 8. Results of microhardness measurements of selected samples.

	HARDNESS HV
Sample No.	Parent Mat. 1	Parent Mat. 2	HAZ1	HAZ2	Padding Weld 1	Padding Weld 2	HAZ1	HAZ2	Parent Mat. 1	Parent Mat. 2	Average
1	243	250	374	278	334	338	361	275	230	232	289
	231	243	376	294	334	334	359	264	225	225
	251	238	349, 417, 339	292, 300, 292	349	341	320, 335, 315	270, 282, 254	226	224
8	243	235	507	252	250	245	457	237	245	238	296
	251	240	536	248	255	248	498	254	261	231
	249	238	502, 491, 501	240, 255, 237	230	251	503, 490, 516	248, 251, 245	259	228
12	283	285	434	411	392	439	288	301	275	245	344
	290	287	389	453	477	476	260	277	267	251
	287	276	463, 456, 464	464, 451, 460	446	493	291, 280, 282	314, 280, 313	263	272
2	282	273	286	284	469	422	286	276	242	250	318
	291	267	307	313	447	462	289	285	248	249
	296	260	309, 460, 289	295, 310, 287	531	493	303, 289, 282	295, 275, 297	256	260
9	247	303	460	608	210	230	538	483	246	245	368
	245	380	513	631	231	202	501	376	247	250
	252	407	537, 588, 591	570, 626, 626	232	225	547, 580, 501	528, 505, 634	247	250
7	215	271	282	291	224	236	281	316	264	280	260
	213	280	272	283	231	234	240	284	260	280
	228	270	244, 260, 252	341, 257, 286	231	203	276, 238, 270	304, 319, 252	262	266

.

Figure 7 shows a summary of the results obtained and the determination of the influence of parameters on hardness. The variation in hardness results indicates differences in microcrystalline structure. Further investigations should include sample etching in a suitable solution, e.g., nital, microscopic, and mechanical tests to understand the reasons for these differences in hardness [28].

Table 8 contains the results of microhardness measurements of selected samples. Sample 1 shows significant variation in hardness in heat-affected zone 1 (349–417 HV) and stable values in parent materials 1 and 2 (approximately 230–250 HV). For sample 8, there are very high hardness results (507–502 HV) in heat-affected zone 1, indicating clearly higher mechanical properties compared to the other materials. This may be due to the perlite structure encountered. In sample 12, in heat-affected zones 1 and 2, the results show high hardness values, similar to that of padding weld 2, which may suggest good wear resistance. Sample 9 has the highest average hardness value (368 HV), mainly due to very high hardness values in heat-affected zone 2 (608–626 HV) and heat-affected zone 1 (460–537 HV). Sample 7 shows lower hardness values compared to samples 8, 12, and 9, with values in parent materials 1 and 2 having values in the range 215–280 HV.

4. Conclusions

The study of C45 steel in plasma surfacing has limitations such as sensitivity in the heat-affected zone, the need for post-weld heat treatment, and the potential for cracking and distortion. Future research should focus on optimising welding and surfacing parameters, exploring advanced techniques, investigating alternative filler materials, and understanding the long-term performance of welded joints. Additionally, the use of simulations and environmentally friendly practices could enhance welding outcomes for C45 steel.

Research into the plasma surfacing process reveals a significant influence of process parameters on the mechanical properties of the surface layers. Analysis of the results shows a relationship between microhardness and the mixing coefficient. It is observed that with increasing microhardness, the mixing ratio also increases. This trend may be the result of better integration of the surfacing layer with the substrate, resulting in increased durability and mechanical resistance of the resulting structure. Understanding the influence of surfacing parameters on these properties is crucial for various applications where durability and wear resistance are important. This suggests that optimisation of process parameters can be an effective strategy in achieving the desired mechanical properties of surface layers.

The plasma arc intensity has the greatest influence on the geometry of the coating and the mixing ratio. This confirms the importance of controlling this parameter in the process. However, further research is needed to more fully understand this relationship and further implications. Continued experimentation may provide more detailed information on optimal process parameter settings, which could lead to further improvements and innovations in plasma surfacing.

Practical implications include the need for optimized plasma welding parameters and advanced techniques to ensure high-quality welds. Investigating alternative filler materials and understanding long-term performance are crucial for enhancing the reliability and durability of welded joints. Additionally, leveraging simulation tools and environmentally friendly practices can lead to more efficient and sustainable welding processes, ultimately broadening the applications of C45 steel in various industries. These results underscore the critical role of precise surfacing parameters in achieving desirable weld characteristics such as minimised dilution and enhanced hardness. Practical applications could benefit from these insights by optimising welding conditions to improve weld quality and durability in industrial settings. However, remaining questions persist, such as the long-term durability of surfaced materials under varying environmental conditions and the development of more sustainable welding practices. Addressing these gaps in future studies could further refine plasma welding techniques and broaden their applicability across diverse engineering applications.