PROCEDURE FOR VERIFYING A WELDED JOINT MADE BY FRICTION WELDING WITH A FILL ELEMENT (FEW)

Description

DESCRIPTION of the industrial invention entitled: “Procedure for checking a welded joint made by friction stir welding (FEW)” by: CRF Consortium Company, nationality Italian, Strada Torino 50, 10043 Orbassano (TO) Designated Inventors: Massimo DI PARDO; Alessandro CACCIATORE Filed on: September 13, 2024 **** DESCRIPTION TEXT Field of invention The present invention relates to the testing of joints welded parts obtained by friction welding with element filler, or Friction Element Welding (FEW – hereinafter (this acronym will be used). As is known, this is a welding technique that allows the joint to be created between two elements of metallic material – in general of materials dissimilar to each other - in a stretch of overlapping of them by means of a filler element in shape of a rivet (or in general of a penetrator) operated in high speed rotation. Known technique In a FEW welding of two material elements metallic, generally in sheet / plate or thickness reduced and generally dissimilar, a contributing element in penetrator or rivet shape is affixed in correspondence of an "external" face of one of the two elements (therefore opposite to the other of the two elements, or in other terms not facing the other of the two elements) and It is operated in rotation at very high speed so to soften and penetrate the first element and melt the second element after the penetration of the first element, in so as to establish a structural continuity with it that results in at least a roughly welded joint comparable to a rivet. The filler element It generally has a much larger head than the body through which rotation and movement are imparted which acts as a shoulder element to compress the first element against the second element of the welded joint. The mechanical resistance (and in general the quality of the joint) depends mainly on the temperature profile space-time of the joint, therefore from the space-time evolution time of heat transferred to the joint itself during the realization. Given the nature of the welding technique, the acquisition of data necessary for deductions of any kind presents significant critical issues, both for the speed of evolution of the welding operation itself, both for the need for real-time data processing, resulting therefore it is possible to check the welding very well approximate (therefore unreliable) and / or very late compared to production needs. Purpose of the invention The purpose of the invention is to solve the problem technical described above. In particular, the purpose of the invention is to provide a method for verifying a welded joint made by friction welding with filler element (FEW) executable in real time and compatible with the dynamics of evolution of the technique of welding in question. Summary of the invention The purpose of the invention is achieved by a process having the characteristics forming the object of the claims that follow, which form part integral to the technical teaching administered here in relation to the invention. Brief description of the figures The invention will now be described with reference to the attached figures, provided purely as an example, do not limiting, where: - Figure 1 schematically illustrates the configuration of a welded joint with respect to which it is the process according to the invention can be implemented, - figure 2 is a schematic view according to the arrow II in figure 1, - figure 3 schematically illustrates an equipment of welding that makes a FEW weld, - Figure 4 illustrates a flowchart exemplifying the process according to the invention, while Figure 4A illustrates a further diagram example of deductions according to the procedure of the invention, - Figure 5 illustrates a three-dimensional surface corresponding to a complex of spatial acquisitions temperature data timescales according to the invention, - Figure 6 and Figure 7 illustrate cross-sections of the three-dimensional surface of figure 5 at a temporal instant and a linear spatial coordinate correspondents, - figure 8 illustrates a discretization operation of an analysis domain according to an execution form favorite of the invention, - figure 9 shows a temperature diagram with indication of underlying areas in reference to figure 8, - Figures 10 to 12 illustrate a spatial relationship between temperature diagrams (figures 11, 12) acquired along two spatial coordinates and the welded joint (figure 10), and - Figure 13 shows a temperature diagram multiple example of one or more operable deductions through the process of the invention. Detailed description The reference J in figure 1 and figure 2 illustrates schematically a welded joint made by Friction welding (FEW). The joint welded J comprises a