Is there any link between Achilles and Sigurd/Siegfried?

The Achille's Heel folk motif, i.e. invulnerability except in one spot, is a very common one worldwide. It appears as motif number Z311 in Thompson's Motif-Index of Folk-Literature, where an incomplete list of attestations of the motif is given, ranging from Irish to African folklore, passing through Jewish, Eskimo and Siberian tales.

With such widespread folk motifs it is always hard to argue for a common (pre-historic?) origin, independent developments, or mutual influences. There is however a set of Indo-European myths concerning imperfect invulnerability which bear such striking similarities that they have led scholars to conjecture a common origin, or at least strong mutual influences. The tales of Greek Achilles, Norse Sigurd and Balder, and Ossetian Soslan belong to this category.

The last one in particular seems to be the link that brings the remaining three together, to the point that Fridrik Thordarson, in his paper Die Ferse des Achilleus‐ein skythisches motiv? (in German) proposes a Scythian (the Ossetians being descendents of the Scythians) origin of the myths of Achilles and Sigurd. At the same time, Georges Dumézil in his book Loki draws a very convincing parallel between the tales of the deaths of Balder and Soslan, and argues for a common Proto-Indo-European origin.

Also John Colarusso in his Peoples, Languages and Lore agrees on the similarities between the four myths. He writes on Soslan (North-West Caucasian Sosruquo):

He [Sosruquo] is born aflame from a rock and tempered by the god of the forge [...]. He is vulnerable in his knees or thighs, where he was held by the smith’s tongs, and in this localised soft spot he shows a close parallel with Greek Achilles and Norse Sigurd.

And this is what he has to say on Syrdon (the Ossetian homologue of Norse Loki) who, much like the latter is responsible for the death of Balder, causes Soslan's death by finding out his only weakness:

A figure restricted to the Ossetian corpus is that of Syrdon, [...] a trickster figure similar to Norse Loki. As Loki resents the gods for their glory and loftiness, so Syrdon is maddened by the fame and arrogance of the Narts [i.e. the Ossetian heroes]. [...] The Narts tolerate Syrdon’s mischief up to a point [i.e. the death of Soslan], just as the Norse gods tolerate Loki’s, up to a point [i.e. the death of Balder].

Some time ago I thought about the same. And I think there is a connection between many myths of different cultures or maybe even a form of proto-myth if you will. An example would be the Noah and flood story in Christendom and the Deucalion myth in Greek mythology. In the case of Siegfried and Achilles, I think it is somewhat justified to believe that both stories share at least some elements. Besides what you mentioned, there is also the idea that name 'Hagen from Tronje' actually derives from the word Troy (http://self.gutenberg.org/articles/Hagen_(legend)). However, this might be just a coincidence, as it was quite fashionable during the middle ages to claim some heroic heritage. Having said that, I am far from an expert in the field, but I sort of like it to be true.

It would seem that Achilles being dipped in the River refers to an old Smithy tale of Quenching: dipping the red-hot iron/carbon meat, aka steel, in water or oil to harden it. It is known Achilles wore a suit (of apparently steel) armor. The "weak spot" would be where the smithy held the armor during the quenching process.

Regarding "Indo-European" origins of metal-working, I would highly recommend this paper, found at academia.edu:

Tvaṣṭr, Meluhha of Bhāratam Janam (Rigveda) is Tuisto, divine ancestor of Germanic peoples (Tacitus), legacy of Proto-Indo-Aryan superstrate & Mitanni Treaty

This can be seen as a pilgrimage, a journey of Meluhha artisans/sea-faring merchants in Eurasia, during the Bronze Age, along the Maritime Tin Road from Hanoi, Vietnam to Haifa, Israel.

This journey also explains why many Proto-Indo-Aryan words like those present in Mitanni Treaty occur on Indus Script Corpora which is a veritable catalogus catalogurm of metalwork by metalcasters in the tradition of Tvaṣṭr , Tuisto...Tvaṣtṛi is the Vulcan of the Hindu mythology. He had a son named Triśiras...Meluhha of the Indian sprachbund, is the spoken form of chandas, the prosody of Rigveda. Tvaṣṭr is the metal artificer par excellence, who forges the vajra (thunderbolt) weapon for Indra, a narrative celebrated in exquisite metaphors of Rigvedic chandas..."King Shaushtatar (ruled c. 1430) extended the boundaries of Mitanni through the conquest of Alalakh, Nuzi, Assur, and Kizzuwatna. Egypt, under Tuthmosis III (1479-1425 BCE), defeated the Mitanni at Aleppo after a long period of contention over control of the region of Syria. Later Egyptian dynasties entered into pacts and treaties with Mitanni and the daughter of the Mitanni King Tushratta, the princess Taduhepa, was given in marriage to Amenhotep III (1391-1353 BCE) as part of a treaty which balanced power between the two nations.This treaty was put to the test during a power struggle in Washukanni between Tushratta and a relative of the previous king, Shuttarna, known as Artatama II.

I am of the opinion that "Tadukhepa" is the same name as the Russian "Dudieva" ("khepa" is pronounced Eva or Eve) and she is none other than "Nefertiti." Further, King Shashtatar reminds strongly of Esua. Further, it seems clear chronology must be re-examined stringently: is this king not a Tartar? "Scratch a Russian and underneath you'll find a Tartar."