Weldability

Weldability is a significant factor influencing the weldability of a component and relates to the technologically important material property of entering into an inseparable connection with another or the same material when using a welding process . Further material properties are the castability , the formability and the machinability , which indicate the respective suitability for casting, forming or machining. With regard to resistance spot welding, DVS Leaflet 2902-2 defines: "Welding suitability is available if a spot welding that meets the requirements can be produced under consideration of qualitative and economic aspects." EN ISO 18278-1 defines weldability: The weldability of metallic materials for welding is defined by:

first, the ability to weld;
the ability to make welds continuously;
the ability of the weld to withstand the operational stresses that arise.

Weldability is difficult to quantitatively formulate and is very different for different welding processes. “A steel that is less suitable for fusion welding can be very well suited for resistance welding . So is z. For example, galvanized steel is not always suitable for gas-shielded arc welding because of spatter and pore formation, but it can be well suited for spot welding. There are also differences with regard to the various resistance welding processes. A galvanized sheet with phosphating that is suitable for spot welding can, however, lead to difficulties with projection welding. "

Weldability for fusion welding

When a weld seam is created by thermal effects, different zones are formed in which changes in the mechanical characteristics take place depending on the material properties and thermal conditions. A rough distinction is made:

unaffected base material
Heat affected zone
Weld metal.

While the properties are retained in the base material, the physical material properties change in the heat-affected zone due to grain growth , phase changes , precipitation processes at the grain boundaries or even hardening , as well as in the weld metal due to crystallization (formation of a cast structure ), dissolution phenomena of accompanying elements , precipitation processes, segregation , shrinkage and resulting internal stresses.

A material is suitable for the respective welding process if, despite these material changes, a weld seam can be produced that meets the stress conditions.

steel

In the case of steels , the carbon content and the cooling rate after welding are essential for weldability, as these factors determine the hardness and residual stresses in the weld seam and in the heat-affected zone. Steels with a carbon content of more than 0.22% are only conditionally suitable for welding, as they tend to develop hardness peaks and cracks due to the structural transformations. Hardness peaks arise in the heat affected zone, especially in the border area to the solidified melt, due to the formation of martensite . Measures such as preheating the welded parts or stress relief annealing reduce this risk even with steels with a C content of more than 0.3%.

The interaction of the carbon with other alloy components leads to undesirable internal stresses in the welded part, even with small amounts of carbon. For this reason, the so-called carbon equivalent, which also takes other alloying elements into account, was introduced to assess weldability .

Non-transformational material (Ni, Al, Cu)

The width of the heat affected zone is determined by the welding parameters. At the border line to the former melt, there is strong grain growth that continuously decreases towards the base material. Gas absorption can lead to embrittlement and pores. High thermal conductivity and expansion coefficients can cause severe distortion and high internal stresses.

Work hardened materials

In the area of increased temperature above the recrystallization temperature , the desired work hardening is canceled. This can only be restored by re-cold forming after welding. The drop in strength can be minimized by fusion welding processes with high thermal power density.

Precipitation hardened material

Materials that have been strengthened by precipitation hardening, e.g. B. aluminum-magnesium alloys or micro-alloyed steels change their strength properties in the heat affected zone. There is a dissolution of the excretions and subsequent re-excretion in an undesirable distribution. This leads to a decrease in strength and toughness. Grain boundary precipitations can lead to cracks. The corrosion resistance can be considerably reduced by coarse precipitations.

Highly reactive materials

Materials such as tantalum , titanium , zirconium or molybdenum react violently with the atmosphere during welding. Atmospheric gases are absorbed at relatively low temperatures of over 600 K and thus become brittle. These materials are only suitable for welding in a partial vacuum or under gas protection.

Welding suitability for resistance spot and projection welding

The weldability of a material or a combination of materials for resistance spot welding is determined by the chemical composition, the metallurgical and the surface condition. All other influencing factors are derived from this.