first element of sheet metal S1 (e.g. an aluminum alloy element) and a second S2 sheet metal element (e.g. a sheet metal element) steel) joined by means of a filler element E in correspondence of an overlapping area OA between them (here element S1 is superimposed on element S2). The filler element E is placed on one face of the element S1 opposite to the overlap area OA (therefore not facing element S2, but “external” to the interface between elements S1 and S2) in position at a distance K from the end of element S1 adjacent to element S2 (e.g. 8-10 mm). In figure 3 the WF reference indicates overall a welding equipment configured to make friction welded joints with the filler element E (FEW welding). The WF equipment includes a D die and a punch P placed on opposite sides to the complex of the sheets S1, S2, with the punch P in correspondence of the S1 sheet, and the matrix D which supports sheet metal S2 (and consequently sheet metal S1). The punch P is operationally connected to an SP spindle configured for operate the punch P in rotation around the respective axis. Punch P is configured for coupling torsional with the supply element E so as to operate in rotation of the filler element And when this is placed on the S1 sheet. As is known, the rotational action impressed to element E causes localized heating of the material of the element S1 up to the complete penetration of it and to the S2 element. Hence the rotation continues until it reaches the fusion of the S2 element material with the element material of filler E. In this way, the filler element E becomes integral with the element S2 and permanently joins to it is the S1 element thanks to its umbrella / mushroom shape that includes a head that contacts the surface of the element S1 in correspondence with which it is placed element E at the beginning of the weld. The execution of the welding is accompanied by the creation of a heat-affected region H that receives and absorbs the heat produced by friction during the rotation of the filler element E. Region H is being investigated according to the process of the invention, and for this purpose the WF equipment includes a CT thermal imaging camera with optics centered on a plane MP acquisition arranged in correspondence with the area of overlap, in a position between the S1 elements, S2 (therefore at the interface between the elements S1, S2 themselves) and preferably tangent to element S1. The optics of the TC thermal imaging camera is placed at a distance F from the region H, where the distance F is established according to the characteristics geometry of optics itself. Referring to figure 4, where the number of reference 1 refers overall to a flowchart representative of the process according to the invention, is defined according to the invention a process for the verification of the welded joint J made by welding friction-welded (FEW), where the procedure includes: - acquire (block 2, figure 4), during the joint construction and for each unit of temporal discretization of a time interval that includes said implementation of the J-joint, a temperature distribution over a range of positions spatial along a first linear spatial coordinate and in correspondence of a conjunction of the first element of metal sheet S1 and the second sheet metal element metallic S2 using the filler element E, - identify a maximum temperature value T_MAX (figures 5-7) of the set of temperature distributions acquired along at least one linear spatial coordinate and extract the acquired temperature distribution in correspondence of the temporal discretization unit to the which manifests the maximum temperature value T_MAX (figure 9), - determine (block 4, figure 4) a sum of the values of each area subtended by the temperature distribution extracted in each spatial discretization unit linear (∆𝑥 ) of the interval of spatial positions along the at least one linear spatial coordinate (x, y, z), - determine a location for said accumulation of values compared to a reference value (REF) for at least one linear spatial coordinate (x, y, z). Figure 3 and Figure 3A schematically illustrate the domain on which the verification performed by the process according to the invention, which corresponds essentially to the H region. Along a first coordinate linear spatial x belonging (or, more rarely, parallel) the range of spatial positions is defined in the MP plane which consequently includes the conjunction between the elements S1 and S2 which is realized in correspondence with the region H. Acquisition of the temperature distribution along the linear spatial coordinate x is operated by the TC thermal imaging camera for each discretization unit linear space, which generally corresponds to a segment resulting from the splitting (preferably stepwise constant) of the extension of the position