Mechanical and Microstructural Characterization of Class 800 Complex Phase Steel before and after the Laser Welding Process

Abstract

Complex phase steels, known for their high levels of conformability, energy absorption, and deformation capacity, are among the more advanced high-strength steels. The objective of this study was to compare the mechanical properties of CPW 800-class complex phase steels, with and without laser welding. The analysis involved determining tensile strength, yield strength, elongation, and area reduction through tensile tests, in scenarios both with and without laser welding. Additionally, the number of cycles was assessed via fatigue tests, and absorbed energy was measured using impact tests. The non-parametric Kruskal–Wallis test, at a 5% significance level, revealed that tensile strength, yield strength, area reduction, and absorbed energy were statistically similar regardless of laser welding. However, elongation and the number of cycles showed significant differences. The fractured surface from axial fatigue tests exhibited ductile characteristics, with the additional presence of dimples or alveoli.

1. Introduction

In recent years, advanced high-strength steels (AHSS) have garnered significant research interest due to their excellent combination of strength, ductility, and affordability. New concepts for high-strength steels continue to emerge, driving the development of advanced metallurgical techniques and the modernization of equipment infrastructure [1,2].

Over the past decade, the automotive and steel industries have collaborated to promote the development of a “third generation” of Advanced High-Strength Steel (AHSS). This new generation aims to bridge the gap between the limited conformability of “first generation” Dual Phase (DP) steels and the high cost of “second generation” Twinning-Induced Plasticity (TWIP) steels [3].

The use of Advanced High-Strength Steels (AHSS), such as Dual Phase (DP) steel, Transformation-Induced Plasticity (TRIP) steel, and Complex Phase (CP) steel, has become a dominant trend in vehicle construction [4]. Additionally, martensitic steel is well-known for its high levels of hardness and wear resistance [5]. According to [6], vehicles now contain more than thirty percent AHSS, including DP steels, TRIP steels, and steels with martensitic microstructures. This trend is driven by the increasing challenges automakers face in terms of environmental impact, safety, and fuel economy [7].

In practice, the need to improve fuel economy has been a significant driving force behind weight reduction, pushing the transition to Advanced High-Strength Steels (AHSS) [8,9]. Additionally, new safety requirements have dramatically impacted material trends [10,11]. Nowadays, the traditional approaches of increasing the material thickness or that of the section are generally avoided, as these measures can hinder efforts to improve fuel economy, reduce emissions of polluting gases, and maintain vehicle aesthetics [12].

Additionally, as noted by [13], thermographic investigations of construction materials from the AHSS group, when deformed at high strain rates, are of significant interest across various technological fields, particularly in the automotive industry and other transportation sectors.

In this context, the objective of this work was to compare the mechanical properties of complex phase steels, specifically the CPW 800 class, with and without the application of laser welding.

Laser welding was used in order to assess its influence on the mechanical properties of the material through mechanical and statistical tests, determining whether it improves or deteriorates the properties of the steel.

1.1. Laser Welding Process

The laser, or Light Amplification by Stimulated Emission of Radiation, is characterized by its monochromatic nature, having a well-defined wavelength. All the photon waves that compose the beam are in phase, propagating as a beam of nearly parallel waves [14]. The advantages of using laser welding include a high penetration-to-width ratio (10:1), a capacity for high-speed welding, low heat conduction (resulting in low levels of distortion and residual stresses), and high flexibility, resulting in strands with a good finish and joints with excellent mechanical properties. It does not require electrodes and allows for the welding of dissimilar materials [15]. Additionally, laser welding offers high welding speed and low heat input compared to other welding methods, leading to reduced deformation and a narrow width of the bead and heat-affected zone (HAZ). These characteristics enable the welding of complex parts that would be challenging to weld using other methods. Consequently, the importance of laser welding has become widely recognized [16].

The disadvantages of laser welding include the need for precise joint positioning and alignment, limited penetration depth (up to approximately 20 mm with medium-power equipment), potential porosity due to rapid solidification, low energy efficiency (around 10%), and a high initial investment [17].

