Understanding Differences in Welding Steel vs. Aluminium

Understanding Differences in Welding Steel vs. Aluminium

The challenges to spot welding of aluminum compared to steel include a tenacious and rapidly forming oxide layer of variable thickness and composition, high electrical and thermal conductivities, small increases in resistivity with temperature, a narrow plastic range, low melting temperatures, and a high coefficient of thermal expansion. Used by permission from Menachem Kimchi,Assistant Professor-Clinical, Materials Science Engineering, Ohio State University, this excerpt from a new book by Kimchi and David Philips, Resistance Spot Welding Fundamentals and Applications for the Automotive Industry, explains these differences for those who wish a deeper understanding. This article is the first of a series by Kimchi on welding of Advanced High-Strength Steels, so stay tuned!

Thermal Conductivity and Electrical Resistivity

Figure 1: Electrical Resistivity of Steel and Aluminium
(compared to copper electrodes)

The Resistance Spot Welding process, one of the primary processes used in the automotive industry, works best with metal alloys such as steels that have electrical and thermal conductivities that are much lower than the copper-based electrodes used to weld them. Low electrical conductivity (or high resistivity) provides for easy I2R heating, and low thermal conductivity means heat will be extracted from the weld nugget region more slowly. The longer it takes for heat to extract, the more robust the weld. As shown in Figure 1, steel has a very high resistivity and therefore is ideal for this welding process.

Aluminum exhibits electrical and thermal conductivities that are close to copper, two additional reasons contributing to spot welding challenges with this metal. These properties dictate the need for much higher currents, and much shorter times, and therefore a less robust process. Rules of thumb regarding welding currents and times for aluminium is approximately three times the current temperature, and 1/3 of the process times for welding steel. Consequently, existing equipment cannot be used in welding aluminium because of the higher current required.

Plastic Range of Metal

Figure 2: Typical Plastic Ranges, Steel vs. Aluminium

The plastic range of a metal can be loosely defined as the range of temperatures below its melting temperature in which the metal exhibits significant softening. The significance to spot welding is that wider plastic ranges will create a wider softened region around the weld for a longer time. This region, in conjunction with the electrode pressure, effectively “seals” the rapidly expanding (metals exhibit large volumetric expansions when they melt) molten weld nugget, and prevents it from being ejected from the weld zone (expulsion). As indicated on Figure 2 the typical plastic range of aluminum is significantly less than that of steel. The figure also includes a random heating line to illustrate the fact that a narrow plastic range not only reduces the width of the “seal” around the nugget, but also suggests that the window of welding time to produce a good weld is restricted. In summary, the narrow plastic range of aluminum combined with its low melting temperature means that the process window to create a good weld and avoid expulsion is very small.

Dynamic Resistance

Figure 3: Dynamic Resistance Curve for
Steel Vs. Aluminium

As indicated on Figure 3, the dynamic resistance curve for aluminum is entirely different from the curve for steel. Two facts contribute to this vast difference:
1) The oxide on the surface of the aluminum, and
2) The small change in resistivity as a function of temperature.
Upon initial flow of current, resistances are extremely high due to the oxide layer which has much higher resistivity than the aluminium. This increases the likelihood of initial expulsion and will also result in significant electrode heating. The oxide layer quickly breaks down allowing current to pass more easily as resistance drops rapidly. However, as compared to the dynamic resistance curve for steel, there is no significant increase in resistance later in the cycle. The reason for this is compared to steel, aluminum’s resistivity increases only slightly with temperature as shown in Figure 3. The implication of this difference is that there is limited opportunity to grow the nugget by taking advantage of the rapid increase in resistivity, as is the case with steel.

Coefficient of Thermal Expansion

Figure 4: Aluminium Weld Discontinuities (Porosity)

Aluminum’s thermal expansion coefficient is roughly three times higher than steel. This results in greater volumetric expansion of the metal upon heating, and subsequent greater contraction upon cooling. The consequence is a greater chance not only for expulsion, but weld discontinuities such as porosity and solidification cracking (Figure 4). This may mandate the need for low inertia, fast “follow-up” weld heads which can maintain consistent force during the rapid movement of the expanding and contracting weld region. This requires more equipment and expense for the process.

Aluminium’s Oxide Layer

As discussed previously, aluminum forms an oxide layer that is tenacious and forms rapidly. The highly resistive oxide layer can be a benefit in that it significantly increases contact resistance between the sheets being welded. But maintaining a consistent oxide layer thickness is difficult since it happens naturally and rapidly as it is exposed to the environment. Because it is inconsistent, it causes inconsistencies in the weld.
On the other hand, if the oxide layer is significantly reduced by mechanical (such as grinding) or chemical (such as acid cleaning followed by a conversion treatment) methods immediately prior to welding, the need for extremely high currents will be mandated which will promote electrode sticking and accelerated wear.

Reference: Resistance Spot Welding Fundamentals and Applications for the Automotive Industry, Menachem Kimchi, David Phillips, 2017
AHSS Unique Mechanical Properties

AHSS Unique Mechanical Properties

typical stress-strain curve

Typical Stress-Strain Curve

For many years, steel producers and stamping plants have gathered the mechanical properties of sheet metal. Individuals recorded properties such as yield strength, n-value and R-value to name a few. As new materials are introduced into the stamping plants, new mechanical properties tests are being discussed. The current use of aluminum and Dual Phase steels has highlighted issues that were insufficiently described by the typical or standard values provided by the tensile test: edge cracking and variation in springback from run to run are two examples. Resulting outcomes can encompass excessive scrap, excessive re-working of parts to remove edge cracks, and even result in excessive downtime as die makers try to correct the issue. The introduction of increasingly complex and sophisticated materials will exacerbate the inefficiencies of the current stamping process. The learning curve can be frustrating.

Proper training for your workforce regarding material mechanical properties and the know-how to develop robust stamping recipes for Advanced High-Strength Steel (AHSS) will benefit the plant. The typical tensile test provides the yield strength, ultimate tensile strength, n-value, uniform and total elongation. And typically, a strain analysis is performed, producing a forming limit diagram to measure a material’s formability. But these tests are reflective of global forming behaviors and don’t adequately describe an advanced material’s performance in localized formability. We need to ask if typical mechanical properties provide adequate information to truly assess the impact to the stamping operation – and if not, what new tests will provide insight on how these materials react to certain forming loads.

Instantaneous n-value

Instantaneous n-value

A new output is instantaneous n-value. Instantaneous n-value identifies the strain gradient – how the material will work-harden at initial contact with the die geometry. If only the standard n-value (work hardening exponent) is observed, measured between 10% and 20% strain, the increase in work hardening that occurs during deformation will be missed. Hole expansion and three-point bend tests are now commonly performed. The hole expansion test identifies the materials ability to stretch at the sheared edge. The three-point bend test identifies the stretchablity of a material via the minimum bend radius that can be achieved for that specific material.

Focusing on AHSS, these materials are characterized by multiple phase structures designed to improve formability. These designer steels are being created to improve crash worthiness through higher strength and lighter gauge thicknesses. To build upon the current understanding of material mechanical properties and resulting forming behaviors, training on AHSS products, testing and stamping process countermeasures is encouraged.

 

 

Eager to know more about metallurgy, forming and joining of Advanced High-Strength Steels? Download a free copy of AHSS Application Guidelines from WorldAutoSteel today!