Factors influencing the spot weld suitability according to B. Leuschen

Materials in general

The physical material properties (chemical composition and metallurgical state) are particularly important for the suitability for welding. Ideal material properties for resistance spot welding are:

same or close to each other melting temperature
low electrical conductivity
low thermal conductivity
high deformability (hot deformability)

In this regard, nickel is nearly ideal. However, these conditions are seldom encountered in real welding tasks. Materials with high electrical and thermal conductivity are difficult to weld because the welding heat is dissipated very quickly and the required welding temperature cannot be achieved. Hard and brittle materials have only a small temperature range with plastic properties and tend to break during the welding and cooling process. The deformability in the range of the welding temperature is important. Metals of low ductility in this temperature range tend to develop so-called hot cracks during cooling.

There are three groups of materials:

Group 1

Gold, aluminum, silver copper, brass and bronze alloys. These materials are face-centered cubic in the lattice structure with high ductility due to the large number of dislocation structures . This group of materials has relatively high electrical and thermal conductivity and therefore limited suitability for welding. Group 1 metals combine with those of groups 1, 2 and 3 in the solid state. In the tensile test of these connections, a weld point can be identified in the fracture area, which is often misinterpreted as a fusion weld result.

Group 2

Nickel, titanium, platinum, X10CrNi18-8 and X2CrNi16-10, which, with the exception of titanium, also have a face-centered cubic lattice, whereas titanium occurs with a hexagonal lattice . This group can weld to one another using any type of connection (by melt, in solid state, by diffusion). Metals of group 2 combine in a solid state with partners in metal groups 1 and 2.

Group 3

Chromium, iron, molybdenum, niobium, tantalum, tungsten and martensitic stainless steels (e.g. X12CrS13, X14CrMoS1) with body-centered cubic lattice. The melting temperature is high, the material is hard and brittle and the electrical conductivity is in the medium range. The connection with metals of all groups takes place in the solid state.

According to L. Pfeifer (quoted by M. Krause), the weldability of various materials can be expressed by a so-called welding factor, which summarizes the electrical and thermal conductivity and the melting temperature to a quantitative value of the weldability for resistance spot welding. ${\ displaystyle S}$ ${\ displaystyle \ lambda}$ ${\ displaystyle \ chi}$ ${\ displaystyle T_ {s}}$

{\ displaystyle S = {\ frac {4 {,} 2 \ cdot 10 ^ {6}} {\ lambda \ cdot \ chi \ cdot T_ {s}}}}

With

{\ displaystyle S}

- sweat factor

{\ displaystyle \ lambda}

- electrical conductivity [Sm / mm²]

{\ displaystyle \ chi}

- thermal conductivity [W / (m K)]

{\ displaystyle T_ {s}}

- melting temperature [° C]

{\ displaystyle S <0 {,} 3}

poorly suitable for welding

{\ displaystyle 0 {,} 3 \ leq S \ leq 0 {,} 8}

conditionally suitable

{\ displaystyle S> 0 {,} 8}

well suited

Welding factor of selected materials
material	${\ displaystyle \ chi}$	${\ displaystyle \ lambda}$	${\ displaystyle T_ {s}}$	${\ displaystyle S}$
Unalloyed steel	6.0	48	1535	9.05
Steel, leg.	3.5	50	1500	17.14
aluminum	36	209	659	0.84
Al-Mg	20th	162	625	2.07
copper	56	372	1083	0.186
Brass	8.7	93	925	5.6
lead	4.8	35	327	76

Steel materials

Steel materials have a wide range of chemical, physical and metallurgical properties. The chemical composition influences the structure and thus the strength and hardness properties, crack and lens formation of the connection. The so-called carbon equivalent (CE) is regarded as an indication of weldability . Depending on the material composition, the material strength, the deformability, the structure and the transformation behavior change in the thermal cycle of spot welding. Depending on the alloy composition, steels can have very different thermal and electrical conductivity and thus different welding factors S.

With regard to the weldability for resistance spot welding, DVS bulletin 2902-2 assigns steel materials to four different groups:

Group 1 "well suited"

unalloyed, uncoated hot or cold rolled strips and sheets
micro-alloyed cold-rolled steel sheets
Dual phase steel
Retained austenite steels (TRIP steels)
Complex phase steels

The welding parameters current, time and electrode force must be adapted to the steel properties. Oils and greases on the surface to improve the drawing behavior lead to electrode contamination and thus reduce their service life.