1.2. Laser Welding of Advanced High-Strength Steels (AHSS)

Laser welding is known to introduce considerably less heat into the materials than conventional welding techniques. The increase in the microstructure grains in the HAZ is minimal, as is the width of the HAZ. Furthermore, the process is particularly beneficial in joining different types and thicknesses, and even different materials, providing flexibility in the design and manufacture of automotive structures. However, some problems may occur, such as residual stresses in the joint area due to localized thermal expansion and contraction caused by rapid heating and cooling cycles [18,19]; formation of brittle material in the form of martensite; and hydrogen embrittlement [20]. Welding of AHSS is challenging, as the high-strength characteristics with good formability cannot be sustained under the extensive heating, melting, and solidification occurring during the welding process [21]. Reisgen et al. (2012) [22] studied the laser welding of TRIP steel and DP steel joints and concluded that, for these materials, there is microhardness dispersion as a result of the microstructure in the fusion zone and heat-affected zone. Furthermore, they observed the good quality of the joints in tensile tests, in which fracture occurred in the base metal for these materials. Lun et al. (2017) [23] studied the laser welding of TRIP steels and observed that the fusion region consisted mainly of martensite, with a small amount of austenite between the dendrite nuclei. Furthermore, the performance of the laser-welded joints was very close to that of the base metal in tensile tests. Farabi, Chen, and Zhou (2011) [23] studied the laser welding of DP steels and reported softening in the heat-affected region; the results demonstrated that the higher the degree of strength of the material, the greater the softening thickness. In tensile tests, the materials broke in the softened region, and the type of fracture found was ductile fracture. Similar results were found by Dong et al. (2014) and Farabi et al. (2010) [24,25] for the same material. Kašpar and Němeček (2015) [26] studied laser welding in CP steel and reported that the laser welding process had minimal impact on the tensile strength of the specimens, finding good fatigue properties for the tested specimens and that fatigue cracks initiated at the weld bead, where there was a higher concentration of carbon.

In this regard, studies of laser welding processes in complex phase steels still have a large gap in the opportunities for study, since in the literature there is a greater focus on dual-phase steels and those with the TRIP effect.

1.3. Non-Parametric Test: Laser Welding

The Kruskal–Wallis test statistic measures the extent to which the observed mean ranks (Ri) deviate from their expected value ((N + 1)/2). If this deviation is substantial, the null hypothesis (H0) will be rejected. The test statistic is provided in Equation (1).

$$H={\displaystyle \frac{12}{N(N+1)}}\sum _{i=1}^a{n}_i{\left(R_i-{\displaystyle \frac{N+1}{2}}\right)}^2$$

(1)

An alternative calculation formula, which can sometimes be more convenient, is provided in Equation (2).

$$H={\displaystyle \frac{12}{N(N+1)}}\sum _{i=1}^a{\displaystyle \frac{R_i^2}{n_i}}-3(N+1)$$

(2)

The null hypothesis (H0) should be rejected if the sample data produce a large value for H. The null distribution of H is derived based on the assumption that, under H0, each possible assignment of ranks to treatments is equally likely. Consequently, all possible assignments could be enumerated, and the frequency of each H value could be counted, resulting in tables of critical values for H. However, these tables are typically limited to small sample sizes (n_i) [27]. For larger samples, the following approximation is commonly used when H0 is true, as shown in Equation (3):

$$\begin{array}{l}a=3 \mathrm{a}\mathrm{n}\mathrm{d} n_i\ge 6 \mathrm{t}\mathrm{o} i=1, 2, 3\\ a>3 \mathrm{a}\mathrm{n}\mathrm{d} n_i\ge 5 \mathrm{t}\mathrm{o} i=1, 2, \dots , a\end{array}$$

(3)

Then, H approximately follows a chi-square distribution with a − 1 degrees of freedom. Since large values of H suggest that H0 is false, we will reject H0 if the observed value, as given in Equation (4), is sufficiently large.

$$h\ge {\chi }_{\alpha , a-1}^2$$

(4)

The test has a significance level of approximately α. When observations are tied, an average rank is assigned to each of the tied observations. In the presence of ties, we must replace the test statistic in Equation (2) with the one in Equation (5):

$$H={\displaystyle \frac{1}{S^2}}\left[\sum _{i=1}^a{\displaystyle \frac{R_i^2}{n_i}}-{\displaystyle \frac{{N(N+1)}^2}{4}}\right]$$

(5)

where n_i represents the number of observations in the treatment, N is the total number of observations, and the variance is defined by Equation (6).

$$S^2={\displaystyle \frac{1}{N-1}}\left[\sum _{i=1}^a\sum _{j=1}^{n_i}{R}_{ij}^2-{\displaystyle \frac{{N(N+1)}^2}{4}}\right]$$

(6)

Note that S² represents the variance of the stations. When the number of moorings is moderate, the difference between Equations (1) and (3) is minimal, so the simpler form, Equation (2), can be used [27].