range spatial chosen along the x-coordinate. Furthermore, the acquisition is repeated for each unit of temporal discretization of the time interval that includes the construction of the J joint, therefore an interval of time within which evolution takes place thermal of the H region. Figure 5 illustrates a three-dimensional TS surface corresponding to the complex of acquisitions of temperature along the linear spatial coordinate x operated using the CT thermal imaging camera in the entire time domain. It is obviously it is possible to define a similar surface for the acquisitions along the y-coordinate, and possibly along the z coordinate (the acquisition time instants are clearly aligned with the acquisitions made along the spatial coordinates, therefore the acquisitions on different spatial coordinates are synchronized with each other compared to the other). As described, and with reference to figures 5 to 7, at completion of the space-time acquisition of the temperature with respect to each linear spatial coordinate of interest, the procedure involves identifying the maximum value MAX_T of the set of distributions of temperature along the linear spatial coordinate (here the x-coordinate) and extract the temperature distribution TS acquired at the unit of temporal discretization at which the manifests itself maximum temperature value, corresponding to what illustrated in figure 7 (figure 6 illustrates an extraction of the temporal distribution). The area under the TS distribution thus extracted is compared (block 6, figure 4; hereinafter referred to with Ao, observed area) with a reference area Ar determined as a function of one or more distributions of reference temperature REF (block 8) which identify a welded joint that conforms to the design specifications. The comparison is between the amount of heat absorbed in the H region, which – as will now be described – can be assumed to be proportional to the area Ao subtended by the extracted TS distribution (sum of the underlying areas in correspondence of each discretization unit linear spatial ∆𝑥 ) and the amount of heat absorbed in the region H under reference conditions (compliant joint), therefore – always in light of the following description – a given proportional to the area Ar subtended by the distribution REF, which by analogy corresponds to the accumulation of the values ​​of each area subtended by the temperature distribution of REF reference in each discretization unit linear spatial ∆𝑥 of the interval of spatial positions along the corresponding linear spatial coordinate x, y, z which includes the H connection of the first sheet metal element metal S1 and the second metal sheet element S2 using the filler element E. The reference area Ar is a known (mapped) datum that is not generally recalculated during the implementation of the procedure according to the invention. With reference to figure 8, a model of discretization preferred for the purposes of the procedure according to the invention provides for considering each unit of linear spatial discretization as coinciding with a first dimension 𝐿 of a volume discretization unit 𝑛 𝑉 of the conjunction of the first metal sheet element 𝑛 S1 and of the second metal sheet element S2 by means of the filler element E, in which the first dimension 𝐿 is 𝑛 develops along the linear spatial coordinate x. In the figure 5 four units of are illustrated as an example volumetric discretization 𝑉 , 𝑉 , 𝑉 , 𝑉 (the index n identifies 1 2 3 𝑛 progressively the volumetric discretization units, with progression along the x-coordinate) with first dimension 𝐿 , 𝐿 , 𝐿 , 𝐿 . Each volume discretization unit 1 2 3 𝑛 𝑉 , 𝑉 , 𝑉 , 𝑉 also includes a second dimension 𝐻 , 𝐻 , 𝐻 , 𝐻 1 2 3 𝑛 1 2 3 𝑛 which develops along a linear spatial direction further (y in the figures) orthogonal to the x direction orthogonal to the MP plane, and a third dimension 𝑃 , 𝑃 , 𝑃 , 𝑃 which 1 2 3 𝑛 it develops along a further linear spatial direction (z in the figures) orthogonal to both directions x, y, therefore parallel or belonging to the MP plane). Each unit of volume discretization 𝑉 , 𝑉 , 𝑉 , 𝑉 defines a domain 1 2 3 𝑛 of discrete computing used for the purposes of the process according to the invention, and represents at the I model a discretization of the H region, as exemplified by the dotted box in figure 8. Yes note that the same type of volume discretization (including subsequent determinations described below) It can be performed with respect to the y coordinate, with the first dimension 𝐿 oriented along the y-coordinate. Note 𝑛 that the same applies in the case where one also operates along a second linear spatial coordinate belonging (or, more than sparse, parallel) to the MP plane, like the z coordinate (in that case the first dimension 𝐿 is oriented along the coordinate 𝑛 z). It