Group 2 "suitable"

This group includes cold-rolled steel sheets, the base material of which is inherently very suitable for welding, but which is less suitable for welding due to metallic surface coatings.

Group 3 "conditionally suitable"

This group consists of steels with higher carbon and manganese contents, which tend to harden and become brittle, as well as steel sheets with weldable paintwork or inorganic, metallic coatings and composite materials made of steel and plastic .

Group 4 "not suitable"

plastic-coated and painted sheets
Sorbitic spring steels
Steels with an enamelled surface

Materials in electronics and precision engineering

In electronics and precision engineering, a wide variety of materials are connected by resistance welding, for which the general statements on weldability apply. A summary of the suitability for welding and the selection of the required electrodes are tabulated in DVS Leaflet 2950, whereby the material combinations are assigned to three weldability classes. It is important to note: “Many material pairings of the same type or of a different type that are described as less suitable for welding can be welded with special measures, processes and machines in a quality that is satisfactory for the application” , ie the technological conditions are of great importance for the weldability of these materials. Therefore, there are also different information on the weldability of such material combinations.

aluminum

Aluminum and aluminum alloys are of great importance as construction materials. The weldability for resistance spot welding is significantly influenced by the good conductivity and the high affinity for oxygen. The conductivity decreases with increasing alloy components of Mg, Mn, Cu, Zn and Si. "In addition to the electrical and thermal conductivity, the contact resistance is the most important influencing factor" . The oxide layer on the surface is already formed by the action of atmospheric oxygen, which leads to a considerable increase in contact resistance and high electrode wear. Therefore, surface treatments (mechanical or chemical) are recommended before resistance spot welding.

Individual evidence

↑ ^a ^b ^c DVS: Resistance spot welding of steels up to 3 mm single thickness - spot weld suitability, DVS Merkblatt 2902-2, 2001.

↑ Resistance welding. Weldability. Part 1: Assessment of the weldability for resistance spot , roller seam and projection welding of metallic materials , EN ISO 18278-1: 2004

↑ Harms & Wende GmbH & Co. KG: Das kleine HWH Lexicon , term materials: Influential factors on suitability for welding ( Memento from November 5, 2014 in the Internet Archive )

↑ G. Schulze: The metallurgy of welding: ferrous materials - non-ferrous metallic materials , Springer, Berlin and Heidelberg 2010 DNB 995 795 894

↑ ^a ^b ^c ^d ^e Bargel, H.-J., Schulze, G. [Ed.]: Material science , Springer, Berlin and Heidelberg, 2000 DNB 958980985

↑ ^a ^b B. Leuschen: Contribution to the load-bearing behavior of aluminum and aluminum-steel resistance spot welded joints under different loads , Diss. RWTH Aachen, 1984

^ ^A ^b D. Steinmeier: Laser and Resistance Weldability Issues, Part I - Bulk Material Properties , microJoining Solutions - microTips ™

↑ L. Pfeifer: Resistance pressure welding - a proven process with many possible uses . Welding and cutting, 38 (1986), H. 1

↑ ^a ^b M. Krause: Resistance pressure welding: Basics - Process - Application , Dt. For welding technology, DVS-Verl. (1993)

↑ DVS: Resistance spot welding of thin sheets made of low-alloy steels - cold-rolled high-strength steels , data sheet DVS 2935-1

↑ DVS: Resistance spot welding of thin sheets made of low-alloy steels - Cold-rolled multi-phase steels (AHSS) , data sheet DVS 2935-2

↑ DVS: Resistance projection and arc stud welding of thin sheet steel with a thick plastic coating on one side , DVS data sheet 2927

↑ DVS: Resistance welding in electronics and precision engineering - overview and fundamentals , information sheet DVS 2950

↑ Miyachieurope "Weldability Micro Resistance Welder" ( Memento from April 7, 2014 in the Internet Archive )

↑ DVS: Resistance spot and roller seam welding of aluminum and aluminum alloys from 0.35 to 3.5 mm individual thickness - weldability , data sheet DVS 2932-1

literature

DIN technical report ISO / TR 581: 2007-04: Weldability - Metallic materials - General principles

[2902-2-1] DVS: Resistance spot welding of steels up to 3 mm single thickness - spot weld suitability, DVS Merkblatt 2902-2, 2001.