2. Materials and Methods

The material used in this work was class 800 complex phase steel (CPW 800). Its chemical composition, in percentage by weight (%w), is as follows: C (0.120), Si (0.560), Mn (0.570), P (0.013), S (0.006), Cr (0.003), Ni (0.018), Mo (0.005), Al (0.034), Ti (0.096), V (0.005), Nb (<0.005), B (<0.005), and Fe (balance).

Figure 1 illustrates the steps followed to perform the experimental procedure, with descriptions of each step provided in sequence. OM refers to optical microscopy, and SEM stands for Scanning Electron Microscopy. The non-parametric Kruskal–Wallis statistical test was applied to compare the mechanical properties of the CPW 800 class in cases with and without the use of laser welding.

2.1. Laser Welding

To complete all tests in this work and analyze the mechanical properties, laser welding was performed to join four 30 × 30 cm flat plates (welded in pairs) of CPW 800 steel, each with a thickness of 2.3 mm. The sequence used in the laser welding is shown in Figure 2.

Initially, all parameters of the machine used for laser welding of the plates were determined: welding speed of 2.5 m/min; power of 80 W; and laser focus height of 1.0 mm, where point 0 is on the surface of the sheet, resulting in the laser focus being 1 mm below the surface and leaving 1.3 mm to the bottom of the material, since it is 2.3 mm thick. Welding positioning was set to 0.05 mm—the welding process joins two CPW 800 steel sheets, and these sheets are positioned at a distance of 0.05 mm. These parameters were determined through pre-tests and the examination of previous results.

The monitoring setup involved using suction cups on both sides to attach the plates and fit the two together, side by side, allowing them to undergo the welding process. The plates then passed through two rollers, which “smoothed” (flattened) them to ensure they were perfectly aligned, even when plates of different thicknesses were used. The plates were monitored by two cameras, one positioned above and the other below, which moved synchronously with the plates to prevent any positioning issues during welding. Finally, the joined plates were welded using the laser, within a setup incorporating the entire monitoring system.

2.2. Mechanical Tests

In order to perform tensile, axial fatigue, and impact tests, sub-sized test specimens were used. These specimens, with a thickness of 2.3 mm, were obtained from laser-welded flat plates, with the weld strand precisely centered in each specimen. The use of these specimens with sub-sized dimensions is necessary, since the work evaluates precisely the steel sheets that are applied within the automotive industry. This configuration for mechanical tests is recurrent in the study of these materials and the evaluation of their mechanical properties, as shown in the work presented by [28,29,30,31,32,33,34,35]. In addition, the evaluations of the properties obtained are not comparable with those for different specimens with different dimensions, but are specific to sub-sized specimens of CPW 800 steel.

After laser welding, the test specimens were removed from the plates, and their dimensions were established according to the relevant standards. The flat laser-welded test specimens were extracted in the rolling direction of the plate. They were then cut using wire electrical discharge machining (EDM) in order to ensure a better trim without any burrs and to guarantee dimensional accuracy.

For the mechanical properties, the tensile strength limit, yield strength limit, elongation, and area reduction were determined from the tensile-vs.-deformation chart. In accordance with the ASTM 8 M standard, eight laser-welded test specimens were prepared. For axial fatigue tests (R = 0.1), 15 laser-welded specimens were made following the ASTM E466 standard. Three axial fatigue tests were conducted for each tensile level (98%, 96.5%, 95%, 90%, and 80%) of the average yield stress found during the tensile test. In accord with the ASTM E 23 standard, 16 laser-welded test specimens were prepared. The notch of the laser-welded test specimen CPW 800 was made using wire EDM.

The temperatures used for the impact test were −40 °C, 0 °C, 28 °C, and 60 °C. Four test specimens were used for each temperature, with a total of 16 specimens analyzed. These temperatures were chosen to reflect the conditions faced by vehicles made from this material, as car manufacturers supply both warmer and colder regions. For −40 °C, the test specimens were placed inside a polystyrene box with a saturated mixture of acetone and dry ice. For 0 °C, they were placed inside a polystyrene box with ice. For 60 °C, they were placed inside a muffle furnace maintained at 60 °C. The 28 °C tests were conducted at laboratory room temperature, as recorded on the specific day of the test.