is of interest to note that the above discretization described, although operating in a completely general way over the entire space-time domain of acquisition, of preference is operated only on the TS distribution extracted in correspondence of the temporal discretization unit to the which manifests the maximum temperature value MAX_T. this is because the TS distribution thus extracted can be considered representative of the entire process of creation of the welded joint J, expressing it peak temperature MAX_T reached during execution of the welded joint J, therefore necessarily representing the entire heat input of the J-joint during the its realization. With reference to the schematization of figure 8 regarding a spatial discretization linear developing along the x coordinate (and referred to the TS distribution extracted as it expresses the value MAX_T), it is possible to define a quantity of heat 𝑄 𝑛 transferred to the n-th discretization unit 𝑉 during 𝑛 welding (therefore during the creation of the joint according to a relationship that establishes a proportionality direct relationship between the quantity of heat and the temperature, in particular: 𝑄 = 𝐶𝑠 ∙ 𝑚 ∙ (𝑇 − 𝑇 ) (1) 𝑛 𝑛 𝑛 𝑓 𝑖 𝑛 in which: 𝑄 is the amount of heat transferred to the unit of 𝑛 n-th volume discretization; (𝑇 − 𝑇 ) are, respectively, the acquired temperatures 𝑓 𝑖 𝑛 at the final (f) and initial (i) delimiting temporal instants the temporal acquisition of a corresponding unit of volumetric discretization 𝑉 in the acquisition context 𝑛 spatial distribution of the TS over the entire domain of spatial acquisition along the spatial coordinate corresponding (x in this case, but similarly applies to ye / oz); 𝐶𝑠 is the specific heat of the discretization unit 𝑛 n-th volume 𝑉 ; and 𝑛 𝑚 is the mass of the volume discretization unit 𝑛 nth. The mass 𝑚 can in turn be expressed as a function 𝑛 of the dimensions of the unit 𝑉 (𝐻 ∙ 𝑃 ∙ 𝐿 ) and of the mass 𝑛 𝑛 𝑛 𝑛 specific volume 𝛾 in the following terms: 𝑚 = 𝑉 ∙ 𝛾 = 𝐻 ∙ 𝑃 ∙ 𝐿 ∙ 𝛾 𝑛 𝑛 𝑛 𝑛 𝑛 so 𝑄 = 𝐶𝑠 ∙ 𝐻 ∙ 𝑃 ∙ 𝐿 ∙ 𝛾 ∙ (𝑇 − 𝑇 ) (2) 𝑛 𝑛 𝑛 𝑛 𝑛 𝑓 𝑖 𝑛 More in detail, grouping the constant terms the quantity of heat 𝑄 can be expressed in the following 𝑛 terms: 𝑄 = 𝐾 ∙ (𝑇 − 𝑇 ) ∙ 𝐿 (3) 𝑛 𝑛 𝑓 𝑖 𝑛 𝑛 where 𝐿 is the first dimension developing along the 𝑛 linear spatial coordinate x; 𝐾 is a constant equal to the 𝑛 product of a second dimension 𝐻 and a third dimension 𝑛 𝑃 of the n-th volume discretization unit, 𝑛 orthogonal to each other and to the first dimension, of a value 𝛾 of a unit density of discretization volumetric, and of a specific heat 𝐶𝑠 of the unit of 𝑛 volumetric discretization. Preferably, the first dimension 𝐿 is constant for 𝑛 each n-th volume discretization unit 𝑉 , the 𝑛 second dimension 𝐻 is constant for each n-th unit 𝑛 of volume discretization 𝑉 , and the third dimension 𝑃 is 𝑛 𝑛 constant for each n-th discretization unit volumetric 𝑉, and in which also the specific heat 𝐶𝑠 and the 𝑛 𝑛 density 𝛾 are constant for each unit of volumetric discretization 𝑉 . Consequently, it is possible 𝑛 group the factors of the product 𝐶𝑠 ∙ 𝐻 ∙ 𝑃 ∙ 𝛾 in the constant 𝑛 𝑛 𝑛 𝐾 , and rewriting each n-th dimension 𝐿 (constant) 𝑛 𝑛 as ∆𝑥 ( 𝐿 = 𝑥 − 𝑥 = ∆𝑥) we can finally rewrite the 𝑛 𝑖+1 𝑖 quantity of heat 𝑄 as: 𝑛 𝑄 = 𝐾 ∙ (𝑇 − 𝑇 ) ∙ ∆𝑥 (4) 𝑛 𝑛 𝑓 𝑖 𝑛 It then becomes possible to determine the total quantity of heat 𝑄 absorbed by the spatial domain of the TS distribution along the linear spatial coordinate x during a time interval at whose extremes they manifest themselves the temperatures 𝑇 and 𝑇 as the sum of values ​​of each 𝑓 𝑖 area under the extracted temperature distribution TS (figure 9) in each spatial discretization unit linear ∆𝑥 of an interval of spatial positions along the x-coordinate (or, preferably, x, y, and possibly z) which includes the conjunction of the first element of metal sheet and the second metal sheet element using the filler element E made in correspondence of the H region. In formulas: 𝑀−1 𝑄 = 𝐾 ∙ ∑ (𝑇 − 𝑇 ) ∙ ∆𝑥 = 𝐾 ∙ 𝐴 (5) 𝑛=1 𝑓 𝑖 𝑛 𝑛 corresponding, in light of the discretization linear spatial and volumetric operation, to the integral 𝑀−1 𝑄 = 𝑄 𝑑𝑥 (6) ∫ 𝑛 where (𝑇 − 𝑇 ) is the temperature detected by the sensor 𝑓 𝑖 𝑛 on the n-th observed volumetric discretization unit 𝑉 𝑛 In particular, since 𝑇 is the temperature at which it occurs 𝑖 reference the sensor reported in degrees Celsius is assumed preferably, as it is a reference provided by the TC thermal imaging camera, 𝑇 =0°C and consequently 𝑖 (𝑇 − 𝑇 ) = 𝑇 (temperature detected by the sensor on the volume n- 𝑓 𝑖 𝑓𝑛 𝑛 th along the x-axis). More generally, the amount of heat absorbed depends on the temperature differences (𝑇 − 𝑇 ), 𝑓 𝑖 𝑛 and since 𝑇 is the same for all units of 𝑖 volumetric discretization 𝑉 (the acquisition along the 𝑛 spatial coordinate x is simultaneous for the entire domain spatial, it can be chosen with arbitrary value – conveniently equal to 0 °C - since for the purposes of the invention it only detects the proportional relationship between heat absorbed and subtended area, which is not influenced by a value of constant reference temperature. Clearly the the same choice is made for the temperature value T_i also for the REF distribution, in order to obtain data of area Ao and Ar directly comparable. This temperature varies from volume to volume and is a function of the x coordinate, that is, T=f(x). Based on these considerations we can write 𝑀−1 𝑄 = 𝑘 𝑇 𝑑𝑥 (7) ∫ 𝑥 in which: 𝑄 is called a cumulation of values; 𝐿 is called the first constant dimension for each n- th volume discretization unit; 𝐴 is the n-th area subtended by the TP curve; 𝑛 (𝑇 ) is the temperature acquired at the O 𝑛 n-th spatial position along the spatial coordinate x (e / oze / oy) corresponding to the discretization unit volumetric 𝑉 ; 𝑛 𝑀 − 1 is the number of discretization units linear spatial, and 𝑀 is the number of spatial positions delimiting the linear spatial discretization units; 𝐾 is a constant equal to the product of the second dimension 𝐻 and the third dimension 𝑃, both constant for each volume discretization unit, of a value 𝛾 of a unit density of discretization volumetric, and of the specific heat 𝐶𝑠 of the unit of volumetric discretization (the index n is omitted from the values ​​outside the sum as they are constant along the range of spatial positions). Figures 10, 11, 12 exemplify the comparison with reference to an extracted temperature distribution TS (because it shows a respective MAX_T temperature) from complex of space-time acquisitions along the x-coordinate and an “orthogonal” temperature distribution extracted (because it shows a respective temperature MAX_T) from the complex of space-time acquisitions along the y-coordinate, in addition to showing the title an example of a thermographic (CT) frame of the H region as acquired by the CT thermal imaging camera. From the figures in question it is immediately evident how the area of Ar reference associated with the reference distribution REF is specific for each linear spatial coordinate x, y, z (so each spatial coordinate is associated with a corresponding reference distribution REF). The comparison made in block 6 has a binary yes / no outcome. and it is developed in these terms: having been calculated the reference area Ar subtended by the REF curve, it is possible define a confidence band within which the comparison was performed between the reference area Ar and the area observed Ao and corresponding to the area under the temperature distribution TS extracted above As regards one or more distributions of reference REF and the reference area Ar, it is possible use a single REF reference curve (and the relative area Ar) obtained in advance and associated with a J joint whose conformity could be verified, or it is possible – and preferable – to define the area Ar as a value average over a certain number of curves associated with J joints compliant, but still having a certain degree of dispersion. In the latter case, once the curves have been determined include in the sample (therefore the curves associated with joints J compliant) the respective underlying areas are calculated as described, the average value is extracted and used as a reference area Ar. The comparison between Ao and Ar is developed by assuming a tolerance band – for example equal to 10% and centered on Ar – and it occurs if the absolute value of the difference Ao-Ar is contained within the half-width of the band tolerance. With reference to figure 4, still block 6, and to figure 4A where the reference ok indicates the amplitude of the tolerance band and REF the area value of Ar reference, in the example with 10% tolerance band, this means checking whether the absolute value of the Ao-Ar difference is greater or less than 0.05, therefore |Ao-Ar| < 0.05 (theoretically there is the possibility of equality |Ao-Ar| = 0.05, but in practice it is difficult to achieve this (check). In general terms, TB is the width of the tolerance band, the verification can be expressed in terms |Ao-Ar| < TB / 2. If the condition is verified it means that in each case you fall within the established tolerance band and the J joint is compliant (block 10, OK). If the condition is not verified (block 12) we proceed to a further check at block 14, in particular a verification of the fact that the difference with sign Ao-Ar (therefore no longer in absolute value) is greater than 0.05 (Ao – Ar > 0.05). In general terms the test corresponds to Ao-Ar > TB / 2. If the verification is successful (block 16) it means that you are outside the tolerance band and at values ​​greater than its upper limit, therefore in a range of absorbed heat