2.3. Non-Parametric Kruskal–Wallis Statistical Test

Non-parametric statistical tests were conducted to analyze the two conditions of the complex phase steel (CPW 800), namely, before and after the laser welding process, for each mechanical test performed. The objective of the non-parametric Kruskal–Wallis test is to determine the median of results for each mechanical test. This test was used due to the small number of test specimens: 8 for the tensile test, 15 for the axial fatigue test, and 16 for the impact test. This study evaluates the hypotheses H0 (the data or samples are the same) and H1 (the data or samples are different). To accept H0, the p-value must be greater than 0.05.

2.4. Microstructural Analysis

2.4.1. Optical Microscopy

The objective of this analysis was to identify the heat affected zone (HAZ), weld metal zone (WMZ), and base metal (BM), as well as the phases present in the weld bead region of laser-welded CPW 800 steel. For this purpose, metallographic preparation was performed using techniques standardized by ASTM E 3-10, including sectioning, inlaying, sanding, and polishing. The samples were chemically etched with a 3% nital reagent.

The photomicrographs were captured in bright field lighting using a NIKON EPIPHOT 200 Optical Microscope (OM) connected to a computer and equipped with a ZEISS AXIO CAM ICC3 digital camera. The images were processed using ImageJ software (version 1.54).

2.4.2. Scanning Electron Microscopy

The objective of these analyses was to observe the fractured surfaces of the laser-welded specimens in greater detail after axial fatigue and impact tests. For the axial fatigue test, specimens which ruptured at different stress levels, including 95% of the yield stress, were analyzed. For the specimens ruptured during the impact test, a sample tested at 60 °C was used. Before placing the samples in the SEM vacuum chamber, the fractured surfaces were cleaned with acetone to remove impurities, using SONICLEAN 2PS ultrasound equipment. The fractured surfaces resulting from the axial fatigue and impact tests were then examined with a ZEISS EVO LS 15 Scanning Electron Microscope (SEM).

3. Results and Discussion

3.1. Microstructural Characterization of CPW 800 Steel: Base Metal and Welded Region

In this stage, the qualitative aspects of the microstructure obtained from two variations of chemical attacks, nital 2% and LePera, are analyzed and discussed. The results are presented and discussed through qualitative analysis by observing the structure to identify the phases present using the chemical reagents.

Figure 3a shows the metallography of the CPW 800 complex phase steel sample in the longitudinal orientation, subjected to a 2% nital etch for 8 s, at 1000× magnification. Figure 3b displays the metallography of the CPW 800 steel in the transverse orientation, etched with 2% nital for 9 s, at 1000× magnification. In this image, the following phases are identified: Ferrite (F), Martensite (M), Retained Austenite (RA), Pearlite (P), and Bainite (B).

Using the 2% nital chemical reagent, it is possible to outline the grain contours of the structure, revealing two shades of gray: a lighter shade corresponding to ferrite, and a darker shade corresponding to either retained martensite or retained austenite, bainite, and/or pearlite. This makes it challenging to precisely identify the outlined structure. The observed structure is complex, with phases appearing simultaneously, which is characteristic of typical multiphase structures. The most effective method for this chemical etching involved dipping a portion of cotton in the 2% nital solution and rubbing it against the sample surface at room temperature, specifically 23 °C, in the laboratory where the analysis was conducted.

In Figure 4a,b, the CPW 800 complex phase steel sample, after being subjected to chemical etching with LePera reagent for 25 s, is viewed in a longitudinal orientation at a magnification of 1000×. The LePera reagent, based on sodium metabisulfite and picric acid, is ideal for highlighting the bainitic structure (B), which appears light brown or dark brown [36,37,38]. The phases containing martensite and retained austenite (RA) cannot be easily differentiated, as these substances appear in light blue or white shades, while ferrite (F) appears in light green to light blue tones. The application of this reagent must be preceded by a pre-etching with 2% nital to delineate the grain contours of the structure, enabling the subsequent etching with LePera reagent to highlight the microconstituents with greater contrast [39]. The average pre-etching time with 2% nital varied between 20 and 25 s.

At this stage of analyzing the CPW 800 complex phase steel sample (subjected to chemical etching with 2% nital) using the Scanning Electron Microscopy (SEM) technique, as shown in Figure 5, it is possible to identify the phases present in the material. Figure 5a,b illustrate the following phases: Ferrite (F), Martensite (M), Retained Austenite (RA), and Bainite (B). The resolution of the BSE (backscattered electrons) image is lower than that of SE (secondary electrons), as the regions cover a larger area than those associated with secondary electron release on the analyzed surfaces [40,41].