attributable to distributions of temperatures representing “hot” J junctions, therefore in which excess heat absorption has taken place. This is illustrated by a HOT distribution in the figures 10 to 13 and from a similar reference in figure 4A. If the verification fails (block 18) it means that one is outside the tolerance band and at values less than its lower limit, therefore in an interval of absorbed heat attributable to distributions of temperatures representing “cold” J junctions, therefore in where a heat absorption defect has occurred. This It is schematized by a COLD distribution in the figures 10 to 13 and from a similar reference in figure 4A. In this last eventuality (COLD) the J joint is however not compliant (block 20, KO). If the following is valid, first eventuality (HOT, block 16), the joint is not automatically compliant, but is susceptible to further verification (block 22, VER). In any case, if the difference Ao-Ar has sign positive and value higher than HOT+TB / 2, which corresponds to the cumulative amplitude of the TB / 2 and HOT bands, therefore it corresponds at a limit value of excess heat absorbed HOT*, the Joint J is still non-compliant. A value can be defined of cumulative mirror amplitude COLD* (COLD+TB / 2 in value absolute) for “cold” joints, with respect to which one can deduce that if the difference Ao-Ar is negative and has value absolute lower than COLD+TB / 2 the J joint however is not compliant, but this is in any case overcome by the fact that the non-conformity occurs even outside the TB band. In summary, according to the process of the invention a tolerance band TB is defined with respect to the value of reference Ar, and the welded joint (J) is declared non-compliant if the cumulative values ​​(area Ao) is less than reference value Ar reduced by a first fraction of width of the TB tolerance band, especially half amplitude TB / 2, for at least one linear spatial coordinate xe / oye / o z. The welded joint J is declared compliant 10 if the cumulative Ao values ​​are within the band of TB tolerance for at least one linear spatial coordinate xe / oye / oz, while the welded joint J is sent back to further check if the cumulation of Ao values ​​is greater than said reference value Ar increased by a second one's complementary bandwidth fraction TB compared to the first fraction of amplitude, in particular half width TB / 2. Thanks to the process according to the invention, exploiting a simple proportional relationship between heat absorbed in the H region and the temperature developed during welding the J joint, and also taking advantage of the space-time discretization of the phenomenon described here It is possible to perform real-time J-joint verification with low computational burden and high reliability, providing a solution to the technical problem stated at the beginning to the description. Of course, the manufacturing details and shapes of execution may be widely varied with respect to as described and illustrated without going beyond from the scope of this invention as defined from the attached claims.

Claims

CLAIMS 1. Method for testing a welded joint (J) made by friction stir welding (FEW), the welded joint (J) comprising a first metal sheet element (S1) and a second metal sheet element (S2) joined by the filler element (E) at an overlapping area between them (OA), the method comprising: - acquiring (2), during the making of the joint and for each time discretization unit of a time interval comprising said making of the joint (J), a temperature distribution in a range of spatial positions along at least one linear spatial coordinate (x, y, z) and at a junction (H) of the first metal sheet element (S1) and the second metal sheet element (S2) by the filler element (E),- identify a maximum temperature value (T_MAX) of the set of temperature distributions (TS) acquired along said at least one linear spatial coordinate and extract the temperature distribution (TS) acquired in correspondence with the temporal discretization unit at which said maximum temperature value (T_MAX) occurs, - determine (4) a cumulative value of each area subtended by the temperature distribution extracted in each linear spatial discretization unit (Δχ) of the interval of spatial positions along the at least one linear spatial coordinate (x, y, z), - determine a location of said cumulative value with respect to a reference value (Ar) for the at least one linear spatial coordinate (x, y, z)., 2. Method according to claim 1, wherein said at least one linear spatial coordinate (x, y, z) comprises a first linear spatial coordinate (x) belonging to or parallel to an acquisition plane (MP) arranged on a surface of said first metal sheet element (S1) facing said second metal sheet element (S2) in correspondence with the overlapping area (OA).