The phase mapping in the CPW 800 complex phase steel sample with 2% nital chemical etching, as carried out with the aid of the SEM technique, clearly shows the phases present and provides a good definition of the grain contours. As evidenced in the literature review, the presence of ferritic, bainitic, and martensitic and/or austenitic phases can be noted, each with a different morphology, which facilitates their identification.

Figure 6 shows the metallography of the welded region of the CPW 800 material. In this image, the small area affected by the welding process and the changes in the microstructure are clear. In the Molten Zone (MZ) region, slightly elongated grain boundaries characteristic of fusion structures were observed. The entire region affected by the welding process has a length of approximately 1300 µm, indicating the minimal influence of the process on the sheets of CPW 800 material. It was observed that the Thermally Affected Zone (TAZ) formed is very small in laser welding. The CO₂ laser used for this welding process heats and cools the joining points of the sheets simultaneously, thus avoiding excessive heating, which generally occurs in other welding processes. In the base metal (BM) region, more homogeneous grain boundaries can be observed, due to the material’s multi-constituted structure, combined with fine precipitates.

3.2. Tensile Test

Table 1. Comparison of the average mechanical properties of CPW 800 material in its non-welded state and after laser welding, following the specified methodology.

Tensile Strength Limit (MPa)			Yield Limit (MPa)		Elongation (%)		Reduction of Area (%)
	CPW800	Welded	CPW800	Welded	CPW800	Welded	CPW800	Welded
Mean	824 ± 21	817 ± 27	725 ± 18	745 ± 18	17 ± 1	25 ± 5	60 ± 6	64 ± 6

presents the average tensile test results for eight specimens of non-welded CPW 800 material and laser-welded CPW 800 material, following the specified methodology.

Figure 7 compares the behavior of the specimens in the tensile test. Figure 7a,b,d, respectively, show the behavior graphs for tensile strength, yield limit, and area reduction. Despite differences in values, when compared with Table 1, the means and standard deviations indicate that the laser welding process had no significant influence on these parameters. This conclusion is supported by the statistical results of the non-parametric Kruskal–Wallis test, as shown in

Table 2. Results of the Kruskal–Wallis non-parametric test for the parameters studied in the tensile test.

Parameter	Median		p-Value
	CPW800	Welded	CPW800	Welded
Tensile Strength Limit (MPa)	820	814	0.834	0.834
Yield Limit (MPa)	721	743	0.140	0.141
Elongation (%)	17	28	0.003	0.003
Reduction of Area (%)	60	67	0.598	0.600

. The values obtained for tensile strength, yield limit, and reduction of area can be attributed to the advantages of laser welding. These advantages include high energy density, high welding speed, small heat-affected zones (HAZ), low material loss, precise control of heat input, and high levels of automation. Additionally, compared to traditional welding methods, laser welding improves the microstructure and reduces the tendency for segregation in the weld zone, resulting in high-quality weld joints [42,43,44,45,46,47]. However, the results for elongation (%) presented in Figure 7c, Table 1 and Table 2, show an increase compared to the non-welded CPW 800 material. The obtained values exceeded expectations for CPW 800. A possible explanation could be the phenomenon of softening in the region around the weld, which has been investigated for TRIP, DP, and HSLA steels [48].

The problem of softening due to the welding process has been studied in recent years; however, for CPW 800 steel, it still needs to be further discussed. Biro et al. [49] investigated these characteristics for dual-phase steels and reported that the heat input required for softening the HAZ decreased along with an increase in the carbon content in the martensite [50]. Park et al. [51] and Kong et al. [52] evaluated the softening characteristics for CP steel, but as a function of the boron concentration. For nickel superalloys, heating to the tempering temperature during welding provides softening of the HAZ metal several millimeters away from the weld [53]. Lambert-Perlade et al. [54] and Lan et al. [55] investigated the phase transformation of the HAZ and the effect on the toughness of the microstructure in low-alloy structural steels after simulated welding. However, the microstructure and softening behavior of laser-welded 960 MPa grade high-strength steel joints have recently been reported, and the microstructural evolution of HAZ still needs to be further investigated [56].

To evaluate the softening, a hardness test was performed with a Vickers diamond indenter, square-based pyramidal, with an angle of 136° between the planes, an application of a load of 1.96 N (200 g), and an indentation time of 15 s. Eight indentations were made in the base material and in the welded material to obtain the average hardness of the region. For the CPW 800 steel, 260 ± 0.73 HRV was obtained, and for the welded CPW 800 material, 230 ± 0.94. Therefore, according to the references and the hardness test performed, softening due to welding can be an explanation for the elongation of the CPW 800 steel.

The tensile test results indicate increases in both yield strength and elongation. The statistical test confirms that the welding process influences these parameters, thus suggesting that the material experienced an increase in ductility after undergoing the welding process.

These results are in line with those of Różański et al. (2016) [57], where the studied CP steel welded joint exhibited tensile resistance properties equal to or greater than those of the base material. Regarding elongation, Sun et al. (2020) [58] found that annealing CP800 steels increases the bainite and martensite content, thereby improving tensile strength and elongation properties. The temperatures reached in the HAZ and the observed behavior in the test suggest that similar changes may have occurred in the joint, leading to improved elongation properties, as shown in the results.

3.3. Fatigue Test

Axial fatigue tests were conducted on fifteen specimens of CPW 800 steel, standardized according to ASTM E 466, to compare welded and non-welded materials. The specimens ruptured at the weld zone precisely. Stress levels of 98%, 96.5%, 95%, 90%, and 80% of the average yield stress found in the tensile test (as shown in Table 1) were applied. The results of the axial fatigue test are presented in

Table 3. Result of the number of cycles obtained in a fatigue test, varying the tension and comparing the CPW 800 material with and without welding.

	98%	96.5%	95%	90%	80%
CPW 800	95,159	216,378	168,138	1,000,000	>1,000,000
	112,521	126,967	291,583	1,000,000	>1,000,000
	131,117	136,155	214,850	1,000,000	>1,000,000
Mean	112,932	159,833	224,857	1,000,000	>1,000,000
SD	17,982	49,184	62,328	0	0
Welded	17,143	27,744	28,284	21,346	42,016
	17,383	16,223	25,303	19,223	46,720
	32,778	19,246	16,035	31,192	42,148
Mean	22,435	21,071	23,207	23,920	43,628
SD	8958	5973	6388	6386	2679

and graphically plotted as Stress (S) vs. Log (N) in Figure 8.

From the analysis of the S-logN curve (Figure 8a) for CPW 800 complex phase steel, it can be concluded that this material exhibits a higher fatigue resistance limit compared to other advanced high-strength steels such as Dual Phase (DP) steel and Transformation-Induced Plasticity (TRIP) steel, which are also used for structural purposes in vehicles. CPW 800 steel demonstrated a fatigue resistance limit of approximately 650 MPa, compared to 570 MPa for TRIP 780 and 580 MPa for DP 780. This indicates that complex phase steel can be effectively used in vehicle components that require good fatigue resistance, thereby increasing its usability. The superior fatigue limit of CPW 800 steel, compared to DP 780 and TRIP 780, is attributed to its microstructure, which exhibits a homogeneous hardness of its micro-constituents. This homogeneity leads to better performance under fatigue demands [59,60,61,62].

As shown in Figure 8b, laser-welded specimens exhibited lower levels of fatigue life compared to the non-welded specimens, even when subjected to lower stress levels. Even under lower voltage conditions, fatigue life never reached the infinite life threshold of (>10⁶) cycles. Moreover, fatigue life remained relatively constant at approximately 10^4.4 cycles, regardless of stress variations. This reduction in fatigue life can be attributed to the microstructural changes induced by the welding process. As will be shown, the weld bead forms a martensitic structure. Although this microstructure is harder and more resistant, it weakens the region, leading to a decrease in the material’s fatigue life.

Table 4. Results of the Kruskal–Wallis non-parametric test for the parameters studied in the fatigue test.

Parameter	Median		p-Value
	CPW800	Welded	CPW800	Welded
Number of Cycles	216,378	25,303	0.000	0.000

presents the non-parametric test results for the axial fatigue test (number of cycles). At a significance level of 5%, the results for specimens with and without laser welding were statistically different. Therefore, it can be concluded that the laser welding process has a significant influence. The pronounced influence of laser welding is evident, as the specimens ruptured in the weld bead precisely. This may be attributed to welding failures, such as low energy efficiency (~10%), material roughness, and mechanical defects.

As seen in Section 3.1, CP800 steel presents a refined microstructure (Figure 3, Figure 4 and Figure 5). After the laser welding process, the microstructure is altered, even with the laser beam acting on a small thickness, as shown in Figure 6. This alteration causes a rearrangement of the structure, implying a decrease in resistance and fatigue life. This conclusion is in line with the results of Zhou et al. (2024) [63], who observed that the strength and fatigue lifetime of complex phase steel increase with the decrease in grain size.