3. A method according to claim 2, wherein said at least one linear spatial coordinate further comprises at least a second linear spatial coordinate (y, z) orthogonal to the first linear spatial coordinate (x).

4. Method according to claim 3, wherein said at least one second linear spatial coordinate comprises: - a second linear spatial coordinate (y) orthogonal to the first linear spatial coordinate and to the acquisition plane (MP), or - a third linear spatial coordinate (z) orthogonal to the first linear spatial coordinate (x) and parallel to or belonging to the acquisition plane (MP).

5. Method according to any of the preceding claims, wherein each linear spatial discretization unit (Δχ) coincides with a first dimension (L1,L2,L3,Ln ) of a volumetric discretization unit (V1,V2,V3,Vn) of the junction of the first metal sheet element (S1) and the second metal sheet element (S2) by means of the filler element (E), said first dimension (L1,L2,L3,Ln ) developing along the corresponding at least one linear spatial coordinate (x, y, z).

6. Process according to claim 5, wherein for each volumetric discretization unit (V1, V2, V3, Vi) a quantity of heat Qn transferred during welding is defined as: Qn = Kn · (7} - Ti)n · Ln where: Qn is the quantity of heat transferred to the n-th volumetric discretization unit; Ln is said first dimension; (7 - T<)n are, respectively, the temperatures acquired at the final and initial time instants delimiting the spatial acquisition of a corresponding volumetric discretization unit Vn for said extracted temperature distribution (TS); Kn is a constant equal to the product of a second dimension Hn and a third dimension Pn of the n-th volumetric discretization unit, orthogonal to each other and to the first dimension, of a value γ of a density of the volumetric discretization unit, and of a specific heat Csn of the volumetric discretization unit (V1,V2,V3,Vn).

7. A method according to claim 6, wherein the first dimension Ln is constant for each n-th volumetric discretization unit (V1, V2, V3, Vn), the second dimension Hn is constant for each n-th volumetric discretization unit (V1,V2,V3,Vn), and the third dimension Pn is constant for each n-th volumetric discretization unit (V1,V2,V3,Vn), and wherein further said specific heat Csn and said density γ are constant for each volumetric discretization unit (V1, V2, V3, Vi) 8. Method according to claim 7, wherein said sum of values ​​is determined as M-1 Q = K^(Tr — Tòn^^ n=1 where: Q is said sum of values; Δχ is said linear spatial discretization unit coinciding with the first constant dimension for each nth volumetric discretization unit (V1,V2,V3,Vn); (Ti - Tl)n are, respectively, the temperatures acquired at the final and initial time instants delimiting the spatial acquisition of a corresponding volumetric discretization unit Vn for said extracted temperature distribution (TS); M-1 is the number of linear spatial discretization units, and M is the number of spatial positions delimiting the linear spatial discretization units (Δχ);K is a constant equal to the product of the second dimension H and the third dimension P, both constant for each volume discretization unit (V1, V2, V3, Vn), of a value γ of a density of the volume discretization unit, and of a specific heat Cs of the volume discretization unit.; 9. Method according to any of the preceding claims, wherein said reference value corresponds to an average value of areas subtended by a plurality of reference temperature distributions (REF) in each linear spatial discretization unit (Δχ) of the range of spatial positions along the corresponding linear spatial coordinate (x, y, z) comprising the conjunction (H) of the first metal sheet element (S1) and the second metal sheet element (S2) by means of the filler element (E), wherein the reference temperature distributions (REF) are associated with compliant welded joints (J).

10. Method according to claim 1 or claim 9, comprising: - defining a tolerance band (TB) with respect to said reference value, - declaring the welded joint non-compliant (20) if said sum of values ​​is lower than said reference value (Ar) reduced by a first fraction of the width of said tolerance band, in particular half the width, for at least one linear spatial coordinate (x, y, z), - declaring the joint compliant (10) if said sum of values ​​is included within said tolerance band (TB) for at least one linear spatial coordinate (x, y, z), and - sending the welded joint (J) back for further verification if the sum of values ​​(Ao) is higher than said reference value (Ar) increased by a second fraction (TB / 2) of the width of the tolerance band (TB) complementary to one with respect to the first fraction of the width, in particular half the width (TB / 2).