3.4. Impact Test

The results from the Charpy impact test, conducted on ASTM E 23 (sub-sized) specimens, are presented in

Table 5. Results of absorbed energy in the impact test.

Temperature		−40 °C	0 °C	28 °C	60 °C
Absorbed Energy (J)	CPW 800	24	24	27	25
		25	25	25	25
		26	27	25	27
		24	25	25	27
	Mean	25	25	26	26
	ST	1	1	1	1
Absorbed Energy (J)	Welded	19	20	29	17
		22	29	33	26
		23	31	27	29
		23	20	22	24
	Mean	22	25	28	24
	ST	2	6	5	5

as absorbed energy (J). The graphical representation of these results is shown in Figure 9.

Comparing the results obtained, it can be observed that the energy absorbed between temperatures of −40 °C to 60 °C is practically the same (within the dispersion range of the results). This indicates that complex phase steel does not show significant differences in impact energy absorption within the evaluated range, ensuring the reliability of structural calculations used during vehicle design, regardless of temperature variations in potential collision scenarios. When compared to other classes of advanced high-strength steel, such as DP steels and TRIP steels, complex phase steel demonstrates superior impact resistance [29,64,65]. Therefore, complex phase steel can be effectively used in vehicles, particularly in components requiring high impact resistance, such as bumper supports, which are currently produced using DP or TRIP steels.

This increases the usability percentage of complex phase steel in automotive applications.

Table 6. Results of the Kruskal–Wallis non-parametric test for the parameters studied in the impact test.

Absorbed Energy (J)	CPW800		Welded
	Median	p-Value	Median	p-Value
−40 °C	25	0.019	23	0.021
0 °C	25	1.000	25	1.000
28 °C	25	0.297	28	0.312
60 °C	26	0.559	25	0.564

presents the non-parametric test results for the impact test (absorbed energy). At a significance level of 5%, the results are statistically the same, indicating that there is no difference between using and not using the laser welding process. This suggests that laser welding had no influence on the parameter analyzed, likely due to the high-quality weld beads, high welding speed, and high flexibility [66].

3.5. Fractographic Analysis of Fractured Samples from the Fatigue Test by Scanning Electron Microscopy

Figure 10 shows images obtained from the fractured region during axial fatigue tests, using Scanning Electron Microscopy (SEM), for the stress level of 95% of the yield stress of CPW 800 steel, with laser welding.

Figure 10a shows the initial region of the sample fractured by axial fatigue after laser welding, at a magnification of 100×. Regions of nucleation and propagation of the fatigue crack were observed for a short duration due to the high level of tension. In Figure 10b, the final region of the same sample is shown, also at 100× magnification, where the ductile characteristics of CPW 800 steel were observed. This indicates that significant plastic deformation occurred prior to fracture during the fatigue test. This is usually accompanied by low levels of hardness and resistance, and considerable tolerance to discontinuities in the material. In Figure 10c, the initial region of the sample fractured by axial fatigue after laser welding is shown at a magnification of 500×. Regions of nucleation and propagation of the fatigue crack were observed due to the high level of tension, i.e., 95% of the yield stress. Figure 10d shows the final region of the sample fractured by axial fatigue after laser welding, at a magnification of 500×. Ductile fracture was observed, resulting from the application of excessive force to the metal, which has the ability to deform permanently or plastically before fracture. The presence of shallow dimples or alveoli over the entire fracture surface due to overload was also noted [67].

Figure 10e shows the final region of the sample fractured by axial fatigue after laser welding, at a magnification of 1000×. Ductile fracture was observed, with alveoli present across the entire fracture surface. The material’s ductile characteristics indicate significant plastic deformation in the fracture region, where the applied shear stress must exceed the shear strength before other fracture modes can occur. Ductile fracture originates from microcavities or dimples (close to the center of the necking region) caused by deformation on the fracture surface [67].

In Figure 10f, the central region of the sample fractured by axial fatigue with laser welding is shown at a magnification of 1000×. The presence of striations in the CPW 800 steel was observed, indicating stable crack growth. At this stage, the crack advances progressively with each stress cycle. This stage is highly characteristic of the fatigue process, in which specific marks, such as striations, develop. The final fracture occurs during the last stress cycle, when the crack reaches a critical size, leading to unstable propagation and catastrophic failure [68,69].

Figure 11 refers to the fatigue-fracture surfaces of the CPW 800 material specimens (without the laser welding process) subjected to a stress level of 95% of the yield stress.

Figure 11a shows the fracture at 95% of the yield stress, showing the ductile rupture mode of the sample (end region of the sample), at a magnification of 100×. Figure 11b shows the fracture at 95% of the yield stress, showing the ductile rupture mode (end region of the sample), at a magnification of 500×. Figure 11c shows the fracture at 95% of the yield stress, showing the region with fatigue striations, at a magnification of 1000×. And Figure 11d shows the fracture at 95% of the yield stress, showing the shear band region, at a magnification of 1000×.

From the images obtained with the aid of the SEM, the ductile appearance of the fracture surface of the material can be observed, along with the presence of microcavities and small dimples, indicating the presence of fine precipitates in the material. Additionally, scattered cracks are noted on the fracture surfaces. In each case, the fracture originated from a different point on the surface of the specimens, with no fractures occurring due to inclusions. The presence of wide fatigue bands was attributed to the high loads applied to the specimen during the test (95% of the yield stress), and characterizes the onset of fatigue cracking.

According to Bathias (2001) [68], in the giga-cycle regime, internal defects or variations in the grain size of the material compete with surface defects to cause fatigue fractures. From a probabilistic point of view, it is evident that the greatest presence of defects is concentrated inside the material, compared to its surface. However, if the defect density is higher at the surface, competition may occur between the surface and the interior of the material, leading to fracture initiation at the surface. This phenomenon was observed in the present work. None of the analyses carried out using the scanning electron microscope (SEM) revealed fractures starting in inclusions.

Furthermore, numerous models have been proposed for the nucleation of microcracks, such as those observed. Based on the fractographs presented in Figure 10 and Figure 11, which refer to the axial fatigue tests of complex phase steel, it is evident that the mechanisms operated at the nucleation sites. The model proposed by Wood (1956) [69] suggests that the formation of microcracks is identical to the continuous growth of intrusions. As these intrusions increase and facilitate the formation of new intrusions, microcracks will nucleate.

The fatigue behavior of metals is sensitive to their microstructure. The formation of slip bands and the initial propagation of microcracks are influenced by grain size, type of crystalline structure, material texture, and obstacles to dislocation movement, such as carbides, precipitates, and inclusions [70,71,72,73]. Analyzing the fatigue resistance limit results from the axial fatigue tests, it is evident that complex phase steel has advantages over two-phase steel and TRIP. This superior performance is directly linked to the microstructural characteristics of complex phase steel, which has a much finer grain refinement compared to other advanced high-strength structural steels. This refinement is one of the hardening mechanisms for metals, specifically the grain size factor [70,71,72,73].

Fatigue resistance can be increased by reducing ductility, which is the material’s ability to deform. By avoiding the formation of persistent slip bands and increasing the material’s hardness, fatigue resistance can be significantly improved [71]. The presence of an unwanted phase or a large difference in hardness values between two phases or particles (such as carbides, precipitates, or inclusions) can drastically reduce the fatigue resistance of a material [73]. This explains the fatigue results observed for the material after the laser welding process, where there was an increase in ductility and the formation of a new microstructure in the molten and heat-affected zone.

4. Conclusions

To compare the mechanical properties of CPW 800 steel (complex phase steel) with and without laser welding, experimental tests were conducted, including tensile tests, axial fatigue tests, and impact tests. A non-parametric Kruskal–Wallis statistical test was applied. At a 5% significance level, the following conclusions were drawn:

• Tensile test: Considering Tensile Strength Limit, Yield Limit, and Reduction of Area, the results with and without welding are statistically the same.
• Tensile test: Considering Elongation, the results with and without welding are statistically different.
• Axial fatigue test: Considering the number of cycles, the results with and without welding are statistically different.
• Impact test: Considering absorbed energy, the results with and without welding are statistically the same.

Laser welding on complex phase steel contributed to an improvement in the mechanical properties of this material, as demonstrated through mechanical and statistical tests. For the automotive industry using CP steel, the use of laser welding is highly beneficial due to its good weld bead finish, higher mechanical strength, high flexibility, and low thermal input, making it economically viable to use.

Regarding the characterization of the fractured surface of the laser-welded CPW 800 steel material (complex phase steel), it can be concluded that, at a stress level of 95% of the yield stress, the material exhibits ductile characteristics, with the additional presence of dimples or alveoli, as well as striations.